Part 12

There are two agencies by which the pollen is carried to the pistil; first, by the wind, and, secondly, by insects (or, occasionally, by humming birds). Flowers that are fertilized through the agency of insects are the most beautiful in existence, displaying all the varied hues and gorgeous patterns that are found in the organic world. On the other hand, the flowers which are fertilized through the agency of the wind are incomparably less beautiful than those of insect production, and they do not secrete sweet juices or nectar. The earliest flowers in geologic time consisted only of those essential portions, the stamens and pistils, and had no colored whorl of petals within another colored whorl of sepals. The poorly developed nectaries secreted only small quantities of honey. The food-seeking insects visited these primitive flowers for the pollen and nectar, even as they do now. The nectaries in the plants were so situated that the insects could not get at the honey without rubbing off some of the pollen and carrying it to other plants which it visited for honey. In this way the nectar-seeking insects would often carry pollen from the flower of one plant to the stigma of another plant’s flower, and thus secure cross-fertilization. Undoubtedly often the stigma of a plant’s flower was fertilized by pollen from the same flower; but it is known that seeds produced by cross-fertilization are much more vigorous and hardy than those produced by self-fertilization in the same flower; therefore, those plants that varied most in the direction best adapted for securing cross-fertilization would have decided advantages in the struggle for existence. Their fertilized eggs (seeds) would be most vigorous, and would most readily secure nutriment and withstand adverse circumstances. The variations of the primitive flowers most calculated to secure cross-fertilization would be those that made the flower more conspicuous by the appearance of bright colorations and delightful odors, such as would attract the attention and visits of nectar-seeking insects. These variations being transmitted, by heredity, to the descendants and accentuated as the ages passed would ultimately lead to all the wonderful adaptations of flower and insect structure to one another that are found in nature.

Natural Selection has acted in a double manner here, preserving those flowers that had the most delightful odors or the most conspicuous and enticing colorations and patterns; and at the same time selecting those insects that varied most in the direction of keenness of scent, acuteness of vision, and color perceptions. It will be instructive to briefly describe the fertilization of the primrose and an orchis by bees. The _primrose_ (Plate IX) has two sorts of flowers that are never found on the same plant; one has stamens high up the corolla tube and a short style with its stigma below the anthers; the other has the stamens far down the tube and a long style with the stigma above the anthers. In both cases the nectar is at the very bottom of the corolla tube. When a bee visits a short-styled flower, it extends its proboscis to the bottom of the tube and in withdrawing it brings away some pollen cells at its base. If it should next visit another short-styled primrose, it would not fertilize it because the pollen on the base of the proboscis could not reach the stigma; it would only gather still more pollen. But when it visits a long-styled primrose, it will necessarily deposit some pollen cells on the stigma, inasmuch as that is at the commencement of the corolla tube. If the bee should first visit a long-styled form of flower, it will carry off pollen on the tip of its proboscis, and when it visits a short-styled primrose will deposit the pollen on the stigma.

In the _orchis_ the stigma is placed below the anther. The stigma is in the front part of the flower and at the base of the lip, the latter being prolonged into a long tube full of nectar. The stigmatic surface is composed of very viscid matter. A bee when seeking the nectar pushes its head against the anther, and in so doing detaches the two sticky glands to which the club-shaped masses of pollen cells are attached (Plate X). It carries these away on the front part of its head. So long as the masses of pollen cells remain erect on the bee’s head, they cannot reach the stigma of any other orchis that it visits. As the sticky glands on the head contract, the pollen masses incline forward and become horizontal, so that they touch the next sticky stigma that is visited. The greater stickiness of the stigma detaches the pollen masses from the bee’s head, so that the flower is fertilized. It takes so long for the pollen masses on the bee’s head to assume the horizontal position that the insect has visited all the flowers on one orchis and then visits another plant. By this time the pollen masses are in the proper position (horizontal) to fertilize the flowers of another plant. In this way cross-fertilization is secured and the vigor of the plant maintained.

