CHAPTER IX

EVOLUTION AS THE RESULT OF EXTERNAL AND INTERNAL FACTORS

We come now to a consideration of other theories that have been advanced to account for the evolution of new forms; and in so far as these new forms are adapted to their environment, the theories will bear directly on the question of the origin of adaptive variations. One school of transformationists has made the external world and the changes taking place in it the source of new variations. Another school believes that the changes arise within the organism itself. We may examine these two points of view in turn.

The Effect of External Influences

We have already seen that Lamarck held as a part of his doctrine of transformation that the changes in the external world, the environment, bring about, directly, changes in the organism, and he believed that all plants and many of the lower animals have evolved as the result of a reaction of this sort. This idea did not originate with Lamarck, however, since before him Buffon had advanced the same hypothesis, and there cannot be much doubt that Lamarck borrowed from his patron, Buffon, this part of his theory of evolution.

This idea of the influence of the external world as a factor inducing changes in the organism has come, however, to be associated especially with the name of Geoffroy Saint-Hilaire, whose period of activity, although overlapping, came after that of Lamarck. The central idea of Geoffroy’s view was that species of animals and plants undergo change as the environment changes; and it is important to note, in passing, that he did not suppose that these changes were always for the benefit of the individual, _i.e._ they were not always adaptive. If they were not, the forms became extinct. So long as the conditions remain constant, the species remains constant; and he found an answer in this to Cuvier’s argument, in respect to the similarity between the animals living at present in Egypt and those discovered embalmed along with mummies at least two thousand years old. Geoffroy Saint-Hilaire said, that since the climatic conditions of Egypt had remained exactly the same during all these years, the animals of Egypt would also have remained unchanged.

Geoffroy’s views were largely influenced by his studies in systematic zoology and by his conception of a unity of plan running through the entire animal kingdom. His study of embryology and paleontology had led him to believe that present forms have descended from other organisms living in the past, and in this connection his discovery of teeth in the jaws of the embryo of the baleen whale and also his discovery of the embryonic dental ridges in the upper and in the lower jaws of birds, were used with effect in supporting the theory of change or evolution. Lastly, his remarkable work in the study of abnormal forms prepared the way for his conception of sudden and great changes, which he believed organisms capable of undergoing. He went so far in fact, in one instance, as to suppose that it was not impossible that a bird might have issued fully equipped from the egg of a crocodile. Such an extreme statement, which seems to us nowadays only laughable, need not prejudice us against the more moderate parts of his speculation.

His study of the fossil gavials found near Caen led him to believe that they are quite distinct from living crocodiles. He asked whether these old forms may not represent a link in the chain that connects, without interruption, the older inhabitants of the earth with animals living at the present time. Without positively affirming that this is the case, he did not hesitate to state that a transformation of this sort seemed possible to him. He said: “I think that the process of respiration constitutes an acquirement so important in the ‘disposition’ of the forms of animals, that it is not at all necessary to suppose that the surrounding respiratory gases become modified quickly and in large amount in order that the animal may become slowly modified. The prolonged action of time would ordinarily suffice, but if combined with a cataclysm, the result would be so much the better.”

He supposed that in the course of time respiration becomes difficult and finally impossible as far as certain systems of organs are concerned. The necessity then arises and creates another arrangement, perfecting or altering the existing structures. Modifications, fortunate or fatal, are created which through propagation are continued, and which, if fortunate, influence all the rest of the organization. But if the modifications are injurious to the animals in which they have appeared, the animals cease to exist, and are replaced by others having a different form, and one suited to the new circumstances.

The comparison between the stages of development of the individual and the evolution of the species was strongly impressed on the mind of Geoffroy. He says: “We see, each year, the spectacle of the transformation in organization from one class into another. A batrachian is at first a fish under the name of a tadpole, then a reptile (amphibian) under that of a frog.” “The development, or the result of the transformation, is brought about by the combined action of light and of oxygen; and the change in the body of the animal takes place by the production of new blood-vessels, whose development follows the law of the balancing of organs, in the sense, that if the circulating fluids precipitate themselves into new channels there remains less in the old vessels.” By preventing tadpoles from leaving the water, Geoffroy claims that it has been shown that they can be prevented from changing into frogs. The main point that Geoffroy attempts to establish is no doubt fairly clear, but the way in which he supposes the change to be effected is not so clear, and his ideas as to the way in which new change may be perpetuated in the next generation are, from our more modern point of view, extremely hazy. It is perhaps not altogether fair to judge his view from the standpoint of the origin of adaptive structures, but rather as an attempt to explain the causes that have brought about the evolution of the organic world.

During the remainder of the nineteenth century there accumulated a large number of facts in relation to the action of the external conditions in bringing about changes in animals and plants. Much of this evidence is of importance in dealing with the question of the origin of organic adaptation.

The first class of facts in this connection is that of geographical variation in animals and plants. It will be impossible here to do more than select some of the most important cases. De Varigny, in his book on “Experimental Evolution,” has brought together a large number of facts of this kind, and from his account the following illustrations have been selected. He says: “When the small brown honey-bee from High Burgundy is transported into Bresse—although not very distant—it soon becomes larger and assumes a yellow color; this happens even in the second generation.” It is also pointed out that the roots of the beet, carrot, and radish are colorless in their wild natural state, but when brought under cultivation they become red, yellow, etc. Vilmorin has noted that the red, yellow, and violet colors of carrots appear only some time after the wild forms have been brought under cultivation. Moquin-Tandon has seen “gentians which are blue in valleys become white on mountains.” Other cases also are on record in which the colors of a plant are dependent on external conditions.

The sizes of plants and animals are also often directly traceable to certain external conditions; the change is generally connected with the amount of food obtainable. “Generally speaking,” De Varigny says, “insular animals are smaller than their continental congeners. In the Canary Islands the oxen of one of the smallest islands are smaller than those on the others, although all belong to the same breed, and the horses are also smaller, and the indigenous inhabitants are in the same case, although belonging to a tall race. It would seem that in Malta elephants were very small,—fossil elephants, of course,—and that during the Roman period the island was noted for a dwarf breed of dogs, which was named after its birthplace, according to Strabo. In Corsica, also, horses and oxen are very small, and _Cervus corsicanus_, the indigenous deer, is quite reduced in dimensions; ... and lastly, the small dimensions of the Falkland horses—imported from Spain in 1764—are familiar to all. The dwarf rabbits of Porto Santo described by Darwin may also be cited as a case in point.”

These facts, interesting as they are, will, no doubt, have to be more carefully examined before the evidence can have great value, for it is not clear what factor or factors have produced the decrease in size of these animals.

The following cases show more clearly the immediate effect of the environment: “Many animals, when transferred to warm climates, lose their wool, or their hairy covering is much reduced. In some parts of the warmer regions of the earth, sheep have no wool, but merely hairs like those of dogs. Similarly, as Roulin notices, poultry have, in Columbia, lost their feathers, and while the young are at first covered with a black and delicate down, they lose it in great part as they grow, and the adult fowls nearly realize Plato’s realistic description of man—a biped without feathers. Conversely, many animals when transferred from warm to cold climates acquire a thicker covering; dogs and horses, for instance, becoming covered with wool.”

