The Evolution Theory, Vol. 2 of 2

by August Weismann

This is due to the fact that the variations of direct origin sooner or later obtain an influence in determining the viability of their possessors, either increasing or diminishing it. It is this, in association with the unlimited multiplication of individuals, which gives a basis to the principle of transformation, which it is the immortal merit of Charles Darwin and Alfred Russel Wallace to have introduced into science: the principle of selection. We have seen that this principle may have a much more comprehensive meaning than was attributed to it by either of these two naturalists; that there is not merely a struggle between individuals which brings about their adaptation to their environment, by preserving those which vary in the most favourable way and rejecting those which vary unfavourably, but that there is an analogous struggle between the parts of these individuals, which, as Wilhelm Roux showed, effects the adaptation of the parts to their functions, and that this struggle must be assumed to occur even between the determinants and biophors of the germ-plasm. There is thus a germinal selection, a competition between the smaller and larger particles of the germ-plasm for space and food, and that it is through this struggle that there arise those definitely and purposefully directed variations of the individual, which are transmissible because they have their seat in the immortal germ-plasm, and without which an adaptation of individuals in the sense and to the extent in which we actually observe it would be altogether inconceivable. I have endeavoured to show that the whole evolution of the living world is guided essentially by processes of selection, in as far as adaptations of the parts to one another, and of the whole to the conditions of life, cannot be conceived of as possible except through these, and that all fluctuations of the organism, from the very lowest up to the highest, are forced into particular paths by this principle, by 'the survival of the fittest.' This ends the whole dispute as to whether there are indifferent 'characters' which have no influence on the existence of the species, for even the characters most indifferent for the 'person' would not exist unless the germinal constituents (determinants) which condition them had been victorious in the struggle for existence over others of their kind, and even the 'indifferent' characters, which depend solely upon climatic or other external influences, owe their existence to processes of germinal selection, for those elements of the determinants concerned were victorious which throve best under such influences. But should variations thus produced by external influence increase so far that they become prejudicial to the survival of their bearers, then they are either set aside by personal selection or, if that be no longer possible, they lead to the extinction of the species. Thus the multitude of small individual variations, which probably occur in every species, but which strike us most in Man—the differences in the development of mouth, nose, and eyes, in the hair, in the colour of skin, &c., as far as they are without significance in the struggle for existence—depend upon processes of germinal selection, which permitted the greater development of one group of determinants, or of one kind of biophor in one case, of another in another. The proportionate strength of the elements of the germ-plasm is not readily lost at once, but is handed on to successive generations, and thus even these 'indifferent' characters are transmitted.

It is obvious that, if the principle of selection operates in nature at all, it must do so wherever living units struggle together for the same requirements of life, for space and food, and these units need not be persons, but may represent every category of vital units, from the smallest invisible units up to the largest. For in all these cases the conditions of the selection-process are given: individual variability, nutrition, and multiplication, transmission of the advantage attained, and, on the other hand, limitation of the conditions of existence—especially food and space. The resulting struggle for existence must, in every category of vital units, be most acute between the individual members of each category, as Darwin emphasized in the case of species from the very first, and persistent variations of a species of living units can only be brought about by this kind of struggle. Strictly speaking, therefore, we should distinguish as many kinds of selection-processes as there are categories of living units, and these could not be sharply separated from one another, apart from the fact that we have to infer many of them, and cannot recognize their gradations. Here, as everywhere else, we must break up the continuity of nature into artificial groups, and it seems best to assume and distinguish between four main grades of selective processes corresponding to the most outstanding and significant categories of vital units, namely: Germinal, Histonal, Personal, and Cormal Selection.

