The Evolution Theory, Vol. 2 of 2

by August Weismann

That variation in an upward direction really can persist for a long time is shown by artificial selection as practised by Man in regard to his domesticated animals and cultivated plants. At first general variability, or at least variability in many directions, sets in as a result of the greatly altered conditions of life; the ordinary fluctuations of the determinants are intensified by the greater fluctuations in the nutritive stream, and it becomes possible for Man consciously or unconsciously to select for breeding whatever he prefers among the chance variations that arise in individual parts or in whole complexes of parts, and he may thus give rise to a long-continued, often apparently unlimited, augmentation of variations in the same direction, although he cannot exercise any direct influence upon the germ-plasm or its determinants. When a determinant has assumed a certain variation-direction it will follow it up of itself, and selection can do nothing more than secure it a free course by setting aside variations in other directions by means of the elimination of those that exhibit them.

That artificial selection can cause the increase of a part has long been established, but in what way this is possible, and how it can be theoretically explained has hitherto been very obscure, for even if we take the favourable case that both parents possess the desired variation, it cannot be supposed that the characters of the parents are, so to speak, added together in the child; all we can say is that the probability that the children will also exhibit the character in question—for instance, a long or crooked nose—becomes greater. Certainly an increase of the character may result if in both parents the determinants K are present in excess as compared with the heterodynamous determinants K´ and K´´, for in that case there is an increased probability that, through reducing divisions and amphimixis, there will again be a preponderance of the determinants K composing the germ-plasm of the child, and further, that these determinants K will dominate strongly as compared with the few K´'s. It may thus happen that the long nose of the two parents will give rise to a still longer nose in the child, or that parents of considerable bodily size may have still bigger children, but such increase would be confined to one generation, and would not lead to a permanent increase of the character; permanent increase cannot depend merely on the number of the determinants K and on their supremacy over their converse, the determinants K´; it must also depend on their own variation, and this again can depend only on germinal selection and not upon personal selection, although the former can be materially assisted by the latter.

That inheritance from both parents is only a secondary consideration in regard to the increase of a part by artificial selection is made evident by the fact that many secondary sexual characters have been modified, although the breeder selected only in regard to one parent. Nevertheless in this very domain the greatest results have been achieved; witness the Japanese breed of cocks with tail-feathers six feet long. This astonishing result has been reached by the strictest selection of the cocks in which the feathers were a little longer than those of other cocks, and the increase in the length of feathers depended—according to our theory—simply on the fact that, by the selection of the determinants which were already varying in the direction of increased length, this process of increase was guarded from interruption by chance unfavourable conditions of nutrition. The continuance of variation in the upward direction in which it had already started is not effected directly by personal selection, but is so indirectly, for without this constant fresh intervention of selection the increase would be apt to come to a standstill, or the variation might even take a contrary direction. There are two other factors operative to which we have not yet given sufficient attention. They are, the multiplicity of the ids in every germ-plasm, and sexual reproduction.

If—as we must assume—each germ-plasm is made up of several or many ids, there must be several or many determinants of each part of the organism, for each id contains potentially the whole organism, though with some individuality of expression. The child is thus not determined by the determinants of a single id, but by those of many ids, and the variations of any part of the body do not depend on the variations of a single determinant X, but on the co-operation of all the determinants X which are contained in the collective ids of the relevant germ-plasm. Thus it is only when a majority of the determinants have varied upwards or downwards that they dominate collectively the development of the part X´ and cause it to be larger or smaller.

We have assumed passive fluctuations in nutrition to be the first cause in individual variation, and it is obvious that the action of this first cause of dissimilarity must be greatly restricted by the multiplicity of the ids and the corresponding homologous determinants. For although passive fluctuations in nutrition should occur continually in the case of all determinants, this would not imply that they would follow the same direction in all the determinants X of all ids, for some determinants X might vary upwards, and others downwards, and these might counteract each other in ontogeny; so that in many cases the fluctuations of the individual determinants will not be felt in their products at all. But since there are—as we shall see later—only two directions of variation, upwards and downwards, plus and minus, it must also sometimes happen that a majority take one direction, and this affords the basis on which germinal selection can build further, and on which it is materially supported by reducing division and the subsequent amphimixis.

For reducing division removes half of the ids and thus of the determinants from the mature germ-cell, and according as chance leaves together or separates a majority of X-determinants varying in the same direction, this particular germ-cell will contain the primary constituents of a plus- or of a minus-variation of X, and it is possible that the presence of a majority or a minority may be entirely due to the reduction. The germ-plasm of the parent may contain, for instance, the determinant X in its twenty ids 12 times in minus-variation form, 8 times in plus-variation form; and the reducing division, according to our view, may separate these into two groups of which one contains eight plus- and two minus-variations, the other ten minus-variations, or the one six plus- and four minus-variations, the other two plus- and eight minus-variations, and so on. Now every germ-cell which contains a majority of plus- or minus-variations—and this must be the case with most of them—may unite, if it attains to amphimixis, with a germ-cell which also contains a majority of plus or minus X-determinants, and if similar majorities let us say plus—meet together, the plus-variation of X must be all the more sharply emphasized in the child.

Thus, although the individual determinants X may not be incited to further variation by their co-operation with others varying in the same direction, the collective effect of the plus-determinants will be greater, and adherence to the same direction of variation in the following generation will be assured, for if in the germ-plasm of the parent there be, for instance, sixteen out of twenty determinants possessing the plus-variation, a minus-majority can no longer result from reducing division.

It is upon this that the operation of natural selection, that is, personal selection, must depend—that the germ-plasms in which the favourable variation-direction is in the majority are selected for breeding, for it is this and nothing else that natural selection does when it selects the individuals which possess the preferred variations. The ascending process is thus considerably advanced, because the opposing determinants are more and more eliminated from the germ-plasm, till the preferred variations of X are left, and among these, as ascent in the direction begun continues, the opposing variations are again set aside by germinal selection, and so on. Reducing divisions and amphimixis are thus powerful factors in furthering the transformations of the forms of life, although they are not the ultimate causes of these.

Now that we have made ourselves familiar with the idea of germinal selection we shall attempt to gain clearness as to what it can do, and how far the sphere of its influence extends, and, in particular, whether it can effect lasting transformations of species without the co-operation of personal selection, and what kind of variations we may ascribe to it alone.

First, I must return for a moment to the question we have already briefly discussed—whether the variation of a determinant upwards or downwards must so continue without limit. We might be inclined to think that the great constancy which many species exhibit was a plain contradiction of this, for if every minute variation of a determinant necessarily persisted without limit in the same direction, we should expect to find all the parts of the organism in a state of continual unrest, some varying upwards, some downwards, always ready to break the type of the species. Must there not be some internal self-regulation of the germ-plasm which makes it impossible that every variation which crops up can persist unlimitedly? Must there not be some kind of automatic control on the part of the germ-plasm, which is always striving to re-establish the state of equilibrium that has once been attained by the determinant system whenever it is disturbed?