A number of kinds of snails that were supposed to belong to different species have been found, on further examination, to be only varieties due to the environment. “Locard has discovered through experiments that _L. turgida_ and _elophila_ are mere varieties—due to environment—of the common _Lymnæa stagnalis_.” He says, “These are not new species, but merely common aspects of a common type, which is capable of modification and of adaptation according to the nature of the media in which it has to live.” It has also been shown by Bateson that similar changes occur in _Cardium edule_, and other lamellibranchs are known to vary according to the nature of the water in which they live.

In regard to plants, the influence of the environment has long been known to produce an effect on the form, color, etc., of the individuals. “The common dandelion (_Taraxacum densleonis_) has in dry soil leaves which are much more irregular and incised, while they are hardly dentate in marshy stations, where it is called _Taraxacum palustre_.

“Individuals growing near the seashore differ markedly from those growing far inland. Similarly, species such as some Ranunculi, which can live under water as well as in air, exhibit marked differences when considered in their different stations, as is well known to all. These differences may be important enough to induce botanists to believe in the existence of two different species when there is only one.”

An interesting case is that of _Daphnia rectirostris_, a small crustacean living sometimes in fresh water, at other times in water containing salt and also in salt lakes. There are two forms, corresponding to the conditions under which they live, and it is said that the differences are of a kind that suffice to separate species from each other. In another crustacean, _Branchipus ferox_, the form differs in a number of points, according to whether it lives in salt or in fresh water. Schmankewitsch says that, had he not found all transitional forms, and observed the transformation in cultures, he would have regarded the two forms as separate species. The oft-quoted case of Artemia furnishes a very striking example of the influence of the environment. _Artemia salina_ lives in water whose concentration varies between 5 and 12 degrees of saltness. When the amount of salt is increased to 12 degrees, the animal shows certain characteristics like those of _Artemia milhausenii_, which may live in water having 24 to 25 degrees of saltness. The form _A. salina_ may be further completely changed into that of _A. milhausenii_ by increasing the amount of salt to the latter amount.

Among domesticated animals and plants—a few instances of which have been already referred to—we find a large number of cases in which a change in the environment produces definite changes in the organism. Darwin has made a most valuable collection of facts of this kind in his “Animals and Plants under Domestication.” He believes that domesticated forms are much more variable than wild ones, and that this is due, in part, to their being protected from competition, and to their having been removed from their natural conditions and even from their native country. “In conformity with this, all our domesticated productions without exception vary far more than natural species. The hive-bee, which feeds itself, and follows in most respects its natural habits of life, is the least variable of all domesticated animals.... Hardly a single plant can be named, which has long been cultivated and propagated by seed, that is not highly variable.” “Bud-variation ... shows us that variability may be quite independent of seminal reproduction, and likewise of reversion to long-lost ancestral characters. No one will maintain that the sudden appearance of a moss-rose on a Provence rose is a return to a former state, ... nor can the appearance of nectarines on peach trees be accounted for on the principle of reversion.” It is said that bud-variations are also much more frequent on cultivated than on wild plants.

Darwin adds: “These general considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability.”

In some cases it has been observed that, in passing from one part of a continent to another, many or all of the forms of the same group and even of different groups change in the same way. Allen’s account of the variations in North American birds and mammals furnishes a number of striking examples of this kind of change. He finds that, as a rule, the birds and mammals of North America increase in size from the south northward. This is true, not only for the individuals of the same species, but generally the largest species of each genus are in the north. There are some exceptions, however, in which the increase in size is in the opposite direction. The explanation of this is that the largest individuals are almost invariably found in the region where the group to which the species belongs receives its greatest numerical development. This Allen interprets as the hypothetical “centre of distribution of the species,” which is in most cases doubtless also its original centre of dispersal. If the species has arisen in the north, then the northern forms are the largest; but if it arose in the south, the reverse is the case. Thus, most of the species of North America that live north of Mexico are supposed to have had a northern origin, as shown by the circumpolar distribution of some of them and by the relationship of others to Old World species; and in these the largest individuals of the species of a genus are northern. Conversely, in the exceptional cases of increase in size toward the south, it can be shown that the forms have probably had a southern origin.

The Canidæ (wolves and foxes) have their largest representatives, the world over, in the north. “In North America the family is represented by six species, the smallest of which (speaking generally) are southern and the largest northern.” The three species that have the widest ranges (the gray wolf, the common fox, and the gray fox) show the most marked differences in size. The skull, for instance, of “the common wolf is fully one-fifth larger in the northern parts of British America and Alaska than it is in northern Mexico, where it finds the southern limit of its habitat. Between the largest northern skull and the largest southern skull there is a difference of about thirty-five per cent of the mean size. Specimens from the intermediate region show a gradual intergradation between the extremes, although many of the examples from the upper Missouri country are nearly as large as those from the extreme north.” The common fox is about one-tenth larger, on the average, in Alaska than it is in New England. The gray fox, whose habitat extends from Pennsylvania southward to Yucatan, has an average length of skull of about five inches in the north, and less than four in Central America—about ten per cent difference.

The Felidæ, or cats, “reach their greatest development as respects both the number and the size of the species in the intertropical regions. This family has sent a single typical representative, the panther (_Felis concolor_), north of Mexico, and this ranges only to about the northern boundary of the United States. The other North American representatives of the family are the lynxes, which in some of their varieties range from Alaska to Mexico.” Although they vary greatly in different localities in color and in length and texture of pelage, they do not vary as to the size of their skulls. On the other hand the panther (and the ocelots) greatly increases in size southward, “or toward the metropolis of the family.”

Other carnivora that increase in size northward are the badger, the marten, the fisher, the wolverine, and the ermine, which are all northern types.

Deer are also larger in the north; in the Virginia deer the annually deciduous antlers of immense size reach their greatest development in the north. The northern race of flying squirrels is one-half larger than the southern, “yet the two extremes are found to pass so gradually one into the other, that it is hardly possible to define even a southern and a northern geographical race.” The species ranges from the arctic regions to Central America.

In birds also similar relations exist, but there is less often an increase in size northward. In species whose breeding station covers a wide range of latitude, the northern birds are not only smaller, but have quite different colors, as is markedly the case in the common quail, the meadow-lark, the purple grackle, the red-winged blackbird, the flicker, the towhee bunting, the Carolina dove, and in numerous other species. The same difference is also quite apparent in the blue jay, the crow, in most of the woodpeckers, in the titmice, numerous sparrows, and several warblers and thrushes. The variation often amounts to from ten to fifteen per cent of the average size of the species.

Allen also states that certain parts of the animal may vary proportionately more than the general size, there being an apparent tendency for peripheral parts to enlarge toward the warmer regions, _i.e._ toward the south. “In mammals which have the external ears largely developed—as in the wolves, foxes, some of the deer, and especially the hares—the larger size of this organ in southern as compared with northern individuals of the same species, is often strikingly apparent.” It is even more apparent in species inhabiting open plains. The ears of the gray rabbit of the plains of western Arizona are twice the size of those of the Eastern states.

