Part 31

But discretion is indispensable for a fruitful result; or, leaving our metaphor, facts must be connected together by theories, if science is to advance. Just as theories are valueless without a firm basis of facts, so the mere collection of facts, without relation and without coherence, is utterly valueless. Science is impossible without hypotheses and theories: they are the plummets with which we test the depth of the ocean of unknown phenomena, and thus determine the future course to be pursued on our voyage of discovery. They do not give us absolute knowledge, but they afford us as much insight as it is possible for us to gain at the present time. To go on investigating without the guidance of theories, is like attempting to walk in a thick mist without a track and without a compass. We should get somewhere under these circumstances, but chance alone would determine whether we should reach a stony desert of unintelligible facts or a system of roads leading in some useful direction; and in most cases chance would decide against us.

In this sense I trust that the sign-post or compass which I offer may be accepted. Even though it should be its fate to be replaced by a better one at a later period, it will have fulfilled its object if it enables science to advance for even a short distance.

Footnotes for Essay V.

Footnote 176:

C. Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’ München u. Leipzig, 1884.

Footnote 177:

‘Ueber die Berechtigung der Darwin’schen Theorie.’ Leipzig, 1868, p. 27.

Footnote 178:

l. c., Preface, p. vi.

Footnote 179:

Since the above was written many other morphological peculiarities of plants have been rightly explained as adaptations. Compare, for instance, the investigations of Stahl on the means by which plants protect themselves against the attacks of snails and slugs (Jena, 1888).—A. W., 1888.

Footnote 180:

l. c., pp. 117, 286.

Footnote 181:

Compare the second and fourth of the preceding Essays, ‘On Heredity’ and ‘The Continuity of the Germ-plasm as the Foundation of a Theory of Heredity.’

Footnote 182:

Compare Rauber, ‘Homo sapiens ferus oder die Zustände der Verwilderten.’ Leipzig, 1885.

Footnote 183:

‘Sitzungsberichte der baierischen Akademie der Wissenschaften,’ vom 18 Nov. 1865. Compare also his ‘Mechanisch-physiologische Theorie der Abstammungslehre,’ p. 102, etc.

Footnote 184:

Jordan, ‘Remarques sur le fait de l’existence en société des espèces végétales affines.’ Lyon, 1873.

Footnote 185:

S. Hermann’s ‘Handbuch der Physiologie,’ Theil II; ‘Physiologie der Zeugung,’ by V. Hensen.

Footnote 186:

E. van Beneden, ‘Recherches sur la maturation de l’œuf, la fécondation et la division cellulaire.’ Gand u. Leipzig, 1883, pp. 404 et seq.

Footnote 187:

Rolph, ‘Biologische Probleme.’ Leipzig, 1882.

Footnote 188:

Cienkowsky, ‘Arch. f. mikr. Anat.,’ ix. p. 47. 1873.

Footnote 189:

Hensen, ‘Physiologie der Zeugung,’ p. 139.

Footnote 190:

Coalescence takes place in the so-called bud-like conjugation of _Vorticellidae_ and _Trichodinidae_, etc.

Footnote 191:

Compare (1) Bardeleben, ‘Zur Entwicklung der Fusswurzel,’ Sitzungsber. d. Jen. Gesellschaft, Jahrg. 1885, Feb. 6; also ‘Verhandl. d. Naturforscherversammlung zu Strassburg,’ 1885, p. 203; (2) G. Baur, ‘Zur Morphologie des Carpus und Tarsus der Wirbelthiere,’ Zool. Anzeiger, 1885, pp. 326, 486.

Footnote 192:

In frogs the sixth toe exists in the hind legs as a rudimentary prehallux. Compare Born, Morpholog. Jahrbuch, Bd. I, 1876.

Footnote 193:

I here make use of the same illustration which I employed in my first attempt to explain the effects of _panmixia_. Compare the second Essay ‘On Heredity.’

Footnote 194:

[E. Ray Lankester has suggested (Encycl. Britann., art. ‘Zoology,’ pp. 818, 819) that the blindness of cave-dwelling and deep-sea animals is also due to the fact that ‘those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected.’ Such a sifting process would certainly greatly quicken the rate of degeneration due to panmixia alone.—E. B. P.]

Footnote 195:

Adler, ‘Zeitschrift f. wiss. Zool.,’ Bd. XXXV, 1881.

