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

by T. W. Rolleston

Were evolution to depend on the occurrence, by pure chance, of a few appropriate variations among a vast multitude of indifferent or disadvantageous ones, is it conceivable that we should find in nature anything like the infinite wealth of closely and beautifully adapted structure which is actually present? In particular, how are we to account for the cases in which a number of parts are so modified as to work together in harmonious co-adaptation? Each of these parts, according to Weismann, originates quite independently of the others. Take the case of the Indian leaf-butterfly already referred to.[70] The first beginnings of the midrib on Weismann’s theory had nothing to do with the rest of the rib, nor had any of the veinings with this, or with one another; and the contour of the leaf, sending out a little projection like a stalk exactly where the midrib starts, originated quite independently of that marking, and equally so of the leaf it mimics! To explain co-adaptations like this on Weismann’s theory is really much the same as to suppose that a picture could be painted by simply plastering the scrapings of a palette on a canvas, if only one continued the process long enough. And the marvel in question, the co-adaptation of various parts, has not been attained once or twice but, to a greater or less degree, in every organism possessing any structural complexity.

The difficulty, of course, has not escaped Weismann. His explanation depends on some conception of the potentialities of conjugation and intercrossing which I confess I cannot understand. He finds the key to the mystery in the mingling and constant recombination of determinants from different individuals produced by promiscuous intercrossing. “It is only through amphimixis [conjugation] that simultaneous harmonious adaptation of many parts becomes possible.”[71] But surely this continual mingling and recombination would, primâ facie, be just as likely to break up co-adaptations already forming as to give rise to new ones? Amphimixis, as we have seen, is one of the most potent forces against which the evolution of a new species has to contend. Evolution has to make head against the constant tendency of intercrossing to obliterate individual distinctions. True, if parents exhibiting the same heritable variation unite, their offspring will have that variation in a strongly marked form, and will transmit it further. But this, to be of value for evolution, presupposes the same variation occurring simultaneously in a number of individuals within reach of each other. Weismann had indeed good reason to ascribe to the action of intercrossing “a wealth and diversity of organic architecture otherwise unattainable,” but were it not supplemented by an architectural instinct of nature, the only architecture attainable would be that of the child when it empties its bricks on the floor.

Consider the theory of germinal selection in the light of the following very curious case.[72] Most people have seen an example of the kind of spectacles having what are called bifocal lenses. Each lens is divided across the centre, and the focal lengths of the upper and the lower halves are different. They are intended for persons who see indistinctly both at near and at far distances—the upper half of the lens is used for looking at distant objects and the lower for reading, etc., so as to avoid the inconvenience of having a different pair of glasses for each requirement. Now there is a fish, named Anableps (the Uplooker), living in estuaries on the east coast of South America which actually has its eye-lenses constructed on this principle. The pupil of the eye is divided laterally by prolongations from the iris. The significance of this extraordinary arrangement is that the fish is in the habit of swimming near the surface, and often has its eyes wholly or partly out of water, presumably to look out for attacks from birds of prey. The upper half of the eye has become adapted for vision in the air and the lower for vision in the water.

According to Weismann, the habits and needs of the fish could have had no influence whatever in producing this peculiar adaptation as an inherited characteristic of a species. Any other fish or mammal would have been just as likely as Anableps to begin the development of a bifocal eye. How does it come, then, that from the thousands of species of eyed animals one, and one only, possesses this bifocal eye, and that precisely the one which so greatly needs it? Weismann’s answer would doubtless be that, in the case of other creatures, Natural Selection would not have acted in protecting the individuals which possessed the bifocal eye and penalizing those which did not. But can we imagine that this principle acted very strongly when the bifocal arrangement in Anableps was in a mere rudimentary stage, as it must at first have been? And should we not occasionally see at least traces of the arrangement in the eyes of other creatures, if its full development in Anableps was merely the result of Natural Selection laying hold of and perfecting an originally quite fortuitous variation?