In birds the bill especially, but also the claws and tail, is larger in the south. In passing from New England southward to Florida the bill in slender-billed forms becomes larger, longer, more attenuated, and more decurved; while in short-billed forms the southern individuals have thicker and larger bills, although the birds themselves are smaller.

The remarkable changes and gradations of color in birds in different parts of North America are very instructive, and the important results obtained by American ornithologists form an interesting chapter in zoology. The evidence would convince the most sceptical of the difficulty of distinguishing between Linnæan species. It is not surprising to find in this connection a leading ornithologist exclaiming, “if there really are such things as species.” The differences here noted are mainly from east to west. We may briefly review here a few striking cases selected from Coues’s “Key to North American Birds.”

The flicker, or golden-winged woodpecker (_Colaptes auratus_), has a wide distribution in eastern North America. It is replaced in western North America (from the Rocky Mountains to the Pacific) by _C. mexicanus_. In the intermediate regions, Missouri and the Rocky Mountain region, the characters of the two are blended in every conceivable degree in different specimens. “Perhaps it is a hybrid, and perhaps it is a transitional form, and doubtless there are no such things as species in Nature.... In the west you will find specimens _auratus_ on one side of the body, _mexicanus_ on the other.” There is a third form, _C. chrysoides_, with the wings and tail as in _auratus_, and the head as in _mexicanus_, that lives in the valley of the Colorado River, Lower California, and southward.

In regard to the song-sparrow (_Melospiza_), Coues writes: “The type of the genus is the familiar and beloved song-sparrow, a bird of constant characters in the east, but in the west is split into numerous geographical races, some of them looking so different from typical _fasciata_ that they have been considered as distinct species, and even placed in other genera. This differentiation affects not only their color, but the size, relative proportions of parts, and particularly the shape of the bill; and it is sometimes so great, as in the case of _M. cinerea_, that less dissimilar looking birds are commonly assigned to different genera. Nevertheless the gradation is complete, and affected by imperceptible degrees.... The several degrees of likeness and unlikeness may be thrown into true relief better by some such expressions as the following, than by formal antithetical phrases: (1) The common eastern bird commonly modified in the interior into the duller colored (2) _fallax_. This in the Pacific watershed, more decidedly modified by deeper coloration,—broader black streaks in (3) _hermanni_, with its diminutive local race (4) _samuelis_, and more ruddy shades in (5) _guttata_ northward, increasing in intensity with increased size in (6) _rafina_. Then the remarkable (7) _cinerea_, insulated much further apart than any of the others. A former American school would probably have made four ‘good species,’ (1) _fasciata_, (2) _samuelis_, (3) _rafina_, (4) _cinerea_.”

Somewhat similar relations are found in three other genera of finches. Thus Passerella is “imperfectly differentiated”; Junco is represented by one eastern species, but in the west the stock splits up into numerous forms, “all of which intergrade with each other and with the eastern bird. Almost all late writers have taken a hand at Junco, shuffling them about in the vain attempt to decide which are ‘species’ and which ‘varieties.’ All are either or both, as we may elect to consider them.” In the distribution of the genus Pipilo similar relations are found. There is an eastern form much more distinct from the western forms than these are from each other.

Finally may be mentioned the curious variations in screech-owls of the genus Scops. This owl has two strikingly different plumages—a mottled gray and a reddish brown, which, although very distinct when fully developed, yet “are entirely independent of age, season, or sex.” There is an eastern form, _Scops asio_, that extends west to the Rocky Mountains. There is a northwestern form, _S. kennicotti_, which in its red phase is quite different from _S. asio_, but in its gray plumage is very similar. The California form, _S. benderii_, is not known to have a red phase, and the gray phase is quite different from that of _S. asio_, but like the last form. The Colorado form, _S. maxwellæ_, has no red phase, “but on the contrary the whole plumage is very pale, almost as if bleached, the difference evident in the nestlings even.” The Texas form, _S. maselli_, has both phases, and is very similar to _S. asio_. The Florida form is smaller and colored like _S. asio_. The red phase is the frequent, if not the usual, one. The flammulated form, _S. fiammula_, is “a very _small species_, with much the general aspect of an ungrown _S. asio_.” This is the southwestern form, easily distinguished on account of its small size and color from the other forms.

These examples might be greatly increased, but they will suffice, I think, to convince one of the difficulty of giving a sharp definition to “species.” The facts speak strongly in favor of the transmutation theory, and show us how a species may become separated under different conditions into a number of new forms, which would be counted as new different species, if the intermediate forms were exterminated.

In discussing the nature of the changes that bring about variability, Darwin remarks: “From a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have varied. We see this with the many domestic races of quadrupeds and birds belonging to different orders, with goldfish and silkworms, with plants of many kinds, raised in various quarters of the world. In the deserts of northern Africa the date-palm has yielded thirty-eight varieties; in the fertile plains of India it is notorious how many varieties of rice and of a host of other plants exist; in a single Polynesian island, twenty-four varieties of the breadfruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives. The mulberry tree of India and Europe has yielded many varieties serving as food for the silkworm; and in China sixty-three varieties of the bamboo are used for various domestic purposes. These facts, and innumerable others which could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability—different changes acting on different organisms.”

Darwin thinks that a change in climate alone is not one of the potent causes of variability, because the native country of a plant, where it has been longest cultivated, is where it has oftenest given rise to the greatest number of varieties. He thinks it also doubtful that a change in food is an important source of variability, since the domestic pigeon has varied more than any other species of fowl, yet the food has been always nearly the same. This is also true for cattle and sheep, whose food is probably much less varied in kind than in the wild species.

Another point of interest is raised by Darwin. He thinks, as do others also, that the influence of a change in the conditions is cumulative, in the sense that it may not appear until the species has been subjected to it for several generations. Darwin states that universal experience shows that when new plants are first introduced into gardens they do not vary, but after several generations they will begin to vary to a greater or less extent. In a few cases, as in that of the dahlia, the zinnia, the Swan River daisy, and the Scotch rose, it is known that the new variations only appeared after a time. The following statement by Salter is then quoted, “Every one knows that the chief difficulty is in breaking through the original form and color of the species, and every one will be on the lookout for any natural sport, either from seed or branch; that being once obtained, however trifling the change may be, the result depends on himself.” Jonghe is also quoted to the effect that “there is another principle, namely, that the more a type has entered into a state of variation, the greater is the tendency to continue doing so, and the more it has varied from the original type, the more is it disposed to vary still further.” Darwin also quotes with approval the opinion of the most celebrated horticulturist of France, Vilmorin, who maintained that “when any particular variation is desired, the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction; for the fixed character of the species being once broken, the desired variation will sooner or later appear.”

Darwin also cites a few cases where animals have changed quite quickly when brought under domestication. Turkeys raised from the eggs of wild species lose their metallic tints, and become spotted with white in the third generation. Wild ducks lose their true plumage after a few generations. “The white collar around the neck of the mallard becomes much broader and more irregular, and white feathers appear in the duckling’s wings. They increase also in size of body.” In these cases it appears that several generations were necessary in order to bring about a marked change in the original type, but the Australian dingoes, bred in the Zoological Gardens, produced puppies which were in the first generation marked with white and other colors.