Histonal Selection includes all the processes of selection which take place between the elements of the body (soma), as distinguished from the germ-plasm, of the Metazoa and Metaphyta, not only between the 'tissues' in the stricter sense, but also between the parts of the tissues, that is, the lower vital units of which they are composed, and which Wilhelm Roux, when he published his Kampf der Teile ('Struggle of the Parts'), called 'molecules.' It occurs between all the parts of the tissues down to the lowest vital units, the biophors. We must also reckon under histonal selection the processes of selection which take place between the elements of the simplest organisms, and through which these have gradually attained to greater complexity of structure and increased functional capacity. As long as no special hereditary substance had been differentiated, variations which arose in the simplest organisms through selection-processes of this kind were necessarily transmitted to the descendants, but after this differentiation had taken place this could no longer occur—'acquired' modifications of the soma were no longer transmitted, and the importance of histonal selection was limited to the individual. But this form of selection must be of the greatest importance in regard to the adaptations of the parts which develop from the ovum, especially during the course of development, and it is also indispensable all through life in maintaining the equilibrium of the parts, and their adaptation to the varying degree of function required from them (use and disuse). But its influence does not reach directly beyond the life of the individual, since it can only give rise to 'transient' modifications, that is, to changes which cease with the individual life.

In contrast to this is Germinal Selection, which depends upon the struggle of the parts of the germ-plasm, and thus only occurs in organisms with differentiation of somatoplasm and germ-plasm, especially in all Metazoa and Metaphyta—forming in these the basis of all hereditary variations. But not every individual variation to which germinal selection gives rise persists and spreads gradually over the whole species, for, apart from the cases we have already mentioned, in which indifferent variations favoured by external circumstances gain the victory, this happens only if the variations in question are of use to their bearer, the individual. Any variation whatever may arise in a particular individual purely through germinal selection, but it is only the higher form of selection—Personal Selection—that decides whether the variation is to persist and to spread to many descendants so that it ultimately becomes the common property of the species. Germinal and personal selection are thus continually interacting, so that germinal selection continually presents hereditary variations, and personal selection rejects those that are detrimental and accepts those that are useful. I will not repeat any exposition of the marvellous way in which personal selection reacts upon germinal selection, and prevents it from continuing to offer unfavourable variations, and compels it to give rise to what is favourable in ever-increasing potency. Although it apparently selects only the best-adapted persons for breeding, it really selects the favourable id-combinations of the germ-plasm, that is, those which contain the greatest number of favourably varying determinants. We saw that this depends upon the multiplicity of ids in the germ-plasm, since every primary constituent of the body is represented in the germ-plasm, not once only, but many times, and it is always half of the homologous determinants contained in the germ-plasm of an individual which reach each of its germ-cells, always, moreover, in a different combination. Thus, with the rejection of an individual by personal selection, a particular combination of ids, a particular kind of germ-plasm is in reality removed, and thus prevented from having any further influence upon the evolution of the species. By this means germinal selection itself is ultimately influenced, because only those ids remain unrejected in the germ-plasm whose determinants are varying in directions useful to the species. Thus there comes about what until recently was believed to be impossible: the conditions of life give rise to useful directions of variation, not directly, certainly, but indirectly.

We may distinguish as a fourth grade of selection Cormal Selection, that is, the process of selection which effects the adaptation of animal and plant stocks or corms, and which depends on the struggle of the colonies among themselves. This differs from personal selection only in that it decides, not the fitness of the individual person, but that of the stock as a whole. It is a matter of indifference whether the stocks concerned are stocks in the actual material sense, or only in the metaphorical sense of sharing the common life of a large family separated by division of labour. In both cases, in the polyp-stock as well as in the termite or ant-colony, the collective germ-plasm, with all its different personal forms, is what is rejected or accepted. The distinction between this cormal selection and personal selection is, therefore, no very deep one, because here too it is in the long run the two sexual animals which are selected, not indeed only in reference to their visible features, but also in reference to their invisible characters, those, namely, which determine in their germ-plasm the constitution of their neuter progeny or, in the case of polyps, their asexually reproducing descendants.