A case still more curious and convincing occurs in connexion with the hermaphroditism exhibited by a whole class of animals belonging to many different orders, but alike in the one respect that it is specially desirable for them to have both sexes comprised in the same individual. These are animals capable only of sluggish movement, the different sexes of which have therefore some difficulty in finding each other out. Terrestrial snails and slugs are an example. All these creatures are double-sexed; any two snails which meet can conjugate, since each can act either as male or as female at will. Oysters are another instance, though in this case the two sexes follow each other at different periods in the life-history of each individual. Clearly, this faculty gives to snails and slugs twice as many opportunities of reproducing their kind as if the sexes were distinct. It is certain from general biological considerations that they were distinct originally. One can easily understand how, if any small group of the original species from which all the present tribes are descended, happened to throw up these bisexual peculiarities, their progeny would multiply faster than the rest and might ultimately exterminate them by the operation of natural selection. But exactly the same might be said of any other tribe of unisexual animals. Any of these might, a priori, on the “mechanical conception of the economy of life,” be just as reasonably expected to develop bisexuality; for no one supposes that there is any physical connexion between sluggishness and hermaphroditism, or swiftness and distinction of the sexes; and the causes which have operated to extend and confirm the type in sluggish and sedentary animals would have the same effect in swift ones. Yet this remarkable adaptation occurs just wherever there is special need for it; there always and there only. What mechanism can account for such a phenomenon as this? No; the dice are loaded. Nature gains her end slowly and not without hesitations and failures, but the phenomena are wholly unlike the results of the play of uncontrolled and fortuitous forces. Imagine a blindfolded archer shooting arrows upwards, downwards, and all around him in every direction as it may take his fancy. There is, unknown to him, a target some distance off. If he went on long enough it is conceivable, though by no means necessary, that some arrow would hit the bull’s eye. But the facts plainly point not to the above analogy, but rather to an aim at a desired object. Some of the arrows miss, some light near the mark, others hit it precisely. The flight, on the whole, is in the right direction, as the immense proportion of complete or partial successes plainly proves.

The two pillars of Weismann’s theory of evolution are germinal variation and natural selection. The one is supposed to originate ceaseless changes of structure, the other to eliminate those changes which are useless[73] or unfavourable and to foster and confirm the favourable. We have seen, if the foregoing considerations are sound, that fortuitous variations do not provide the material with which natural selection can build up a universe of organic life like ours. We have now to turn our attention to the other prop of the system and to inquire whether natural selection can play and does play the part which Darwin and his school assign to it in the economy of nature.

Natural selection is supposed to depend for its efficacy on the existence of a state of strenuous competition for nourishment, or for the avoidance of foes, in the type out of which the favourable variations emerge. But in recent times the fact of any such competition has been gravely doubted. Let us look back to the beginnings of animal life in the world. The first primitive animal organisms found themselves swimming in a boundless sea of nourishment and had no foes at all! Yet they developed into higher and higher grades of life. Competition did not aid in the development of these higher grades—it was they which ultimately created the state of competition. What Nature then achieved without competition she is equally able to perform now. Even now when the earth is swarming with varied life competition plays a much smaller part than was taken for granted in the first flush of Darwinism. Creatures of the same type but on different grades of organization, like the hive-bee and the humble bee, are constantly found side by side, drawing their nourishment from the same sources, but each holding its own without difficulty. Facts like these were not unobserved by Darwin, who met them by the supposition that competition came chiefly into play at exceptional periods, during a drought, an inundation, a severe winter, or the like, in which the less fitted members of the race perished wholesale. But, as Kropotkin, in his interesting work, Mutual Aid among Animals, has remarked,

“If the evolution of the animal world were based exclusively, or even chiefly, upon the survival of the fittest during periods of calamities; if natural selection were limited in its action to periods of exceptional drought, or sudden changes of temperature, or inundations, retrogression would be the rule in the animal world. Those who survive a famine, or a severe epidemic of cholera, or small-pox, or diphtheria, such as we see them in uncivilized countries, are neither the strongest, nor the healthiest, nor the most intelligent. No progress could be based on such survivals—the less so as all survivors usually come out of the ordeal with an impaired health, like the Transbaikalian horses just mentioned, or the Arctic crews, or the garrison of a fortress which has been compelled to live for a few months on half rations, and comes out of its experience with a broken health, and subsequently shows a quite abnormal mortality.”[74]

Kropotkin’s book shows good reason to believe that the principle of mutual aid and support plays at least as great a part in the animal world as does that of mutual competition and extermination.

That the competition of organisms, animal and vegetable, for nourishment and for protection may favour certain types, and depress or even exterminate others, is of course indisputable. We see it when the Japanese worker and the Californian meet in industrial rivalry on the Pacific slopes—we see it when the willows planted by New Zealand rivers destroy the weed which infested them, by absorbing the nourishment from the river-bed on which it lived.[75] What we have to consider, however, is the efficacy of competition in giving predominance and permanence to a type differing but slightly in the initial stages from that of the rest of the species, and differing but in a very few individuals. We have to consider, in fact, whether natural selection is not a consequence rather than a cause of evolution. On no mechanical theory of evolution can we suppose that the first leaf-markings of the butterfly, Kallima paralecta, were either at all pronounced in their mimicry, or that they originated simultaneously in any large group of the original species from which Kallima paralecta sprang. Therefore, with very small advantage in the way of protection from enemies, and with the constant and powerful influence of intercrossing ever tending to obliterate the distinctive leaf-marks, how could natural selection alone enable the new, the mimicking type, to assert and develop itself, as it has done not only in this particular species of butterfly but in hundreds of species of the Lepidoptera and other insects?