The following cases from De Varigny are also very striking. The dwarf trees from Japan, for the most part conifers, which may be a hundred years old and not be more than three feet high, are in part the result “of mechanical processes which prevent the spreading of the branches, and in part of a starving process which consists in cutting most roots and in keeping the plant in poor soil.”

As an example of the sudden appearance of a new variation the following case is interesting. A variety of begonia is recorded as having appeared quite suddenly at a number of places at the same time. In another case a narcissus which had met with adverse circumstances, and had then been supplied with a chemical manure in some quantity, began to bear double flowers.

Amongst animals the following cases of the appearance of sudden variations are pointed out by De Varigny. “In Paraguay, during the last century (1770), a bull was born without horns, although his ancestry was well provided with these appendages, and his progeny was also hornless, although at first he was mated with horned cows. If the horned and the hornless were met in fossil state, we would certainly wonder at not finding specimens provided with semi-degenerate horns, and representing the link between both, and if we were told that the hornless variety may have arisen suddenly, we should not believe it and we should be wrong. In South America also, between the sixteenth and eighteenth centuries the niata breed of oxen sprang into life, and this breed of bulldog oxen has thriven and become a new race. So in the San Paulo provinces of Brazil, a new breed of oxen suddenly appeared which was provided with truly enormous horns, the breed of franqueiros, as they are called. The mauchamp breed of sheep owes its origin to a single lamb that was born in 1828 from merino parents, but whose wool, instead of being curly like that of its parents, remained quite smooth. This sudden variation is often met with, and in France has been noticed in different herds.”

The ancon race of sheep originated in 1791 from a ram born in Massachusetts having short crooked legs and a long back. From this one ram by crossing, at first with common sheep, the ancon race has been produced. “When crossed with other breeds the offspring, with rare exception, instead of being intermediate in character, perfectly resemble either parent; even one of twins has resembled one parent and the second the other.”

Two especially remarkable cases remain to be described. These are the Porto Santo rabbit and the japanned peacock. Darwin has given a full account of both of these cases. “The rabbits which have become feral on the island of Porto Santo, near Madeira, deserve a fuller account. In 1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board which had produced young during the voyage, and he turned them all out on the island. These animals soon increased so rapidly that they became a nuisance, and actually caused the abandonment of the settlement. Thirty-seven years subsequently, Cada Mosto describes them as innumerable; nor is this surprising, as the island was not inhabited by any beast of prey, or by any terrestrial mammal. We do not know the character of the mother rabbit; but it was probably the common domestic kind. The Spanish peninsula, whence Zarco sailed, is known to have abounded with the common wild species at the most remote historical period; and as these rabbits were taken on board for food, it is improbable that they should have been of any peculiar breed. That the breed was well domesticated is shown by the doe having littered during the voyage. Mr. Wollaston, at my request, brought two of these feral rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home three more specimens in brine and two alive. These seven specimens, though caught at different periods, closely resemble each other. They were full-grown, as shown, by the state of their bones. Although the conditions of life in Porto Santo are evidently highly favorable to rabbits, as proven by their extraordinarily rapid increase, yet they differ conspicuously in their small size from the wild English rabbit.... In color the Porto Santo rabbit differs considerably from the common rabbit; the upper surface is redder, and is rarely interspersed with any black or black-tipped hairs. The throat and certain parts of the under surface, instead of being pure white, are generally gray or leaden color. But the most remarkable difference is in the ears and tail. I have examined many fresh English rabbits, and the large collection of skins in the British Museum from various countries, and all have the upper surface of the tail and the tips of the ears clothed with blackish gray fur; and this is given in most works as one of the specific characters of the rabbit. Now in the seven Porto Santo rabbits the upper surface of the tail was reddish brown, and the tips of the ears had no trace of the black edging. But here we meet with a singular circumstance: in June, 1861, I examined two of these rabbits recently sent to the Zoological Gardens and their tails and ears were colored as just described; but when one of their dead bodies was sent to me in February, 1863, the ears were plainly edged, and the upper surface of the tail was covered with blackish gray fur, and the whole body was much less red; so that under the English climate this individual rabbit had recovered the proper color of its fur in rather less than four years.”

Another striking case of sudden variation is found in the peacock. It is all the more remarkable because this bird has hardly varied at all under domestication, and is almost exactly like the wild species living in India to-day. Darwin states: “There is one strange fact with respect to the peacock, namely, the occasional appearance in England of the ‘japanned’ or ‘black-shouldered’ kind. This form has lately been named, on the high authority of Mr. Slater, as a distinct species, viz. _Pavo nigripennis_, which he believes will hereafter be found wild in some country, but not in India, where it is certainly unknown. The males of these japanned birds differ conspicuously from the common peacock in the color of their secondary wing-feathers, scapulars, wing-coverts, and thighs, and are, I think, more beautiful; they are rather smaller than the common sort, and are always beaten by them in their battles, as I hear from the Hon. A. S. G. Canning. The females are much paler-colored than those of the common kind. Both sexes, as Mr. Canning informs me, are white when they leave the egg, and they differ from the young of the white variety only in having a peculiar pinkish tinge on their wings. These japanned birds, though appearing suddenly in flocks of the common kind, propagate their kind quite truly.”

In two cases, in which these birds had appeared quite suddenly in flocks of the ordinary kind, it is recorded that “though a smaller and weaker bird, it increased to the extinction of the previously existing breed.” Here we have certainly a remarkable case of a new species suddenly appearing and replacing the ordinary form, although the birds are smaller, and _are beaten in their battles_.

Darwin has given an admirably clear statement of his opinion as to the _causes of variability_ in the opening paragraph of his chapter dealing with this topic in his “Animals and Plants.” Some authors, he says, “look at variability as a necessary contingent on reproduction, and as much an original law as growth or inheritance. Others have of late encouraged, perhaps unintentionally, this view by speaking of inheritance and variability as equal and antagonistic principles. Pallas maintained, and he has had some followers, that variability depends exclusively on the crossing of primordially distinct forms. Other authors attribute variability to an excess of food, and with animals, to an excess relatively to the amount of exercise taken, or again, to the effects of a more genial climate. That these causes are all effective is highly probable. But we must, I think, take a broader view, and conclude that organic beings, when subjected during several generations to any change whatever in their condition, tend to vary; the kind of variation which ensues depending in most cases in a far higher degree on the nature of the constitution of the being, than on the nature of the changed conditions.”

Most naturalists will agree, in all probability, with this conclusion of Darwin’s. The examples cited in the preceding pages have shown that there are several ways in which the organisms may respond to the environment. In some cases it appears to affect all the individuals in the same way; in other cases it appears to cause them to fluctuate in many directions; and in still other cases, without any recognizable change in the external conditions, new forms may suddenly appear, often of a perfectly definite type, that depart widely from the parent form.

For the theory of evolution it is a point of the first importance to determine which of these modes of variation has supplied the basis for evolution. Moreover, we are here especially concerned with the question of how adaptive variations arise. Without attempting to decide for the present between these different kinds of variability, let us examine certain cases in which an immediate and adaptive response to the environment has been described as taking place.