Footnote 196:

Compare my paper, ‘Parthenogenese bei den Ostracoden,’ in ‘Zool. Anzeiger,’ 1880, p. 82. Purely negative evidence, unless on an immense scale, is quite rightly considered to be of no great value in most cases. But the condition of these animals renders the accumulation of such evidence unusually easy, because the presence of males in a colony of Ostracodes can be proved by a very simple indirect test. Thus if a colony contains any males the _receptacula seminis_ of all mature females are filled with spermatozoa, and on the other hand we may be quite sure that males are absent, if after the examination of many mature females, no spermatozoa can be found in any of their _receptacula_.

Footnote 197:

We cannot, however, be absolutely certain of this, for it is conceivable that males may still occur in colonies other than those examined.

Footnote 198:

It has now been shown by Blochmann that males appear for a very short time towards the close of summer, as in the case of _Phylloxera_.—A. W., 1888.

APPENDICES.

Appendix I. Further considerations which oppose Nägeli’s explanation of transformation as due to internal causes[199].

When I describe Nägeli’s theory of transformation as due to active causes lying within the organism, as a phyletic force of transformation, I do not mean to imply that it is one of those mysterious principles which, according to some writers, constitute the unconscious cause which directs the transformation of species. Nägeli’s idioplasm, which changes from within itself, is conceived as a thoroughly scientific, mechanically operating principle. This cause is undoubtedly capable of theoretical conception: the only question is whether it has any real existence. According to Nägeli, the growing organic substance, the idioplasm, not only represents a _perpetuum mobile_ rendered possible as long as its substance continually receives from without the matter and force which are necessary for continuous growth, but it also represents a _perpetuum variabile_ due to the action of internal causes[200]. But this is just the doubtful point, viz., whether the structure of the idioplasm itself compels it to change gradually during the course of its growth, or whether it is not rather the external conditions which compel the ever slightly varying idioplasm to change in a certain direction by the summation of small differences. It has been shown above that we do not gain anything by adopting Nägeli’s theory, because the main problem which organic nature offers for our solution, viz. adaptation, remains unsolved. Hence this theory does not explain the phenomena of nature, and I believe that there are also certain facts which are directly antagonistic to it.

If the idioplasm really possessed the power of spontaneous variability ascribed to it by Nägeli; if, as a result of its own growth, it were compelled to undergo gradual changes, and thus to produce new species, we should expect that the duration of species, genera, orders, &c. would be of approximately equal length respectively, at least in forms of equal structural complexity. The time required by the idioplasm to undergo such changes as would constitute transformation into a new species ought to be always the same at equal heights in the scale of organization, that is, with equal complexity in the molecular structure of the idioplasm. It appears to me to be a necessary consequence of Nägeli’s theory that the causes of transformation lie solely in this molecular structure of the idioplasm. If nothing more than a certain amount of growth, and consequently a certain period of time during which the organism reproduces itself with a certain intensity, is required to produce a change in the idioplasm, then we must conclude that the alteration in the latter must take place when this certain amount of growth has been reached, or after this certain period has elapsed. In other words, the time during which a species exists—from its origin as a modification of some older species, until its own transformation into a new one—must be the same in species with the same degree of organization. But the facts are very far from supporting this consequence of Nägeli’s theory. The duration of species is excessively variable: many arise and perish within the limits of a single geological formation, while others may be restricted to a very small part of a formation; others again may last through several formations. It must be admitted that we cannot estimate the exact position of extinct species in the scale of organization, and the differences may therefore depend upon differences of organization: or they may be explained by the supposition that certain species may have become incapable of transformation, and might, under favourable conditions, continue to exist for an indefinite period. But this reply would introduce a new hypothesis in direct antagonism to Nägeli’s theory, which assumes that the variability of idioplasm takes place as the consequence of mere growth, and necessarily depends upon molecular structure. Nägeli himself asserts that the essential substance (idioplasm) of the descendants of the earliest forms of life is in a state of perpetual change, which would continue even if the series of successive generations were indefinitely prolonged[201]. Hence there can be no rest in the process of change which the idioplasm must undergo; and this is as true of each single species as it is of the organic world taken as a whole. We could, perhaps, find shelter in the insufficiency of our geological knowledge, but the number of ascertained facts is too great for this to be possible. Thus it is well known that the genus _Nautilus_ has lasted from Silurian times, through all the three geological periods, up to the present day: while all its Silurian allies (_Orthoceras_, _Gomphoceras_, _Goniatites_, &c.) became extinct at a comparatively early period.