It may now perhaps be appreciated how intimate are the relations between the form and habits of insects and the structure and coloration of flowers. Our standards of beauty have largely been created for us through Insect and Sexual Selection. If insects had not been developed on the earth, plants would not be ornamented by beautiful flowers, but would show only such poor and inconspicuous ones as are found on grasses, on oak, ash, and fir trees. Grant Allen has beautifully written that, “while man has tilled only a few level plains, a few great river-valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread itself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. His buttercup, his dandelion, and his meadowsweet grow thick in every English field. His thyme clothes the hillside; his heather purples the bleak grey moorlands. High up among the Alpine heights his gentian spreads its lakes of blue; amid the snows of the Himalayas his rhododendrons gleam with crimson light. Even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of Brazilian streams are beautified by his gorgeous water lilies. The insect has turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers to his allurement.”

Natural Selection is the great agency that accounts not only for the color patterns and forms of living creatures, but also for the great majority, if not all, of the useful characteristics of organic creatures, including their internal organization.

One more illustration of the power of Natural Selection may be given, outside of the subject of color patterns. It may be interesting to readers who are not familiar with the elements of zoölogy to know that whales are not fishes but mammals. They belong to the same class that man does. The embryology of a whale reveals that it is descended from ancestors that were _land_ mammals, and that these mammals had a scanty covering of hair, teeth of different shape, broad tails like beavers, short fore and hind legs, and well developed sense organs. The olfactory organ was especially well developed. It is probable that these ancestors of the whale lived in marshy districts and were omnivorous, eating both plant and animal food. They sought their food in both shallow water and in swamps. As the conditions of life became more and more unpropitious on land, they were slowly modified through the ages under the action of Natural Selection into creatures somewhat like dolphins. At first they lived in fresh water, but finally they found their way into the sea and became the rulers of the ocean, from which the giant sea reptiles of earlier epochs had vanished. Hence are explained the adaptive changes of structure: the fore-limbs were modified into flippers enclosed in a fin-like sac, but retaining the bones corresponding to like structures in other _mammals_, as in the arm of man, the wing of the bat, and the fore-leg of the horse. Traces of the hind legs may be detected in a few species; the tail, which acted as a powerful swimming organ, became divided into two lobes; the head became fish-like in shape; the seven bones of the neck, common to most mammals, grew together; the skin became hairless; and the teeth, which appear in the young of the true whale, but are never cut, gave place to hanging fringes of whalebone, in the meshes of which the animal entangles the minute organisms it feeds upon.[16]

ISOLATION OF VARIETIES IN NATURE.

The following analysis of Isolation will be useful:

{ Preferential Mating. { Sexual. { Isolation { { Cross-Sterility. { Geographical.

It has been stated on an earlier page that the commingling of diverse hereditary units accomplished through the fertilization of an ovum by a spermatozooid is the source of many variations in the offspring. In this fertilized ovum the complexity of chemical substance, and, therefore, the complexity of inheritance, gives instability to the embryo, and thus produces variations in the offspring. In this embryo there is a struggle among the hereditary units,—a struggle among the various qualities inherited from both sides,—and a survival of the fittest, a veritable intra-cellular Natural Selection. It is a well-established law in biology that the union of germ cells of very closely related individuals, that is, of consanguineous individuals (in and in breeding) leads to less vigorous and variable offspring, and the parents are less prolific; while the commingling of diverse heritages by the union of germ cells from individuals belonging to strong but different varieties leads to vigorous and quite variable offspring. The union of such individuals is also most prolific. On the other hand the union of individuals belonging to very diverse varieties becomes less and less prolific until the cross-sterility of species is reached, although there are many exceptions to this rule of cross-sterility. The individuals of a species living in a state of nature are constantly varying. With every generation trivial variations take place in all directions and of all kinds. But these variations are all funded in the common stock, for the varieties freely mingle among one another and cross-breeding is constant not only between them, but with the parent stock. Of the variations that are constantly taking place some are advantageous to the creatures, some are disadvantageous, while many are neutral, being neither useful nor harmful. Natural Selection is ever alert, selecting the advantageous variations and eliminating the disadvantageous ones. The neutral variations are not touched by Natural Selection; among these intercrossing of varieties probably affords Nature an opportunity to make almost endless combinations, some of which might be useful to the animals, and others harmful, and in either case would come under the influence of Natural Selection. The commingling of diverse heritages due to the union of more or less pronounced varieties of the species in nature not only leads to a funding of varietal characteristics, but also increases the instability of the offspring, augmenting their plasticity, so that more numerous and diverse variations take place. According to Romanes the reproductive organs are among the most variable in the body. Of the numerous variations taking place in the individuals of a species under nature, some, therefore, affect the reproductive organs in such a way that certain of these individuals are cross-fertile with one another, but _cross-sterile with other varieties and with the parent stock_. This interesting and very important kind of variation is known to occur in some individuals of the human species. It is well known that a man and woman have been cross-sterile with one another, being unable to have children; yet when separating and mating with others they have both been cross-fertile, families being reared by both of them.