Responsive Changes in the Organism that adapt it to the New Environment

There is some experimental evidence showing that sometimes organisms respond directly and adaptively to certain changes in the environment. Few as the facts are, they require very careful consideration in our present examination. The most striking, perhaps, is the acclimatization to different temperatures. It has been found that while few active organisms can withstand a temperature over 45 degrees C., and that for very many 40 degrees is a fatal point, yet, on the other hand, there are organisms that live in certain hot springs where the temperature is very high. Thus, to give a few examples, there are some of the lower plants, nostocs and protococcus forms, that live in the geysers of California at a temperature of 93 degrees C., or nearly that of boiling water. Leptothrix is found in the Carlsbad springs, that have a temperature of 44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in water between 54 and 68 degrees, and in the hot springs in the Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland at 98 degrees.

It is probable from recent observations of Setchel that most of the temperatures are too high, since he finds that the water at the edge of hot springs is many degrees lower than that in the middle parts.

The snail, _Physa acuta_, has been found in France living at a temperature of 35 to 36 degrees; another snail, Paludina, at Abano, Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54 degrees; Anguillidæ at Ischia at 81 degrees; _Cypris balnearia_, a crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of “Pise” at 38 degrees.

Now, there can be little doubt that these forms have had ancestors that were like the other members of the group, and would have been killed had they been put at once into water of these high temperatures, therefore it seems highly probable that these forms have become specially adapted to live in these warm waters. It is, therefore, interesting to find that it has been possible to acclimatize animals experimentally to a temperature much above that which would be fatal to them if subjected directly to it. Dutrochet (in 1817) found that if the plant, nitella, was put into water at 27 degrees, the currents in the protoplasm were stopped, but soon began again. If put now into water at 34 degrees they again stopped moving, but in a quarter of an hour began once more. If then put into water at 40 degrees the currents again slowed down, but began again later.

Dallinger (in 1880) made a most remarkable series of experiments on flagellate protozoans. He kept them in a warm oven, beginning at first at a temperature of 16.6 degrees C. “He employed the first four months in raising the temperature 5.5 degrees. This, however, was not necessary, since the rise to 21 degrees can be made rapidly, but for success in higher temperatures it is best to proceed slowly from the beginning. When the temperature had been raised to 23 degrees, the organisms began dying, but soon ceased, and after two months the temperature was raised half a degree more, and eventually to 25.5 degrees. Here the organisms began to succumb again, and it was necessary repeatedly to lower the temperature slightly, and then to advance it to 25.5 degrees, until, after several weeks, unfavorable appearances ceased. For eight months the temperature could not be raised from this _stationary point_ a quarter of a degree without unfavorable appearances. During several years, proceeding by slow stages, Dallinger succeeded in raising the organisms up to a temperature of 70 degrees C., at which the experiment was ended by an accident.”[27]

Footnote 27:

Quoted from Davenport’s “Experimental Morphology.”

Davenport and Castle carried out a series of experiments on the egg of the toad, in which they tried to acclimatize the eggs to a temperature higher than normal. Recently laid eggs were used; one lot kept at a temperature of 15 degrees C., the other at 24-25 degrees C. Both lots developed normally. At the end of four weeks the temperature point at which the tadpoles were killed was determined. Those reared at a temperature of 15 degrees C. died at 41 degrees C., or below; those reared at 24-25 degrees C. sustained a temperature 10 degrees higher; no tadpole dying in this set under 43 degrees C. “This increased capacity for resistance was not produced by the dying off of the less resistant individuals, for no death occurred in these experiments during the gradual elevation of the temperatures in the cultures.” The increased resistance was due, therefore, to a change in the protoplasm of the individuals. It was also determined that the acquired resistance was only very gradually lost (after seventeen days’ sojourn in cooler water). The explanation of this result may be due, in part, to the protoplasm containing less water at higher temperatures, for it is known that while the white of egg (albumen) coagulates at 56 degrees C. in aqueous solution; with only 18 per cent of water it coagulates between 80 degrees and 90 degrees C.; and with 6 per cent, at 145 degrees C.; and without water between 100 degrees and 170 degrees C.

It has long been known that organisms in the dry condition resist a much higher temperature. The damp uredospore is killed at 58.5 degrees to 60 degrees C.; but dry spores withstand 128 degrees C. It is also known that organisms may become acclimatized to cold through loss of water, but we lack exact experimental data to show to what extent this can be carried.

There are also some experiments that go to show that animals may become attuned to certain amounts of light, but the facts in this connection will be described in another chapter.

Some important results have been obtained by accustoming organisms to solutions containing various amounts of salts. A number of cases of this sort are given by De Varigny. It has been found that littoral marine animals that live where the water may become diluted by the rain, or by rivers, survive better when put into fresh water than do animals living farther from the shore. Thus the oyster, the mussel, and the snail, Patella, withstand immersion in fresh water better than other animals that live farther out at sea. The reverse is also true; fresh-water forms, such as Lymnæa, Physa, Paludina, and others may be slowly acclimatized to water containing more salt. The forms mentioned above could be brought by degrees into water containing 4 per cent of salt, which would have killed the animals if they had been brought suddenly into it. Similar results have been obtained for amœba.

It has been shown that certain rotifers and tardigrades, and also some unicellular animals, that live in pools and ponds that are liable to become dry, withstand desiccation, while other members of the same groups, living in the sea, do not possess this power of resistance. Cases of this sort are usually explained as cases of adaptation, but it has not been shown experimentally that resistance to drying can be acquired by a process of acclimatization to this condition. The case is also in some respects different from the preceding, since intermediate conditions are less likely to be met with, or to be of sufficiently long duration for the animal to become acclimatized to them. It seems more probable, in such cases, that these forms have been able to live in such precarious conditions from the beginning because they could resist the effects of drying, not that they have slowly acquired this power. Finally, there must be discussed the question of the acclimatization to poisons, to which an individual may be rendered partially immune. The point of special importance in this connection is that the animal may be said to respond adaptively to a large number of substances, which it has never met before in its individual history, or to which its ancestors have never been subjected. It may become slowly adapted to many different kinds of injurious substances. These cases are amongst the most important adaptive individual responses with which we are familiar, and the point cannot be too much emphasized that organisms have this latent capacity without ever having had an opportunity to acquire it through experience.

The preceding groups of phenomena, included under the general heading of individual acclimatization, have one striking thing in common, namely, that a physiological adaptation is brought about without a corresponding change in form, although we must suppose that the structure has been altered in certain respects at least. The form of the individual remains the same as before, but so far as its powers of resistance are concerned it is a very different being.

In regard to the perpetuation of the advantages gained by means of this power of adaptation, it is clear in those cases in which the young are nourished during their embryonic life by the mother, that, in this way, the young may be rendered immune to a certain extent, and there are instances of this sort recorded, especially in the case of some bacterial diseases. Whether this power can also be transmitted through the egg, in those instances in which the egg itself is set free and development takes place outside the body, has not been shown. In any case, the effect appears not to be a permanent one and will wear off when the particular poison no longer acts. It is improbable, therefore, that any permanent contribution to the race could be gained in this way. Adaptations of this sort, while of the highest importance to the individual, can have produced little direct effect on the evolution of new forms, although it may have been often of paramount importance to the individuals to be able to adapt themselves, or rather to become able to resist the effect of injurious substances. The important fact in this connection is the wonderful latent power possessed by all animals. So many, and of such different kinds, are the substances to which they may become immune, that it is inconceivable that this property of the organism could ever have been acquired through experience, no matter how probable it may be made to appear that this might have occurred in certain cases of fatal bacterial diseases. And if not, in so many other cases, why invent a special explanation for the few cases?

