August Weismann

“ And what wonderful special adaptations of the eye to near and to distant vision, to vision in the dusk and at night, or in the great ocean-depths, to recognition of mere movement or the focussing of a clear image, have been interpolated in the course of this evolution! ”

“ The beards of hens, the tail-feathers of the long-tailed domestic cocks, the long and short, straight and curved bills of pigeons, the enormously long ruffled feathers of the Jacobin, the multiplication of the tail-feathers in the fan-tail, and innumerable other breed-characters of these playthings of the breeder, prove that when variation-tendencies of any part are once present, that is, when they have arisen through germinal selection, they apparently go on unchecked until their further development would permanently and irretrievably destroy the harmony of the parts. ”

“ It would be impossible to prove by experiment that spontaneous generation could never have taken place; because each negative experiment would only prove that life does not arise under the conditions of the experiment. ”

“ We must therefore assume that, in the higher colony-forming plants, germ-plasm is contained in a great many cells, perhaps in all which are not histologically differentiated, and sometimes even in those which are so, as, for instance, in the leaves of Begonias. I suppose, therefore, that in the higher plants the process of development implies a segregation of the determinant-complexes of the germ-plasm, but that this takes place at a late stage, and that in a much higher degree than among animals the individual or the 'person' carries with it germ-plasm in a latent state. ”

“ But of course species are not exclusively complicated systems of adaptations, for they are at the same time 'variation-complexes,' the individual components of which are not all adaptive, since they do not all reach the limits of the useful or the injurious. All transformations arise from a basis of spontaneous chance variations, just as all forest plants grow from the soil of the forest, but do not all grow into trees, the adaptive forms which determine the essential character of the forest; for many species remain small and low, like the mosses, grasses, and herbs ”

“ 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. ”

“ In some quarters this whole conception of germinal selection has been characterized as the merest figment of imagination, condemned on this ground alone, that it is based on the differences in nutrition between such extremely minute quantities of substance as the chromosomes of nuclear substance within the germ-cell. The quantity of substance is certainly minute, but it needs nutriment none the less, and can we believe that the stream of nourishment for all the invisibly minute vital elements is exactly alike? ”

“ When, in speaking of plant-hybrids, I said that the collective ids of the germ-plasm of a species must be equivalent in regard to the characters of the species, I did not speak quite precisely; in the majority of ids, in many cases in an overwhelming majority, this must be the case, but not actually in all, at least not on the assumption we make that the transformation of species is accomplished under the control of natural selection. ”

“ As soon as personal selection, whether sexual or ordinary, prefers as useful in any sense a saltatory variation, it is not only preserved and becomes a character of a variety, but it may increase, and we have to ask whether such sudden variations are frequently of a useful kind, especially when not individual characters alone, but whole combinations of them are implicated. If we may judge from the sports of the flowers and the leaves of plants, transformations useful to the species as a whole rarely occur suddenly, that is, they occur only in a few out of very numerous sports ”

“ 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 ”

“ The roots of all the transformations of organisms, then, lie in changes of external conditions. Let us suppose for a moment that these might have remained absolutely alike from the epoch of spontaneous generation onwards, then no variation of any kind and no evolution would have taken place. But as this is inconceivable, since even the mere growth of the first living substance must have exposed the different kinds of biophors composing it to different influences, variation was inevitable, and so also was its result—the evolution of an animate world of organisms. ”

“ Many hybrids suggest one parent-form in the leaves, the other in the blossoms. In the same way in the child the form of eye may be that of the father, the colour of the iris that of the mother, the nose maternal, the mouth paternal—in short, the preponderance in heredity swings hither and thither from part to part, from organ to organ, from character to character, and this is even the rule though the oscillations may not always be apparent and are often invisible. ”

“ It seems clear to me that the species is not a life-crystal in the sense that it must, like a rock-crystal, take form in a particular way and in no other for purely internal reasons and by virtue of its physical constitution; the species is essentially a complex of adaptations, of modern adaptations which have been recently acquired, and of inherited adaptations handed down from long ago—a complex which might quite well have been other than it is, and indeed must have been different if it had originated under the influence of other conditions of life. ”

“ 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? ”

“ And in truth, if the body were really able to replace, after artificial injury, parts which are never liable to injury in natural conditions, and to do so in a most beautiful and appropriate manner, then there would be nothing for it but at least to regard the faculty of regeneration as a primary power of living creatures, and to think of the organism as like a crystal, which invariably completes itself if it be damaged in any part. ”

“ If I mistake not, we may say at least so much, that all variations are, in ultimate instance, quantitative, and that they depend on the increase or decrease of the vital particles, or their constituents, the molecules. For this reason I have hitherto always spoken of only two directions of variation—a plus or a minus direction from the average. What appears to us a qualitative variation is, in reality, nothing more than a greater or a less, a different mingling of the constituents which make up a higher unit, an unequal increase or decrease of these constituents, the lower units. ”

“ And it does cease as soon as an organ ceases to be of use to its species, like the eye to the species of crustacean which descends into the dark depths of our lakes, or to the abyssal zones of the ocean, or into a subterranean cave-system. In this case all selection of individuals ceases as far as the eye is concerned; it has no importance in deciding survival in the struggle for existence, because no individual is at a disadvantage through its inferior eyes, for instance, by being in any way hindered in procuring its food. ”

“ There can be no doubt whatever that death is virtually implied in the very constitution of a multicellular organism, and is thus, so to speak, a foreseen occurrence, the inevitable end of a development which begins with the egg-cell and reaches its highest point with the liberation of the germ-cells, that is, with reproduction, and then enters on a longer or shorter period of decadence, leading to the natural death of the individual. ”

“ We must not forget, however, that natural selection works essentially upon a basis of sexual reproduction, which with its reduction of the ids and its continually repeated mingling of germ-plasms, combines the existing variational tendencies, and thus diffuses them more and more uniformly among the individuals of a whole area of occupation. Sexual reproduction, continual intermingling of the individuals selected for breeding, is thus a very effective and important, if not an indispensable, factor in the evolution of the specific type. ”