Variations are commencing species; isolated variations diverge more and more into distinct species. This fact, then, of the occurrence in nature of variations in some of the individuals of a species by which they are cross-fertile with one another, but cross-sterile with other varieties and with the parental stock, shows that Nature has a most effectual means by which varieties may be isolated from one another,—just as effective means as man, the breeder of varieties of domestic animals, possesses in isolating these domestic varieties by physical barriers, as fences, etc. Cross-sterility, therefore, in nature, is a most effective sexual barrier. The special form of it under consideration is what Romanes has called Physiological Selection. The varieties that are isolated by this sexual barrier have got to run the risks of in and in breeding, which Darwin has shown occur in domesticated animals, but which Wallace thinks are much less in a state of nature.

Another very important mode of Nature for isolating varieties is that which arises out of the instinctive preferences of animals. There is a tendency for like to breed with like where varieties are formed. The pale and dark colored herds of fallow deer in the Forest of Dean have never been known to interbreed. In the Falkland Islands all the cattle are known to have descended from the same stock. Here there are differently colored herds of cattle, and those cattle of the same color will interbreed with each other, but not with individuals outside their own color-caste, as Morgan expresses it. When two flocks of heath sheep and merino sheep are mixed together, they do not interbreed. This isolation of varieties by instinctive preferences for those individuals with like color patterns may be spoken of as _preferential mating_ through _recognition marks_. A very obvious mode of isolation in nature is by geographical barriers (including migration). In treating of environment we learned that during the geologic ages of the globe, the physical geography and climate have repeatedly changed. A very cold temperature, a mountain chain, a body of water, a stretch of desert land, may completely prevent interbreeding, on either side of the barrier, between the individuals of a species.

It should now be understood that Nature, like man, produces divergences from the parental stock, and isolates them by various effective agencies. Natural Selection ever carefully watches over these processes, eliminating the unfit variations and selecting the useful ones. Variation, Heredity, Environment, Isolation, and Natural Selection, having been acting and reacting through the ages, have produced, from a common parental stock, all the innumerable divergent and adaptive forms of living creatures that can be traced through the geologic strata to those on the surface of the earth to-day; have produced, therefore, _man_ also as the inflorescence of the topmost branch of the tree of life.

SECTION VI.

EVOLUTION OF MAN.

EVOLUTION OF MAN.

The detailed study of the development (ontogenetic and phylogenetic) of man is so vastly intricate and extensive a subject that it will be impossible in a work of this character to do more than refer to it in brief outline. One of the best and simplest methods to approach the study of such a subject is to acquire some idea of the _development of the frog_.

The fertilized frog’s egg, which is the starting point in the life of a new frog, is deposited in water and hatched by the warmth of spring. After fertilization the egg or cell divides into two cells, these two into four, the four into eight, the eight into sixteen, and so on till a large number of small cells, associated together, is formed (Fig. 10). These cell phases represent different stages in the life of the growing frog. It is thus seen that the frog, in its earliest stages, consists of nothing but many small cells. These cells, through the mysterious powers of heredity, are going to differentiate as development proceeds into the various organs (groups of various kinds of cells) that form the tadpole and finally the frog. For further stages in the development of the egg (oösperm) towards the tadpole, works upon Comparative Embryology should be consulted.

At length the tadpole is hatched from the egg, and then soon swims about in the water (Fig. 17).