A keen and clever controversialist might still bring forward many objections against such an argument. I do not therefore place too much dependence upon the geological facts by themselves, as a disproof of the self-variability of Nägeli’s idioplasm; for it must be admitted that the facts are not sufficiently complete for this purpose. For instance, in the case of _Nautilus_ it might be argued that we do not know anything about the fossil Cephalopods of pre-Silurian times, and that it is therefore possible that the above-mentioned allies of _Nautilus_ may have existed previously for as long a period as that through which _Nautilus_ has lived in post-Silurian time. However this may be, it will be at least conceded that the geological facts do not lend any support to Nägeli’s theory, for we can see no trace of even an approximately regular succession of forms.

Appendix II. Nägeli’s explanation of adaptation[202].

In order to explain adaptation Nägeli assumes that, under certain circumstances, external influences may cause slight permanent changes in the idioplasm. If then such influences act continually in the same direction during long periods of time, the changes in the idioplasm may increase to a perceptible amount, i. e. to a degree which makes itself felt in visible external characters[203]. But such changes alone could not be considered as adaptations, for the essential character of an adaptation is that it must be a purposeful change. Nägeli, however, brings forward the fact that external stimuli often produce their chief effects at that very part of the organism to which the stimuli themselves were applied. ‘If the results are detrimental, the organism attempts to defend itself against the stimulus: a confluence of nutrient fluid takes place towards the part upon which the stimulus has acted, and new tissues are formed which restore the integrity of the organism by replacing the lost structures as far as possible. Thus in plants the healthy tissues begin to grow actively around the seat of an injury, tending to close it up, and to afford protection by impenetrable layers of cork.’ Purposeful reactions of this kind are certainly common in the organic world, occurring in animals as well as in plants. Thus in the human body an injury causes a rapid growth of the surrounding tissues, which leads to the closing-up of the wound; while in the Salamander even the amputated leg or tail is replaced by growth. An extreme example of these purposeful reactions is afforded by the tree-frog (_Hyla_), which is of a light-green colour when seated upon a light-green leaf, but becomes dark brown when transferred to dark surroundings. Hence this animal adapts itself to the colour of its environment, and thus gains protection from its enemies.

Admitting this capability on the part of organisms to react under certain stimuli in a purposeful manner, the question remains whether such a power is a primitive original quality belonging to the essential nature of each organism. The power of changing the colour of the skin in correspondence with that of the surroundings is not very common in the animal kingdom. In the frog this power depends upon a highly complex reflex mechanism. Certain chromatophores in the skin are connected with nerves[204] which pass to the brain and are there brought into relation, by means of nerve-cells, with the nervous centres of the organ of vision. The relation is of such a kind that strong light falling upon the retina constitutes a stimulus for the production of an impulse, which is conducted, along the previously mentioned motor nerves, from the brain to the chromatophores, thus determining the contraction of these latter and the consequent appearance of a light-coloured skin. When the strong stimulus (of light) ceases, the chromatophores expand again, and the skin becomes dark. That the chromatophores do not themselves react upon the direct stimulus of light was proved by Lister[205], who showed that blind frogs do not possess the power of altering their colour in correspondence with that of their environment. It is quite obvious that in this case we are not dealing with a primary, but with a secondarily produced character; and it has yet to be proved that all the purposeful reactions mentioned by Nägeli are not similarly secondary characters or adaptations, and thus very far from being primitive qualities of the organic substance of the forms in which they occur.

I do not by any means doubt that some of the reactions witnessed in organisms do not depend upon adaptation, but such reactions are not usually purposeful. Curiously enough, Nägeli mentions the formation of galls in plants among his instances of purposeful reactions under external stimuli. I think, however, that it can hardly be maintained that the galls are of any use to the plant: on the contrary, they may even be very injurious to it. The gall is only useful to the insect which it protects and supplies with food. The recent and most excellent investigations of Adler[206] and of Beyerinck[207] have shown that the puncture made by the _Cynips_ in depositing its eggs is not the stimulus which produces the gall, as was formerly believed to be the case, but that such a stimulus is provided by the larva which developes from the egg. The presence of this small, actively moving, foreign body stimulates the tissue of the plant in a definite manner, always producing a result which is advantageous to the larva and not to the plant. It would be to the advantage of the latter if it killed the intruding larva, either enclosing it by woody tissue devoid of nourishment, or poisoning it by some acrid secretion, or simply crushing it by the active growth of the surrounding tissues. But nothing of the kind occurs: in fact an active growth of cells (forming the so-called ‘Blastem’ of Beyerinck) takes place around the embryo, while it is still enclosed in the egg-capsule; but the growth is not such as to crush the embryo, which remains free in the cavity, the so-called larval chamber, which is formed around it. It would be out of place to discuss here the question as to how we can conceive that the plant is thus compelled to produce a growth which is at any rate indifferent and may be injurious to it; and which, moreover, is exactly adapted to the needs of its insect-enemy. But it is at all events obvious that this cannot be an example of a self-protecting reaction under a stimulus, and that therefore an organism does not always respond to external stimuli in a manner useful to itself.

