Parallel Paths: A Study in Biology, Ethics, and Art

by T. W. Rolleston

The goal of the breeder is some new form which it pleases him to produce, either for its use, or its beauty, or for its mere singularity. The goal of nature, at least the apparent and immediate goal, is the adaptation of each species to the circumstances of its life. And the first thing that strikes the investigator is the way, often indeed not perfect, but usually most impressive in its apparent thoughtfulness and care, in which the organs of plants and animals are fashioned to secure the most favourable results. But all this is the result of development. The whale is a creature excellently adapted for its present mode of life, but we know that it was once a furry land animal with four legs; the legs are all there still, in modified or rudimentary form, and the fur appears at a certain stage of embryonic development. When we ask, How did this extraordinary transformation come about? what we really mean is, How did the determinants composing the chromatin in the reproductive cells of the original land animal so come to alter as to produce the characteristics of the whale? For new species can only be evolved by means of structural modifications capable of being transmitted by inheritance; and nothing can be inherited except through the action of the determinants. A modification which does not affect the reproductive cells has no significance in the evolution of species.

To this question Darwinism has given us our choice of two answers, which may be termed respectively the Darwin-Lamarck and the Darwin-Weismann theories. Lamarck explained the origin of species by the accumulated effect of the inheritance, through many generations, of modifications acquired by the exercise, or the disuse, of the modified organs. Observing that living protoplasm responds to demands upon it (thus, for instance, a muscle when systematically exercised attracts more nourishment from the blood and grows stronger, and callosities form to protect the skin of the hands of a manual worker), he assumed that modifications so acquired might be transmitted by inheritance. Each new generation, then, would start with a slightly better equipment in this particular respect than the former one had when it started; and so, by slow degrees, a new organ, or one markedly differing from the original form, might be built up. The world, since protoplasmic life first appeared upon it, has gone through many changes, and has always presented a vast variety of climatic and other conditions, calling for the most varied types of organic structure. As animal life gradually spread over the earth and sea, the effort to cope with the different conditions it met with would gradually, by the combined action of exercise, of disuse, and of heredity, produce multitudes of different types; and these are what we know as families, orders, genera, and species. When a species is fairly well adapted to its surroundings and way of life it may go on indefinitely without change. But should any members of it be obliged to migrate, from scarcity of food or any other reason, to some new locality where somewhat different conditions prevail, structural alterations would soon begin to appear to suit those new conditions. Thus the giraffe, if we could trace its ancestry back, would probably be found to have originated in some animal not differing from the vast majority of quadrupeds in the relative proportions of its fore and hind quarters. But some members of this original species—or the whole species, owing to some change in their surroundings—found themselves obliged to rely largely for food on leaves growing at a considerable height. They stretched up to reach them, and a prolongation of the bones of the neck (the giraffe has only the usual seven cervical vertebræ) and of the fore-legs would ensue, especially in the young; this prolongation would be handed on by inheritance, and so by degrees the new type of animal would be evolved. The horn of the rhinoceros, the antlers of the stag, the canine teeth of beasts of prey, the flat grinders of ruminants, the flippers of the whale, the proboscis of the honey-feeding butterfly, the jaws of the ant or the beetle, and a host of other adaptations which seem obviously to owe their origin to the exercise of their functions, occur to the mind in confirmation of this theory.

Besides Adaptation, we have what appears the strikingly confirmatory case of what is called Co-adaptation, where the variation of one organ or structure in an animal puts a strain upon other parts, which accordingly respond by auxiliary adaptations. Such co-adaptations are numerous in every animal structure, and, as we cannot suppose them to have all originated simultaneously and by chance, the conclusion drawn by Lamarckians is that one was produced by use, and, in the course of its development, produced the others in the same way. A typical case is that of the Irish elk. The enormous antlers of this beast, sometimes weighing a hundredweight, must have needed (besides other structural changes) a cervical ligament of immense size and power to support them, and from the peculiar structure of the cervical vertebræ it is demonstrable that such a ligament must have existed. What more natural than to suppose that the antlers were developed by fighting wild beasts of prey, combats between male elks, etc., and that then in their gradual growth, as the species was evolved, the ligament and the bony structure associated with it responded to the increasing strain. That is exactly what would happen in an individual. We have only to assume the heritability of modifications acquired by use to understand how these co-adaptations became constant characters in a species.

Not less apparent cogency for the argument for modifications by use have those cases where the modification has been apparently due to disuse. It is well known that living creatures found in the total darkness of great limestone caverns, like those at Kentucky, are blind, through imperfections of one kind or another in the organs of sight. But the rudimentary structures which remain tell us that these creatures had ancestors which were once fully equipped in this respect, and which had wandered into the caverns from the sunlit outer world. Thus the case of a crab has been noted, in which the stalks on which a crab’s eyes are set were preserved, while the eyes had disappeared: it is, as Darwin observes, as if the stand of a telescope had been retained while the telescope itself had gone. Sometimes the eyes of cave-fishes are covered with a horny layer, sometimes the whole structure is atrophied and withered. But never is an animal found under these conditions which has retained its power of sight. The conclusion seems obvious. In individuals, a muscle or other organ is known to strengthen and develop by use and to atrophy by disuse. As use and disuse appear to be invariably accompanied by precisely the same effects in the species as in the individual, and as there seems no way of accounting for this by any known physiological law without assuming that modifications acquired by the individual are transmitted to its progeny, the case for the inheritability of such modifications appears, at first sight, irresistible.[54]