We may defer the general discussion of the rôle that external factors have played in the adaptation of organisms, until we have examined some of the theories which attribute changes to internal factors. The idea that something innate in the living substance itself has served as the basis for evolution has given rise to a number of different hypotheses. That of the botanist Nägeli is one of the most elaborately worked out theories of this sort that has been proposed, and may be examined by way of illustration.

Nägeli’s Perfecting Principle

Nägeli used the term _completing principle_ (“Vervollkommungsprincip”) to express a tendency toward perfection and specialization. Short-sighted writers, he says, have pretended to see in the use of this principle something mystical, but on the contrary it is intended that the term shall be employed in a purely physical sense. It represents the law of inertia in the organic realm. Once set in motion, the developmental process cannot stand still, but must advance in its own direction. Perfection, or completion, means nothing else than the advance to complicated structure, “but since persons are likely to attach more meaning to the word _perfection_ than is intended, it would perhaps be better to replace it with the less objectionable word _progression_.”

Nägeli says that Darwin, having in view only the condition of adaptation, designates that as more complete which gives its possessor an advantage in the battle for existence. Nägeli claims that this is not the only criterion that applies to organisms, and it leaves out the most important part of the phenomenon. There are two kinds of completeness which we should keep distinctly apart: (1) the completeness of organization characterized by the complication of the structure and the most far-reaching specialization of the parts; (2) the completeness of the adaptation, present at each stage in the organization, which consists in the most advantageous development of the organism (under existing conditions) that is possible with a given complication of structure and a given division of functions.

The first of these conceptions Nägeli always calls “completeness” (Vollkommenheit), for want of a simpler and better expression; the second he calls adaptation. By way of illustrating the difference between the two, the following examples may be given. The unicellular plants and the moulds are excellently adapted each to its conditions of life, but they are much less complete in structure than an apple tree, or a grape vine. The rotifers and the leeches are well adapted to their station, but in completeness of structure they are much simpler than the vertebrates.

If we consider only organization and division of labor as the work of the completing principle, and leave for the moment adaptation out of account, we may form the following picture of the rise of the organic world. From the inorganic world there arose the simplest organic being thinkable, being little more than a drop of substance. If this underwent any change at all, it would have been necessarily in the direction of greater complication of structure; and this would constitute the first step in the upward direction. In this way Nägeli imagines the process once begun would continue. When the movement has reached a certain point, it must continue in the same direction. The organic kingdom consists, therefore, of many treelike branches, which have had a common starting-point. Not only does he suppose that organisms were once spontaneously generated, and began their first upward course of development, but the process has been repeated over and over again, and each time new series have been started on the upward course. The organic kingdom is made up, therefore, of all degrees of organization, and all these have had their origins in the series of past forms that arose and began their upward course at different times in the past. Those that are the highest forms at the present time represent the oldest series that successfully developed; the lowest forms living at the present time are the last that have appeared on the scene of action.

Organisms, as has been said, are distinguished from one another, not only in that one is simpler and another more complicated, but also in that those standing at the same stage of organization are unequally differentiated in their functions and in their structure, which is connected primarily with certain external relations which Nägeli calls adaptations.

Adaptation appears at each stage of the organization, which stage is, for a given environment, the most advantageous expression of the main type that was itself produced by internal causes. For this condition of adaptation, a sufficient cause is demanded, and this is, as Nägeli tries to show later, the result of the inherited response to the environment. In many cases this cause will continue to act until complete adaptation is gained; in other cases, the external conditions give a direction only, and the organism itself continues the movement to its more perfect condition.

The difference between the conception of the organic kingdom as the outcome of mechanical causes on the one hand, or of competition and extermination on the other hand, can be best brought out, Nägeli thinks, by the following comparison of the two respective methods of action. There might have been no competition, and no consequent extermination in the plant kingdom, if from the beginning the surface of the earth had continually grown larger in proportion as living things increased in numbers, and if animals had not appeared to destroy the plants. Under these conditions each germ could then have found room and food, and have unfolded itself without hinderance. If now, as is assumed to be the case on the Darwinian theory, individual variations had been in all directions, the developmental movement could not have gone beyond its own beginnings, and the first-formed plants would have remained swinging now on one side and now on another of the point first reached. The whole plant kingdom would have remained in its entirety at its first stage of evolution, that is, it would never have advanced beyond the stage of a naked drop of plasma with or without a membrane. But, according to the further Darwinian conception, competition, leading to extermination, is capable of bringing such a condition to a higher stage of development, since it is assumed that those individuals which vary in a beneficial direction would have an advantage over those that have not taken such a step, or have made a step backward.

If, on the other hand, under the above-mentioned conditions of unrestricted development, without competition, variations were determined by “_mechanical principles_,” then, according to Nägeli’s view, all plant forms that now exist would still have evolved, and would be found living at the present time, but along with all those that now exist there would be still other forms in countless numbers. These would represent those forms which have been suppressed. On Nägeli’s view competition and suppression do not produce new forms, but only weed out the intermediate forms. He says without competition the plant kingdom would be like the Milky Way; in consequence of competition the plant kingdom is like the firmament studded with bright stars.

The plant kingdom may also be compared to a branched tree, the ends of whose branches represent living species. This tree has an inordinate power of growth, and if left to itself it would produce an impenetrable tangle of interwoven branches. The gardener prevents this crowding by cutting away some of the parts, and thus gives to the tree distinct branches and twigs. The tree would be the same without the watchful trimming of the gardener, but without definite form.

Nägeli states: “From my earlier researches I believe that the external influences are small in comparison to the internal ones. I shall speak here only of the influences of climate and of food, which are generally described as the causes of change, without however any one’s having really determined whether or not a definite result can be brought about by these factors. Later I shall speak of a special class of external influences which, according to my view, bring forth beyond a doubt adaptive changes.”

The external influence of climate and of food act only as transitory factors. A rich food supply produces fat, lack of food leads to leanness, a warm summer makes a plant more aromatic, and its fruit sweeter; a cold year means less odor and sour fruit. Of two similar seeds the one sown in rich soil will produce a plant with many branches and abundance of flowers; the other, planted in sandy soil, will produce a plant without branches, with few flowers, and with small leaves. The seeds from these two plants will behave in exactly the same way; they have inherited none of the differences of their parents. Influences of this sort, even if extending over many generations, have no permanent effect. Alpine plants that have lived since the ice age under the same conditions, and have the characters of true high-mountain plants, lose these characters completely during the first summer, if transplanted to the plains. Moreover, it makes no difference whether the seed or the whole plant itself be transferred. In place of the dwarfed, unbranched growth, and the reduced number of organs, the plant when transferred to the plains shoots up in height, branches strongly, and produces numerous leaves and flowers. The plants retain their new characters as long as they live in the plain without any other new variation being observed in them.