But even if we could accept the suggestion that the purposeful reaction of an organism under stimulation is a primary and not a secondarily produced character, such a principle would by no means suffice for the explanation of existing adaptations. Nägeli attempts to explain certain selected cases of adaptation as the direct results of external stimuli. He looks upon the thick hairy coat of mammals in arctic regions, and the winter covering of animals in temperate regions, as a direct reaction of the skin under the influence of cold. He considers that the horns, claws, and tusks of animals have arisen directly as reactions under stimuli applied to certain parts of the surface of the body in attack and defence[208]. This interpretation is similar to that offered by Lamarck at the beginning of this century. At first sight such a suggestion appears to be plausible, for the acquisition of a thick hairy covering by the mammals of temperate regions is actually contemporaneous with the cold season of the year. But the question arises as to whether the production of a larger number of hairs at the beginning of winter is not merely another instance of a secondary character, like the assumption of a green colour by the tree-frog under the stimulus exerted by strong light.

In the case of the hairy coat it is only necessary to produce a larger number of structures such as had existed previously; but how can it have been possible for the petals of flowers, with their peculiar and complex forms, to have been developed from stamens as a direct result of the insects which visit them in order to obtain pollen and nectar? How could the creeping of these insects and the small punctures made by them constitute stimuli for the production of an increased rate of growth? And how is it possible in any way to explain, by mere increase in growth, the origin of a structure in which each part has its own distinct meaning and plays a peculiar part in attracting insects and in the process of cross-fertilization effected by them? Even if the manifold peculiarities of form could be explained in this way, how can such an explanation possibly hold for the colours of flowers? How could the white colour of flowers which open at night be explained as the direct result of the creeping of insects? How can the suggestion of such a cause offer any interpretation of the fact that flowers which open by day are tinted with various colours, or of the fact that there is often a bright or highly coloured spot which shows the way to the hidden nectary?

There are, moreover, a large number of very striking adaptations in form and colour, for which no stimulus acting directly upon the organism can be found. Can we imagine that the green caterpillar[209], plant-bug, or grasshopper, sitting among green surroundings, is thus exposed to a stimulus which directly produces the green colour in the skin? Can the walking-stick insect, which resembles a brown twig, be subject to a transforming stimulus by sitting on such branches or by looking at them? Or again, if we consider the phenomena of mimicry, how can one species of butterfly, by flying about with another species, exercise upon the latter such an influence as to render it similar to the first in appearance? In many cases of mimicry, the mimicked and the mimicking species do not even live in the same place, as we see in the moths, flies, and beetles which resemble in appearance the much-dreaded wasps.

The interpretation of adaptation is the weak part of Nägeli’s theory, and it is somewhat remarkable that so acute a thinker should not have perceived this himself. One very nearly gains the impression that Nägeli does not wish to understand the theory of natural selection. He says, for instance, in speaking of the mutual adaptation observable between the proboscis, the so-called ‘tongue’ of butterflies, and flowers with tubular corolla[210]:—‘Among the most remarkable and commonest adaptations observable in the forms of flowers, are the corollas with long tubes considered in relation to the long “tongues” of insects, which suck the nectar from the bottom of the long narrow tubes, and at the same time effect the cross-fertilization of the plant. Both these arrangements have been gradually developed to their present degree of complexity—the long-tubed corollas from those without tubes, and from those with short ones, the long “tongues” from short ones. Undoubtedly both have been developed at the same rate so that the length of both sets of structures has always remained the same.’

No objection can be raised against these statements, but Nägeli goes on to say:—‘But how can such a process of development be explained by the theory of natural selection, for at each stage in the process the adaptation was invariably complete. The tube of the corolla and the “tongue” must have reached, for instance, at a certain time, a length of 5 or 10 mm. If now the tube of the corolla became longer in some plants, such an alteration would have been disadvantageous because the insects would be no longer able to obtain food from them, and would therefore visit flowers with shorter tubes. Hence, according to the theory of natural selection, the longer tubes ought to have disappeared. If on the other hand the “tongue” became longer in some insects, such a change would be superfluous and should have been given up, according to the same theory, as unnecessary structural waste. The simultaneous change in the two structures must, according to the theory of natural selection, be due to the same principle as that by which Münchhausen pulled himself out of a bog by means of his own pig-tail.’