So matters stood when Darwin’s Origin of Species carried the argument for evolution a long step further. Accepting fully the views of Lamarck, Darwin attempted, by his doctrine of Natural Selection, first to reinforce those views, secondly to explain much that they could not be made to cover. It is plain that if we assume the existence of a severe competition for livelihood among the members of a species, any favourable variations of structure or instinct which may occur among certain members of the species will give their type an advantage over the normal type in the struggle for existence. They will, on the average, live longer and produce more offspring. Ultimately, as the struggle for life is always most severe among nearly related organisms, which seek a living from the same sources, the less perfectly equipped type will be extinguished, and so on, until a species exhibiting the most complete form of adaptation has been evolved. The variations on which Natural Selection has to work are produced, according to Darwin, not only by the exercise of particular organs as in Lamarck’s theory, but also and more potently by “innate variations” originating from unascertained causes in the reproductive cells. Variations, it is indisputable, are always occurring; probably no two members of any species exactly resemble each other. Among low and primitive organisms, such as the Foraminifera, Dr. W. B. Carpenter (I quote from A. R. Wallace’s Darwinism) found, on careful examination, the range of variation so great that characteristics typical not merely of species but of genera and even of orders were liable to vary,[55] while at the other end of evolution, in man, to give only one instance, Mr. J. Wood is stated by Darwin to have observed no less than five hundred and fifty-eight variations in the muscular structure of thirty-six subjects examined.[56] The cause of these variations is often quite obscure, but it is certain that some kinds of them are capable of arising as the natural response of the organism to changed conditions of food or habitat Conditions such as these, affecting the whole constitution of the organism, have been proved capable of affecting the reproductive cells, and thus of giving rise to hereditary characteristics. Natural Selection, then, by preserving and encouraging the better fitted as opposed to the less fit, acts as a spur to the Lamarckian principles of development by exercise of function, while it also lays hold of and intensifies all kinds of other favourable variations occurring either casually or in consequence of change of habitat, and weeds out the types in which such variations happen to be unfavourable. According to Darwin, therefore, given (1) constant variations of structure arising from use, disuse, or from other known or unknown causes, (2) the capacity to transmit by inheritance these variations whether innate or acquired, (3) a constant struggle for existence among organisms both against each others’ competition and against the general conditions of life[57]—given these simple data, the secret springs of evolution are laid bare, and the vast complexity of natural forms upon the globe is adequately accounted for without calling in the agency of special creations. But variations are the starting-point in the process: Natural Selection can originate nothing—it can only act on what is presented to it by some quite different force. The relative parts played by the various agencies at work are, with characteristic moderation of statement, thus described by Darwin:—

“On the whole I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, and in the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by the natural selection of innate variations.”[58]

To explain evolution, then, we must first explain the occurrence of appropriate variations, strong enough and widespread enough to maintain themselves against the constant reducing influence of promiscuous intercrossing, and they must be variations capable of being transmitted by inheritance. This, we now see, is the true field of the inquiry.

The new factors introduced by Darwin into the process of evolution—Natural Selection and Innate Variations—were destined in our day to have the whole weight of the argument for evolution suddenly thrown upon them. The inheritability of variations acquired by the individual through use and disuse when subjected to fresh investigation by the younger school of biologists has turned out to be open to the gravest doubts, both theoretically, on account of the great difficulty of reconciling it with what has now been ascertained of the nature of the reproductive mechanism in plants and animals, and also on the score of a closer consideration of the facts commonly adduced as evidence for the law. To take these points separately: The reproductive cells in every living creature are now believed to be formed directly from the reproductive cells of its parents. They are not a product of the organism in which they find themselves. They are nourished by its blood, and are therefore liable to be affected by anything which produces a broad general effect on the constitution of the being in whom they are lodged, but it is difficult to see how special modifications of individual parts of that being could affect them so as to influence the determinants in the direction of reproducing that modification. How, for example, could the habit of grubbing for roots in an animal of the pig tribe so affect its reproductive cells as to ensure the birth of an offspring with callosities on their snouts? The physiological mechanism by which such a result could be produced seems hardly conceivable—at any rate no one has yet offered a plausible conception of it. Of course if the fact were indisputably proved one would only have to accept it, and endeavour, if possible, to discover the why and how. But the fact, which once looked so solidly established, is taking on a more and more insubstantial appearance in the light of closer investigation.

The argument against Lamarckism rests on the basis (1) artificial experiment, (2) of observation of nature under normal conditions.

As to the evidence from experiment, opinions fluctuated for some time—Darwin was disposed at one time to deny, at another to admit the alleged proofs it offered. In the present day opinion is overwhelmingly against the validity of these proofs. The cases where artificially produced mutilations are said to have been inherited have, when investigated, turned out to be by no means as clear and trustworthy as was supposed, nor can one place much reliance on a few cases of striking coincidence such as are certain to occur from time to time.[59]