We venture to maintain that everything in the world of organisms that has permanence and significance depends upon adaptation, and has arisen through a sifting of the variations which presented themselves, that is, through selection. Everything is adaptation, from the smallest and simplest up to the largest and most complex, for if it were not it could not endure, but would perish. The principle which Empedocles announced, in his own peculiar and fantastic way, is the dominating one, and I must insist upon what has so often been objected to as an exaggeration—that everything depends upon adaptation and is governed by processes of selection. From the first beginnings of life, up to its highest point, only what is purposeful has arisen, because the living units at every grade are continually being sifted according to their utility, and the ceaseless struggle for existence is continually producing and favouring the fittest. Upon this depends not only the infinite diversity of the forms of life, but also, and chiefly, the closely associated progress of organization.

It cannot be proved in regard to each individual case, but it can be shown in the main that attaining a higher stage in organization also implies a predominance in the struggle for existence, because it opens up new possibilities of life, adaptations to situations not previously utilizable, sources of food, or places of refuge. Thus a number of the lower vertebrates ascended from the water to the land, and adapted themselves to life on dry land or in the air, first as clumsily moving salamanders, but later as actively leaping frogs; thus, too, other descendants of the fishes gained a sufficient carrying power of limbs to raise the lightened body from the ground, and so attained to the rapid walk of the lizards, the lightning-like leaps of the arboreal agamas, the brief swooping of the flying-dragons, and ultimately the continuous flight which we find in the flying Saurians and the primitive birds of the Jurassic period, and in the birds and bats of our own day.

It is obvious that each of these groups, as it originated, conquered a new domain of life, and in many cases this was such a vast one, and contained so many special possibilities, that numerous subordinate adaptations took place, and the group broke up into many species and genera, even into families and orders. All this did not come about because of some definitely directed principle of evolution of a mysterious nature, which impelled them to vary in this direction and in no other, but solely through the rivalry of all the forms of life and living units, with their enormous and ceaseless multiplication, in the struggle for existence. They were, and they are still, forced to adapt themselves to every new possibility of life attainable to them; they are able to do this because of the power of the lowest vital units of the germ to develop numerous variations; and they are obliged to do it because, of the endless number of descendants from every grade of vital unit, it is only the fittest which survive.

Thus higher types branched off from the lower from time to time, although the parent type did not necessarily disappear; indeed it could not have disappeared as long as the conditions of its life endured; it was only the superfluous members of the parent form that adapted themselves to new conditions, and as, in many cases, these required a higher organization, there arose a semblance of general upward development which simulated a principle of evolution always upwards. But we know that, at many points on this long road, there were stations where individual groups stopped short and dropped back again to lower stages of organization. This kind of retreat was almost invariably caused by a parasitic habit of life, and in many cases this degeneration has gone so far that it is difficult to recognize the relationship of the parasite to the free-living ancestors and nearest relatives. Many parasitic Crustaceans, such as the Rhizocephalids, lack almost all the typical characteristics of the crustacean body, and dispense not only with segmentation, with head and limbs, but also with stomach and intestine. As we have seen, they feed like the lower fungi, by sucking up the juices of their hosts, by means of root-like outgrowths from the place where the mouth used to be. Nevertheless, their relationship to the Cirrhipedes can be proved from their larval stages. There are, however, parasites in the kidneys of cuttlefish—the Dicyemidæ—in regard to which naturalists are even now undecided whether they ought to form a lowly class by themselves between unicellular animals and Metazoa, or whether they have degenerated, by reason of their parasitism, from the flat worms to a simplicity of structure elsewhere unknown. They consist only of a few external cells, which enclose a single large internal cell, possess no organs of any kind, neither mouth nor intestine, neither nervous system nor special reproductive organs. But although degeneration cannot be proved in this case, it can be in hundreds of other cases with absolute certainty, as, for instance, in the Crustacea belonging to the order of Copepods, which are parasitic upon fishes, in which we find all possible stages of degeneration, according to the degree of parasitism, that is, to the greater or less degree of dependence upon the host; for organs degenerate and disappear in exact proportion to the need for them, and they thus show us that degeneration also is under the domination of adaptation.