assumed, the value of the hypothesis is undiminished. It was the road to truth.

In our science Haeckel has made the most extensive use of the right of devising hypothetical pedigrees as landmarks for research. It matters nothing that he has repeatedly been obliged to correct himself, or that others have frequently corrected him; the influence of these pedigrees on the progress of the zoology of Descent is manifest to all who survey the field of science, not to mention that in the last ten years a series of researches have conclusively fixed their results in good pedigrees. As we propose to give merely an introduction to the doctrine of Descent, we shall content ourselves with showing how the system or the pedigree is constituted in its application to the single group of the Vertebrata.

As we have seen above, the most important indications. of the pedigree of the species are contained in the evo

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lutionary history of the individual. Only, if all vertebrate animals testified their family connection by agreeing inter se in the distribution of the germ as well as in the fundamentally important organs, the spinal cord and the vertebral column, this token of their descent from inferior animals, which is unconditionally demanded. by the theory, seemed to be entirely wanting. In other words, it seemed that in all vertebrate animals the memory of their original derivation had been obliterated by curtailed development (comp. p. 211). Thus the case remained until Kowalewsky a few years ago studied the development of the lancelet (Amphioxus), the lowest vertebrate animal known, and showed that in this creature the typical phenomena of vertebrate development

FIG. 22. Larva of the Lancelet after Kowalewsky.

are preceded by the phases required by the theory. We have already made acquaintance with this form of development (p. 51, &c.), and we here again point out its profound significance. It is only when the Amphioxus has passed through the phase of the vibrating, sac-like

gastrula larva that the future dorsal side becomes flattened, and the protuberances arise, which shortly after close into the sheath of the spinal marrow, while underneath originates this important cellular column, the chorda dorsalis, or notochord. With this the lancelet becomes a vertebrate animal, and the preceding phases do not (according to the view at one time inculcated by C. E. v. Baer respecting such phenomena) recall the inferior and undeveloped in general by the absence of differentiation, but they agree in genesis and distribution, in the differentiation of their cellular layers, and in their totality, with the Gastrula phases of invertebrate animals.

We are therefore fully justified in regarding these first incidents in the evolution of the Amphioxus as a reminiscence of the roots of the pedigree of the Vertebrata; and this direct indication of the descent of vertebrate from invertebrate animals is supported by a second and no less important discovery by the Russian naturalist. It is, that during their development a number of the Testacea of the division of the Ascidians temporarily possess a spinal cord, and the rudiments of a vertebral column. Kowalewsky's researches have been ratified on all essential points and in many ways extended by Kupfer, and the facts which interest us may be explained by the diagram, Fig. 23, representing the point of the larva of an Ascidian in a somewhat advanced stage. The bulk of the Ascidian larva consists of a body of which our figure shows the whole, and a rudder-like tail. The appendages projecting from the body on the right are organs of adhesion, by means of which the larva fixes itself for its definitive transforma

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tion. Ato the orifice of the mouth is formed; d developes into the branchial cavities and the intestinal

FIG. 23.

a

canal, and we will incidentally remark that in the lance

let also, the anterior end of the primitive intestine becomes the branchial cavity. But with reference to the vertebrate animals, the most important parts of the Ascidian larva are the following. It possesses a true spinal cord with a vesicularly expanded brain (ra). The distribution and position of this organ agrees accurately with the corresponding parts of the vertebrate animal, and Kupfer has even discerned the rudiments of nerves (sss), which, if the observation is confirmed, will still more incontrovertibly establish the homology of the organ in question with the spinal cord of the Vertebrata and the nerves proceeding from it in pairs. But we know that it is not the spinal cord alone, but its combination with the vertebral column which constitutes the characteristic feature of the vertebrate animal. This vertebral column the Ascidian larva likewise possesses (c) in the form of the noto-chord, and, as in the vertebrate animal, this embryonic vertebral column lies between the intestine and the spinal cord. So far goes the accordance; henceforth, the development of this part, so important to the vertebrate animal, becomes retrogressive in the Ascidian. The rudder-like tail, with the spinal cord contained in it, and the noto-chord, are cast off when the animal becomes fixed; the larval brain which promised so well, shrinks into an insignificant nervous ganglion, and the complete animal gives no cause for suspecting its analogy with the Vertebrata.

These laborious observations prove that the Vertebrata are not the sole proprietors of the spinal cord and vertebral column, but received these organs as a heritage from lower grades of organization as their progenitors. does not occur to the Darwinists to regard man as the

It