The Evolution Theory, Vol. 2 of 2

by August Weismann

What I propose to discuss in this lecture should have been considered at an earlier stage, if we had pledged ourselves to adhere strictly to the historical sequence of scientific discovery, for the phenomena which we are about to deal with attained recognition shortly after the revival of the evolution idea, and indeed they formed the first important discovery which was made on the basis of the Darwinian Doctrine of Descent. I have introduced them at this stage because they have to do with phenomena of inheritance and modifications of these, the understanding of which—in as far as we can as yet speak of understanding at all—is only possible on the basis of a theory of inheritance. Therefore, in order to examine these phenomena and their causes, it was necessary first to submit a theory of heredity, as I have done in the germ-plasm theory. We have to treat of the connexion between the development of many-celled individuals and the evolution of the species, between germinal history and racial history, or, as we say with Haeckel, between ontogeny and phylogeny.

Long before Darwin's day individual naturalists had observed that certain stages in the development of the higher vertebrates, such as birds and mammals, showed a likeness to fishes, and they had spoken of a fish-like stage of the bird-embryo. The 'Natural Philosophers' of the beginning of the nineteenth century, Oken, Treviranus, Meckel, and others, had, on the basis of the transmutation theory of the time, gone much further, and had professed to recognize in the embryonic history of Man, for example, a repetition of the different animal stages, from polyp and worm up to insect and mollusc. But von Baer afterwards showed that such resemblances are never between different types, but only between representatives of the same general type, e.g. that of Vertebrata; and Johannes Müller maintained, from the standpoint of the old Creation theory, that an 'expression of the most general and simple plan of the Vertebrates' recurred in the development of higher Vertebrates, giving as an instance that, at a certain stage of embryogenesis, even in Man, gill-arches were laid down and were subsequently absorbed. But why this 'plan' should have been carried out where it was afterwards to be departed from remained quite unintelligible.

An answer to this question only became possible with the revival of the Theory of Descent, and the first to throw light in this direction was Fritz Müller, who, in his work Für Darwin, published in 1864, interpreted the developmental history of the individual, 'the ontogeny,' as a shortened and simplified repetition, a recapitulation, so to speak, of the racial history of the species, the 'phylogeny.' But at the same time he recognized quite clearly—what indeed was plain to all eyes—that the 'racial history' cannot be simply read out of the 'germinal history,' but that the phylogeny is often 'blurred,' on the one hand by the fusing and shortening of its stages, since development is always 'striking out' a more direct course from the egg to the perfect animal, while, on the other hand, it is frequently 'falsified' by the struggle for existence which the free-living larvæ have to maintain.

For the establishment of these views Fritz Müller relied chiefly upon larvæ, and in particular upon those of Crustaceans, and the facts, which were in part new and in part interpreted in a new manner, were so striking that it was impossible to deny their importance. In particular, he drew attention to the fact that in several of the lower orders of Crustaceans the most diverse species have a similar form when they leave the egg, all of them being small, unsegmented larvæ, with a frontal eye and a helmet-like upper lip, and with three pairs of appendages, the two posterior pairs being two-branched swimming-legs beset with bristles. In the size and form of the body, and especially of the chitinous carapace, these larvæ differ in the various systematic groups; thus, for instance, the larvæ of the Copepods are simply oval, while those of the Cirrhipedes are produced anteriorly into two horn-like processes, and so on, but in essentials they are all alike, and for a long time these larval forms had been distinguished by the special name of 'Nauplius' ([Fig. 109]).

The development of the perfect animal begins with the longitudinal growth of the Nauplius; the posterior end lengthens and becomes segmented, between the anterior portion and the tail more segments are interpolated, and on these new pairs of limbs may grow. The number of these segments and limbs varies according to the group to which the animal belongs. Thus the body of the perfect animal in the little Cyprids always consists of eight segments, seven of which bear a pair of limbs apiece; in the Branchiopods, on the other hand, the number of segments varies from twenty to sixty, with ten to over forty pairs of legs; in the Daphnids or water-fleas there are about ten segments, with seven to ten pairs of limbs, and in the Copepods about seventeen segments with eleven pairs of limbs. The difference between the orders depends not only upon the differences in the number of segments and limbs, but quite as much upon the form and development of the segments, and above all of the limbs, and in this connexion it is worthy of note that the additional limbs which grow out usually appear at first as biramose swimming-legs, and are subsequently modified in form. Thus the pairs of jaws, three in number, which appear in the Copepods are developed from such swimming-legs, and so also is the second pair of antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes, &c.

Fig. 108. Nauplius larva of one of the lower Crustaceans. After Fritz Müller. Au, the frontal eye; I, first pair of limbs, corresponding to the future antennæ; II and III, two biramose swimming appendages.

If then we have before us in the 'germinal history' (ontogeny) a fairly precise repetition of the 'racial history' (phylogeny), we may deduce from this that the primitive forms of the Crustacean race were animals which consisted of few segments, and that from these, in the course of the earth's history, the very diverse modern groups of Crustaceans have arisen, by the addition of new segments, and the adaptation of the limbs upon them, which were at first biramose swimming-legs, to different kinds of functions, one becoming an antenna, another a jaw or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin.

Fig. 109. Metamorphosis of one of the higher Crustacea, a Shrimp (Peneus potimirim), after Fritz Müller. A, the nauplius larva with the three pairs of appendages: I, the antennæ; II and III, the biramose swimming-feet. Au, the single eye. B, first Zoæa stage, with six pairs of appendages (I-VI). Skn, area where new segments are being formed.