The Evolution Theory, Vol. 2 of 2

by August Weismann

But we went further than this, and saw that there is a second result implied in amphimixis, namely, that the individual character of the germ-plasm is being continually altered by new combinations of the ids contained in it. We inferred from what I believe to be the demonstrated hypothesis that the germ-plasm is composed of ids, that its reduction to half the original mass must mean a reduction of the ids to half the number, and as the ids contain primary constituents which are individually different, this must effect a new arrangement, a new mingling of these individual peculiarities. The reduction of the germ-plasm to half, that is, the diminution of the number of its ids to half, is a phenomenon generally associated with amphimixis, and has been established in the case of all animals which have hitherto been investigated, and of all the most carefully studied plants, and finally, it has been shown to be very probable in unicellular organisms, for the processes of conjugation in Infusorians and many other Protozoa include phenomena very similar to those of reducing division in the higher animals. The prediction made on theoretical grounds has here been verified by observation, and it is obvious that the assumption of ids, that is, of units in the germ-plasm which are handed on from one generation to the succeeding one, involves a reduction of their number in each amphimixis. Without this the number of ids would be doubled at each amphimixis, and would therefore gradually amount to something enormous. We see therefore why this normally recurrent reduction of ids before each amphimixis was established in the course of evolution, and we see that it inevitably involves that a new combination of ids should be associated with each amphimixis.

If nothing persists unless it be purposeful, that is, necessary, what is the meaning of the fact that arrangements for amphimixis occur over almost the whole known domain of life, from the very simple organisms up to the highest, in unicellular and multicellular organisms, in plants and animals alike? Why is it that this arrangement has been departed from only in a few small groups of forms, while it occurs everywhere else, in almost every generation, so indissolubly associated with reproduction that it has even been regarded—with a lack of clearness—as itself a form of reproduction, and is even now generally called 'sexual reproduction'? And why is it that in many organisms, especially lowly ones, it is not associated with every reproduction, though it recurs at regular or irregular intervals? Such a universal arrangement must undoubtedly be of fundamental importance, and we have to ask wherein this importance lies. That is the problem to the solution of which we must now apply ourselves.

So much we may say at once: The significance of amphimixis cannot be that of making multiplication possible, for multiplication may be effected without amphimixis in the most diverse ways—by division of the organism into two or more, by budding, and even by the production of unicellular germs. Even though these last are usually in various ways so organized that they must undergo amphimixis before they can develop into new organisms, yet there are numerous germ-cells which are not subject to this condition (e.g. spores), and there are—as we have seen—many germ-cells, adapted for amphimixis, which always, or in certain generations, or even only occasionally, emancipate themselves from this condition under certain external influences: I refer to egg-cells which develop parthenogenetically.

If amphimixis is not a universal preliminary condition of reproduction, wherein lies the necessity for its general occurrence among living organisms?

We have already learned that there are two results produced without exception by amphimixis; one of these is the antecedent reduction of the original number of ids by one half, and the consequent new combination of ids which results from this; the other is the union of two such halved germ-plasms from two different individuals. The first we may, with Hartog, compare to the removal of half of a pack of cards previously mixed, the second to the combination of two such halves from different packs. The first process brings nothing new into the complex of primary constituents, but rather removes a part—larger or smaller—of its characters: not necessarily exactly half of these, since each individual kind of id may be represented by doubles or multiples. But the reduction simplifies the composition of the germ-plasm, and might by itself, through the struggle of the ids in ontogeny, lead to a resultant different from the parent, that is, to a new individuality. Through the second process, however, new individual traits are of necessity added, and make the resultants still more markedly diverse, that is, if the ids of both parents attain to expression in the struggle of ontogeny, and this, as we have already seen, is usually the case, though not always and certainly not always in all parts. Thus amphimixis, together with the preparatory reduction of the ids, secures the constant recurrence of individual peculiarities through the ceaseless new combinations of individual characters already existing in the species.

