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conjecture was tested. The further an inquirer has carried his survey of the conditions of organization in any of the larger groups, the less will he be able to divest himself of the genealogical idea in his every act and thought.
All this is so self-evident, that one would scarcely suppose that the use of this method could have been made a subject of reproach to the doctrine of Descent. Nevertheless, it frequently occurs, and the champions of the doctrine of Descent are blamed for often speaking of mere probabilities, forgetting that even in cases in which the probability ultimately proves false, the refuted hypothesis has led to progress. Of this the science of language has recently borne testimony. It is well known that linguistic comparison within the family of IndoGermanic tongues suggested the reconstruction of the primitive language which formed their common basis. Johannes Schmidt " now proves that the fundamental forms disclosed may have originated at widely different periods, and hence that the primitive language, regarded as a whole, is a scientific fiction. Nevertheless, inquiry was essentially facilitated by this fiction, and with it was intimately connected the formation of a pedigree of the Indo-Germanic linguistic family, as a hypothesis supported by many indications. A bifurcation was assumed into a South European language, with Greek, Italian, and Celtic ramifications, and another language, from a second division of which proceeded the fundamental language of North Europe and the Aryan fundamental language. Although Johannes Schmidt has demonstrated that this pedigree is false, as the existence of Slavotic shows the impossibility of the first division
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
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
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