Other characteristics also, which arise from different kinds of external influences due to different localities, such as dampness and shade, a swampy region, or different geological substrata, last only so long as the external conditions last.

These transient peculiarities make up the characters of local varieties. That they have no permanency is intelligible, since they exhibit no new characters, but the change consists mainly in the over- or under-development of those peculiarities that are dependent on external influences. The effect of these influences may be compared to an elastic rod, which, however much it may be distorted by external circumstances, returns again to its original form as soon as released.

Besides these temporary changes, due to external influences, there are many cases known in which the same plant lives under very diverse conditions and yet remains exactly the same. For example, the species of _Rhododendron ferragineum_ lives on archæan mountains and especially where the soil is poor in calcium. Another species, _Rhododendron hirsutum_ is found especially on soil rich in calcium. The difference in the two species has been supposed to depend on differences in the soil, and if so, we would imagine that, if transplanted for a long time, the one should change in the direction of the other. Yet it is known that the rusty rhododendron may be found in all sorts of localities, even on dry, sunny, calcareous rocks of the Apennines and of the Jura, and despite its residence in these localities, since the glacial epoch, no change whatever has taken place.

Single varieties of the large and variable genus of _Hieracium_ have lived since the glacial period in the high regions of the Alps, Carpathians, and in the far north, and also in the plains of different geological formations, but these varieties have remained exactly the same, although on all sides there are transitional forms leading from these to other varieties.

Some parasitic species also furnish excellent illustrations of the same principle. Besides the several species of Orobanchia and of the parasitic moulds, the mistletoe deserves special mention. It lives on both birch and apple trees and on both presents exactly the same appearance; and even if it is true that mistletoe growing on conifers presents certain small deviations in its character, it is still doubtful whether, if transferred to the birch or apple tree, it would not lose these differences, thus indicating that they are not permanent.

It is a fact of general observation that, on the one hand, the same variety occurs in different localities and under different surroundings, and, on the other hand, that slightly different varieties live together in the same place and therefore under the same external conditions. It is evident, then, that food conditions have neither originated the differences nor kept them up. The rarer cases in which in different localities different varieties exist show nothing, because competition and suppression keep certain varieties from developing where it would be possible otherwise for them to exist.

Nägeli says his conclusion may be tested from another point of view. If food conditions, as is generally supposed, have a definite, _i.e._ a permanent, effect on the organism, then all organisms living under the same conditions should show the same characters. Indeed, it has been claimed in some instances that this is actually the case. Thus it is stated that dry localities cause plants to become hairy, and that absence of hairiness is met with in shady localities. This may apply to certain species, but in other cases exactly the reverse is true, and even the same species behaves differently in different regions, as in _Hieracium_. And so it is with all characteristics which are ascribed to external influences. As soon as it is supposed a discovery has been made in this direction, we may rest assured that in other cases the reverse will be found to hold. We have had, in respect to the influence of the outer world on organisms, the same experience as with the rules for the weather,—when we come to examine the facts critically there are found to be as many exceptions as confirmations of the rule.

If climatic influence has a definite effect, the entire flora of a special locality ought to have the same peculiarities, but this stands in contradiction to all the results of experience. The character of the vegetation is not determined by the environment of the plants but by their prehistoric origin, and as the result of competition. Nägeli concludes his discussion with the statement that all of our experience goes to show that the effects of external influences (climate and food) appear at once, and their results last only as long as the influences themselves last, and are then lost, leaving nothing permanent behind. This is true even when the external influences have lasted for a long time,—since the glacial epoch, for instance. We find, he claims, nothing that supports the view that such influences are inherited.

If we next examine the question of changes from _internal causes_, Nägeli claims that here also observation and research fail to show the origin of a new species, or even of a new variety from external causes. In the organic world little change has taken place, he believes, since the glacial epoch. Many varieties have even remained the same throughout the whole intervening time; and while it cannot be doubted that new varieties have also been formed, yet the cause of their origin cannot be empirically demonstrated. The permanent, hereditary characters, of whose origin we know something from experience, belong to the individual changes which have appeared under cultivation in the formation of domestic races. These are for the most part the result of crossing. So far as we have any definite information as to the origin of the changes, they are the result of inner, and never of external, causes. We recognize that this must be the case, since under the same external conditions individuals behave differently—in the same flower-bud some seeds give rise to plants like the parent, others to altered ones. The strawberry with a single leaflet, instead of three, arose in the last century in a single individual amongst many other ordinary plants. From the ten seeds of a pear Van Mons obtained as many different kinds of pears. The most conclusive proof of the action of inner causes is most clearly seen when the branches of the same plant differ. In Geneva a horse-chestnut bore a branch with “filled” flowers, and from this branch, by means of cuttings, this variation has been carried over all Europe. In the Botanic Garden at Munich there is a beech with small divided leaves; but one of its branches produces the common broad undivided leaves. Many such examples have been recorded which can only be explained by assuming that a cell, or a group of cells, like those from which the other branches arose, have become changed in some unknown way as the result of inner causes. The properties that are permanent and inherited are contained in the idioplasm, which the parent transmits to its offspring. A cause that permanently transforms the organism must also transform the idioplasm. How powerless, in comparison to internal causes, the external causes are is shown most conclusively in grafting. The graft, although it receives its nourishment through the stock, which may be another species, remains itself unchanged.

Nägeli makes the following interesting comparison between the development of the individual from an egg, and the evolution, or development, of the phylum. No one will doubt that the egg during the entire time of its process of transformation is guided by internal factors. Each successive stage follows with mechanical necessity from the preceding. If an animal can develop from inner causes from a drop of plasma, why should not the entire evolutionary process have also been the outcome of developmental inner causes? He admits that there is a difference in the two cases in that the plasma that forms the egg has come from another animal, and contains all the properties of the individual in a primordial condition. In the other case we must suppose that the original drop of plasma did not contain at first the primordium of definite structures, but only the ability to form such. Logically the difference is unimportant. The main point is that in the primordium of the germ a special peculiarity of the substance is present which by forming new substances grows, and changes as it grows, and the one change of necessity excites the next until finally a highly organized being is the result.

Nägeli discusses a question in this connection, which, he says, has been unnecessarily confused in the descent theory. Since we are entirely in the dark as to how much time has been required for the formation of phyla, so also are we ignorant as to how long it may have taken for each step in advance. We may err equally in ascribing too much and too little time to the process. It is, moreover, not necessary that for every step the same amount of time should have been required. On the contrary, the probability is that recognizable changes may at times follow each other rapidly, and then for a time come to a standstill,—just as in the development of the individual there are periods of more rapid and others of less rapid change.

A more difficult problem than that relating to the sort of changes the external influences bring about in the organism, is the question as to how they effect the organism, or how they act on it mechanically. This, as is well known, was answered by Darwin, who regards all organization as a problem of adaptation: only those chance variations surviving which are capable of existence, the others being destroyed. On this theory external influences have only a negative or a passive action, namely, in setting aside the unadapted individuals. Nägeli, on the other hand, looks upon some kinds of external conditions as directly giving rise to the adaptive characters of the organism. This is accomplished, he supposes, in the following ways: two kinds of influence are recognized; _the direct action_, which, as in inorganic nature, comes to an end when the external influences come to an end, as when cold diminishes the chemical actions in the plant; and _the indirect action_, generally known as a stimulus, which starts a series of molecular motions, invisible to us, but which we recognize only in their effects. Very often the stimulus starts only a reflex action, usually at the place of application.