The creature that comes from the egg looks nothing whatever like a frog. It has no limbs whatever, and consists mainly of a bulky head and tail. This is the tadpole stage in the development of the frog. It can exist only in water, breathing air therefrom by means of gills. Like the fish, it has a two-chambered heart. At this stage it has no lungs, and the gills consist of an external (Fig. 17, a) and an internal pair. The mouth is small, with only horny toothless jaws, with no tongue. The creature is herbivorous, living on decaying vegetable matter. The vertebræ of the spinal column are bi-concave, as in fishes. The tadpole is essentially a fish, and would be so classed if it did not develop further. An evolving fish does not go beyond this stage. But the developing frog does go beyond this stage to a higher one. As its evolution proceeds through the multiplication and differentiation of the cells that form its body, limbs begin to bud out, first posteriorly (Fig. 17, b) and then anteriorly (Fig. 17, c). The lungs now begin to develop, and the external gills dwindle more and more until they soon disappear, the internal ones persisting for a while longer. The tongue, at this stage, also makes its appearance. The creature now can breathe both air and water. This is the permanent condition of many adult amphibians belonging to a lower order than the mature frog, such as the _siren_, _menobranchus_, etc. The siren in developing also passes through the fish stage, but does not get beyond the siren stage. But the evolving frog does go beyond this stage, for with the growth of the legs the tail dwindles slowly by its gradual absorption (Fig. 17, d). The internal gills now disappear through absorption, and the lungs develop more thoroughly. Great changes take place in the blood-vascular system, the fish-like, two-chambered heart evolving into the three-chambered, amphibian heart. In spite of its dwindling, the tail is still a very conspicuous organ. In this phase of its development the frog can breathe only air, and must frequently come to the surface of the water for that purpose, and soon leaves the water altogether. Now this stage of the creature’s development corresponds to the permanent adult condition of another order of amphibians, which is higher than that to which the siren belongs but lower than the order of the adult frog. This intermediate order has such creatures in it as the _triton_. The triton in developing passes through the fish and siren stages, but does not get higher than the triton stage. But the evolving frog goes even higher than this triton-like condition. Its tail is more and more absorbed until it finally disappears, and then the young but perfect frog appears (Fig. 17, e). During this period the teeth develop and the creature becomes carnivorous, feeding on insects. It is thus seen that the developing frog passes by small gradations from one class (the fish class) to an altogether different and higher class (the amphibian class). When it has evolved to this higher class, it then passes from the lower order (“siren” order) to a higher one (“triton” order), and then to the highest order (“frog” order). The bi-concave vertebræ of the fish-like tadpole have now developed into vertebræ with the cup-and-ball joints of the higher amphibian. It is the same with all the complex organs of the adult frog; they evolve from the much simpler structures of the tadpole.

This study of the frog’s evolution from the fertilized egg is profoundly instructive. It reveals to us, through direct observation, that a creature varies in its form and structure at succeeding intervals of time. These variations diverge more and more, so that specific, generic, and even ordinal and class distinctions are revealed as the development proceeds. Owen, the distinguished comparative anatomist, in speaking of the transmutation of one species into another in the course of geologic history, says, though with a hostile purpose in view, that in the metamorphoses of the amphibians we seem to have such process carried on before our eyes to its extremest extent. Not merely is one specific form changed to another of the same genus; not merely is one generic modification of an order substituted for another, the transmutation is not even limited by passing from one order (Urodela) to another (Anura); it affects a transition from class to class. The fish becomes the frog (amphibian); the aquatic animal changes to the terrestrial one; the water-breather becomes the air-breather; an insect diet is substituted for a vegetable one. And these changes, moreover, proceed gradually, continuously, and without any interruption of active life. Such is the language of Owen in reference to these remarkable transmutations of the developing frog.

The development of the frog is a brief recapitulation, an epitome, through heredity, of the main transmutations of its ancestral forms in geologic time. It is not true that the embryonic phases in the development of a higher form always resemble the adult stages of lower forms. This may or may not be the case; but what always does occur is that the embryonic phases of a higher form resemble the corresponding phases of the lower forms. So far as the frog’s development is concerned, it is very instructive to know that the order of succession of its embryonic forms undoubtedly parallels the order of succession of corresponding forms in past geologic ages. Fishes appeared in the Upper Silurian rocks with amphibian characteristics. In the succeeding Carboniferous Ages the fishes still continued under new forms; but also the lowest forms of amphibians, the most fish-like forms, now appeared. They were somewhat like the sirens, they were perennibranchs. In the next succeeding rocks, the Permian and Triassic, higher, triton-like forms appeared. They were caducibranchs. Finally, in the Tertiary rocks, the highest forms of amphibians are found, such as the frogs.