The Evolution Theory, Vol. 2 of 2

by August Weismann

But if the question be asked, how the first majority of determinants varying in the same direction is brought about, there are two possibilities: first, by chance, and secondly, by influences which cause particular determinants of all the ids to vary in almost exactly the same manner. We shall find illustrations of the latter among climatic varieties; but the cases of the first kind are the more important, for they form the foundation and the starting-point for processes of selection of a higher order, for personal selection. It might seem perplexing that processes of such importance should depend ultimately upon chance; but when we remember that there are only two directions of variation, namely a plus direction or a minus direction, we recognize that the chance of a majority in one direction or another is much greater than that of absolute equilibrium between the two, and there is therefore a very strong probability that in many individuals of the species either the upward or the downward movement of a determinant A will preponderate.

Now as such variation movements, when they are of a certain strength, increase automatically, we can easily see that they must gradually attain to a level at which they acquire selection value, and how then, by personal selection, the ids with favourably varying determinants may be collected together.

Of course it is not possible to state positively the time at which in individual cases a variation acquires a biological significance, that is, selection value. We can only say in a general way that, as soon as it attains this, personal selection either in a positive or a negative sense must intervene; an injurious variation tends to the elimination of its possessor, a useful one increases the probability of its survival.

There must, however, be for every variation a stage of development in which it has as yet no decisive biological importance, and this stage need not by any means be so insignificant that we cannot see it, or can hardly do so: in other words, there are characters which have arisen through germinal selection, which are of purely 'morphological importance.'

It has often been disputed whether there can be any such thing as 'purely morphological characters,' which are indifferent as far as the existence of the species is concerned. This question used to be an important one, because the sphere of operation, and therefore the importance of the Darwin-Wallace selection—personal selection—depends on the answer, since this mode of selection only begins when a character has some biological importance. But as soon as we take germinal selection into consideration the question loses its importance, because we now know that every variation is indifferent to begin with, but every one can, under favourable circumstances, be increased to such a pitch that it attains biological importance, and that personal selection then takes over the task of carrying it on, either in a positive or a negative sense. We may therefore leave this disputed point alone just now, for while germinal selection seems still far from being generally recognized, we have to remember that we are not at all in a position to judge with any certainty as to the biological value of a character. What labour and painstaking investigation it has cost to give a verdict as to this even in a few instances! Innumerable characters appear indifferent, and are nevertheless adaptations. Darwin in his day pointed out the need for caution in this matter, referring to the case of animal coloration as an example; very little attention had been directed to it for a long time because it had been believed to be without significance. And how many diverse kinds of characters among animals and plants, which had likewise been regarded as 'purely morphological,' have on more careful investigation shown themselves of very great biological importance. I need only refer to the shape, position, hair-arrangement, colour, and lustre of flowers, and their relation to cross-fertilization by means of insects, or to the thickness and shape of the leaves of tropical trees with their coating of wax and their gutter-like outlets for carrying off the tropical rain which falls in terrible downpour (Haberlandt, Schimper), or to the limp, perpendicular drooping of the tufts of the young and tender leaves of the same trees, which also secures protection from being battered and torn by the rain.

Fig. 107, C. Leptocephalus stage of an American Eel, with seven pigment spots, of which three are on the left (l) and four on the right (r) side. After Eigenmann.

There are even characters the biological use of which is unknown to us, but in regard to which we can affirm that they have a use. Thus Eigenmann described the larva of an American eel, which differs from other so-called 'Leptocephali' in that a row of seven black spots runs along its side. Apparently all these lie upon the side turned towards us, but in reality they are distributed on both sides, three lying on the left and four on the right, and so arranged that they look like a single row of spots at regular intervals, for the flat little fish is absolutely transparent. The habits of this larva are not yet known, but we may conclude that this appearance of a simple row of spots must have some value for the animal, for such a significant asymmetry could not have arisen for purely internal reasons (Fig. 107, C). It is possible that the fish is thus made to resemble parts of some marine alga, and that it is thereby protected from many enemies; that there is not a complete row upon each side may depend upon the fact that the two rows would be visible at the same time, and that they would blur each other in the eyes of the swimming enemy, and so destroy the resemblance of the picture to its unknown model.

But it cannot be denied that there are characters which have no special biological significance. There are doubtless many such characters, which stand beyond the threshold of good or bad, and which are therefore not affected by personal selection; it is difficult and often impossible to point these out with certainty. The shape of the human nose and of the human ear, the colour of the hair and of the iris, may be such indifferent characters whose peculiarities are to be referred solely to germinal selection. On the other hand, I would not venture to assert that the gay colouring and the complex markings on the wings of our modern Lepidoptera are always and in all cases unimportant, even when we cannot interpret their details either as protective, or as a sign of nauseousness, or as mimetic. The usually very exact similarity of the colour pattern in the individuals of each species seems to point to the intervention of personal selection in some form or other, for in what other way could such a large majority of variations in the same direction have developed in the germ-plasm as this constancy of the character indicates.

We know, of course, that the colours of butterflies and moths can be caused to vary through external and especially climatic influences, but this would only account for simple modifications of colour, and not for the origin of the complex colour patterns that actually occur. I therefore believe with Darwin that sexual selection has had much to do with this by giving a slight preference to the variations produced by spontaneous germinal selection, and thus preventing the majority of varied ids once acquired from being scattered again, but always collecting more of them, and so securing free play for the increase of the new character through intra-germinal processes. In this way have arisen not only the brilliance of our Lycænidæ and of the large Morphidæ of South America, but also many of the coloured spots, streaks, bands, eyes, and other components which have gradually in the course of time evolved into the complex colour pattern of many of our modern butterflies. I should like to remind any one who doubts this of a fact which corroborates the view that personal selection has co-operated in the production of these colours—I refer to the inconspicuous colouring of the females of many of these brilliant males—while in contradistinction to these cases there are other species in which both sexes are alike brilliant, so that it is impossible that mere spontaneous germinal selection can have determined that the females, because of their femaleness, should vary in a different manner from the males.