A stimulus acting for but a short time produces no lasting effect on the idioplasm. A person stung by a wasp suffers no permanent effect from the injury. But if a stimulus acts for a long time, and through a large number of generations, then it may, even if of small strength, so change the _idioplasm_, that a tendency or disposition capable of being seen may be the result. This appears to be the case in regard to the action of light, which causes certain parts of the plant to turn toward it and others away from it; also for the action of gravity, which determines the downward direction of the roots. It may be claimed, perhaps, that these are the results of direct influence and not of an internal response, but this is not the case; for some plants act in exactly the opposite way, and send a stem downward, as in the case of the cleistogamous flowers of _Cardamine chenopodifolia_; and other plants turn away from the light. This means that the idioplasm behaves differently in different plants in response to the same stimulus.

Concerning the more visible effects of adaptation, Nägeli states that in regard to some of them there can be no question as to how they must have arisen. Protection against cold, by the formation of a thick coat of hair, is the direct result of the action of the cold on the skin of the animal. The different weapons of offence and of defence, horns, spurs, tusks, etc., have arisen, he maintains, through stimulus to those parts of the body where these structures arise.

The causes of the other adaptations, especially of those occurring in plants, are less obvious. Land plants protect themselves from drying by forming a layer of cork over the surface. The most primitive plants were water plants, which acclimated themselves little by little to moist, and then to dry, air. When they first emerged from the water the drying acted as a stimulus on the surface, and caused it to harden in the same way as a drop of glue hardens. This hardening in turn acted as a stimulus, causing a chemical transformation of the surface into a corky substance. This effect was inherited, and in this way the power to form cork originated.

Land plants have, in addition to the soft parts, the hard bast and wood which serves the mechanical purpose of supporting the soft tissues and protecting them from being injured. The arrangement of the hard parts is such as to suggest that they are the result of the action of pressures and tensions on the plant, for the strongest cells are found where there is most need for them. It is easy to imagine, Nägeli adds, that this important arrangement of the tissues is the result of external forces which brought about the result in these parts.

Nägeli accounts for the origin of twining plants as follows. Being overshadowed by other plants, the stem will grow rapidly in the damp air. Coming in contact with the stems of other plants, the delicate stem is stimulated on one side, and grows around the point of contact. This tendency becomes inherited, and the habit to twine is ultimately established.

The difference in the two sides of leaves is explained by Nägeli as the result of the difference in the illumination of the two sides. This influence of light on the leaf has been inherited. The formation of the tubular corolla that is seen in many plants visited by insects is explained as the result of the stimulus produced by the insects in looking for the pollen. The increase in the length of the proboscis of the insect is the result of the animal straining to reach the bottom of the ever elongating tube of the corolla. “The tubular corolla and the proboscis of the insect appear as though made for each other. Both have slowly developed to their present condition, the long tube from a short tube and the long proboscis from a short one.” Thus, by purely Lamarckian principles, Nägeli attempts to account for many of the adaptations between the organism and the outer world. But if this takes place, where is there left any room for the action for his so-called perfecting principle? Nägeli proceeds to show how he supposes that the two work together.

As a result of inner causes the organism would pass through a series of perfectly definite stages, J, J^1, J^2. But if, at any stage, external influences produced an effect on the organism so that the arrangement of the idioplasm changes in response, a new adaptation is produced. In this way new characters, not inherent in the idioplasm, may be added, and old ones be changed or lost. “In order not to be misunderstood in regard to the completing or perfecting principle I will add, that I ascribe to it no determinate action in the organism, neither in producing the long neck of the giraffe, nor the prehensile tail of the ape, neither the claws of the crab, nor the decoration of the bird of paradise. These structures are the outcome of both factors. I cannot picture to myself how external causes alone, and just as little how internal causes alone, could have changed a monad into a man.” But Nägeli goes on to say, that if at any stage of organization one of the two causes should cease to act, the other could only produce certain limited results. Thus, if external causes alone acted, the organization would remain at the same stage of completeness, but might become adapted to all kinds of external conditions—a worm, for instance, would not develop into a fish, but would remain a worm forever, although it might change its worm structure in many ways in response to external stimuli. If, on the other hand, only the completing principle acted, then without changing its adaptations the number of the cells and the size of the organs might be increased, and functions that were formerly united might become separated. Thus, without altering the character of the organism, a more highly developed (in the sense of being more specialized) organism would appear.

Nägeli, as we have just seen, has attempted to build up a conception of nature based on two assumptions, neither of which has been demonstrated to be an actual principle of development. His hypothesis appears, therefore, entirely arbitrary and speculative to a high degree. Even if it were conceivable that two such principles as these control the evolution of organisms, it still requires a good deal of imagination to conceive how the two go on working together. Moreover, it is highly probable that whole groups have evolved in the direction of greater simplification, as seen especially in the case of those groups that have become degenerate. To what principle can we refer processes of this sort?

It is certainly a strange conclusion this, at which Nägeli finally arrives, for, after strenuously combating the idea that the external factors of climate and of food have influence in producing new species, he does not hesitate to ascribe all sorts of imaginary influences to other external causes. The apparent contradiction is due, perhaps, to the fact that his experience with actual species led him to deny that the direct action of the environment produces permanent changes, while in theory he saw the necessity of adding to his perfecting principle some other factor to explain the adaptations of the new forms produced by inner causes. Nägeli seems to have felt strongly the impossibility of explaining the process of evolution and of adaptation as the outcome of the selection of chance variations, now in this direction, now in that. He seems to have felt that there must be something within the organism that is driving it ever upward, and he attempts to avoid the teleological element, which such a conception is almost certain to introduce, by postulating the inheritance of the effects of long-continued action of the environment, in so far as certain factors in the environment produce a response in the organism. Nevertheless, this combination is not one that is likely to commend itself, aside from the fact that the assumptions have no evidence to support them. Despite Nägeli’s protest that his principles are purely physical, and that there is nothing mystical in his point of view, it must be admitted that his conception, as a whole, is so vague and difficult in its application that it probably deserves the neglect which it generally receives.

Nägeli’s wide experience with living plants convinced him that there is something in the organism over and beyond the influence of the external world that causes organisms to change; and we cannot afford, I think, to despise his judgment on this point, although we need not follow him to the length of supposing that this internal influence is a “force” driving the organism forward in the direction of ever greater complexity. A more moderate estimate would be that the organism often changes through influences that appear to us to be internal, and while some of the changes are merely fluctuating or chance variations, there are others that appear to be more limited in number, but perfectly definite and permanent in character. It is the latter, which, I believe, we can safely accredit to internal factors, and which may be compared to Nägeli’s internal causes, but this is far from assuming that these changes are in the direction of greater completeness or perfection, or that evolution would take place independently of the action of external agencies.