When sixteen years ago I first inquired into the actual and ultimate significance of sexual reproduction, I thought I had found it in this ceaseless production of new individualities. This seemed to me a sufficient reason for the introduction of amphimixis into nature, since the difference between individuals is the basis of the process of selection, and thus the basis of all the transformations of organisms, which we may refer to natural or sexual selection. Now these differences of selection-value are—as I believed then, and do still—not only by far the most frequent organic changes, but also the most important, since they not only initiate, but control new lines of evolution. Therefore I still regard amphimixis as the means by which a continual new combination of variations is effected a process without which the evolution of this world of organisms so endlessly diverse in form and so inconceivably complex, could not have taken place.

But I do not regard this amphimixis as the real root of variation itself, for that must depend not on a mere exchange of ids, but rather upon a variation of the ids. The ids of a worm of the primitive world could not without variation now make up the germ-plasm of an elephant, even if it be true that mammals are descended from worms. The ids must have been meanwhile transformed times without number by the modification, degeneration, and new formation of determinants. Amphimixis, that is, the union of two germ-plasms, does not of itself cause variation of the determinants, it only arranges the ids (the ancestral plasms) in ever-new combinations. If the origin of variation were limited to that alone, a transmutation of species and genera would only be possible on a very limited scale; there could at most be a narrow circle of variations, just as in the example already given of the packs of cards; even if the taking away and mixing up of the halves were repeated a thousand times, a definite though undoubtedly large number of card-combinations would in the long run recur again and again. But the case is different with the germ-plasm and amphimixis, where there is an infinitely more varied series of results, because the individual cards—the ids—are variable, even between one time of sifting and shuffling and another, and therefore infinitely productive of variety in the course of numerous repetitions of the shuffling.

I have been frequently and persistently credited with maintaining that the germ-plasm is invariable—a misunderstanding of my position, due perhaps to a somewhat too brief and terse statement which I made at an earlier period (1886). I had described the germ-plasm as 'a substance of great power of persistence,' and as varying with difficulty and slowly, basing this statement upon the age-long persistence of many species in which the specific constitution of the germ-plasm must have remained unchanged. The idea of 'germinal selection,' of a ceaseless struggle between the 'primary constituents' of the germ, and of the resulting continual slight and invisible rising and falling of individual characters, had not yet dawned upon me, nor had I at that time formulated the conception of 'determinants.' I was even doubtful at that time whether development, heredity, and variation were not interpretable on the assumption of an undifferentiated substance without primary constituents. But at no time was I unaware that the whole phyletic evolution of the organic world is only conceivable on the assumption of continual variation of the germ-plasm, that it actually depends upon this, even if these variations come about with exceeding slowness, and are thus in a certain sense 'difficult.'

Now that I understand these processes more clearly, my opinion is that the roots of all heritable variation lie in the germ-plasm, and furthermore, that the determinants are continually oscillating hither and thither in response to very minute nutritive changes, and are readily compelled to variation in a definite direction, which may ultimately lead to considerable variations in the structure of the species, if they are favoured by personal selection, or at least if they are not suppressed by it as prejudicial. But selection is continually keeping watch over both kinds of variation, and if the conditions of life do not further the variation or do not even allow it to persist, selection eliminates everything that lessens the purity of the specific type, everything that transgresses the limits of utility, or that might endanger the existence of the species. Thus we understand how the germ-plasm may be variable, and yet at the same time remain unvaried for thousands of years, how it is ready and able to furnish any variation that is possible in a species if that is required by external circumstances, and yet is able to preserve the characters of the species in almost absolute constancy through whole geological ages; in short, how it can be at once readily variable and yet slow to vary.

The importance which amphimixis thus has in connexion with the adaptation of organisms lies, if I mistake not, in the necessity for co-adaptation, that is, in the fact that in almost all adaptations it is not merely a question of the variation of a single determinant, but of the correlated variations of many—often very numerous—determinants, of 'harmonious adaptation,' as we have already said. Many-sided adaptation of this kind seems to me impossible without a continually recurrent sifting and recombining of the germ-plasms, and this can only be effected by amphimixis.