The Last Link/Other Evidence

​Greater and more frequent difficulties arise if we penetrate further into the most remote part of the human phylogeny, and attempt to derive the vertebrate stem from an older stem of invertebrate ancestors. None of those had a skeleton which could be petrified; and the same remark applies to the lowest classes of Vertebrata—to the Cyclostomes and the Acrania. Palæontology, therefore, can tell us nothing about them; and we are limited to the other two great documents of phylogeny—the results of comparative anatomy and ontogeny. The value of their evidence is, however, so great that every competent zoologist can perceive the most important features of the most remote portion of our phylogeny.

Here the first place belongs to the invaluable results which modern comparative ontogeny has gained by the aid of the biogenetic law or the theory of recapitulation. The foundation-stones of vertebrate embryology had been laid by the works of Von ​Baer, Bischoff,^[1] Remak, and Koelliker;^[2] but the clearest light was thrown upon it by the famous discoveries of Kowalevsky^[3] in 1866. He proved the identity of the first developmental stages of Amphioxus and the Ascidians, and thereby confirmed the divination of Goodsir, who had already announced the close affinity of Vertebrates and Tunicates. The acknowledgment of this affinity has proved of increasing importance, and has abolished the erroneous hypothesis that the Vertebrata may have arisen from Annelids or from other Articulata. Meanwhile, from 1866 to 1872, I myself had been studying the development of the Spongiæ, Medusæ, Siphonophora, and other Cœlenterata. Their comparison led me to the statements embodied in the ​'Gastræatheorie,' the first abstract of which was published in 1872 in my monograph of the Calcispongiæ.

These ideas were carried on and expanded during the subsequent ten years by the help of many excellent embryologists—first of all by E. Ray Lankester and Francis Balfour. The most fruitful result of these widely extended researches was the conclusion that the first stages of embryonic development are essentially the same in all the different Metazoa, and that we may derive from these facts certain views on the common descent of all from one ancestral form. The unicellular egg^[4] repeats the stage of our Protozoan ancestors; the Blastula is equivalent to an ancestral cœnobium of Magosphæra or Volvox; the Gastrula is the hereditary repetition of the Gastræa, the common ancestor of all the Metazoa.

Man agrees in all these respects with the other vertebrates, and must have descended with them from the same common root.

​Particularly obscure is that part of our phylogeny which extends from the Gastræa to Amphioxus. The morphological importance of this last small creature had been perceived by Johannes Mueller, who in 1842 gave the first accurate description of it. It would not, of course, be correct to proclaim the modern Amphioxus the common ancestor of all the vertebrates; but he must be regarded as closely related to them, and as the only survivor of the whole class of Acrania. If the Amphioxidæ had through some unfortunate accident become extinct, we should not have been able to gain anything like a positive glimpse at our most remote vertebrate ancestor. On the one hand, Amphioxus is closely connected with the early larva of the Cyclostomes, which are the oldest Craniota, and the pre-Silurian ancestors of the fishes. On the other hand, the ontogeny of Amphioxus is in harmony with that of the Ascidians, and if this agreement is not merely coincidental, but due to relationship, we are justified in reconstructing for both Ascidians ​and Amphioxus one common ancestral group of chordate animals, the hypothetical Prochordonia. The modern Copelata give us a remote idea of their structure. The curious Balanoglossus, the only living form of Enteropneusta, seems to connect these Prochordonia with the Nemertina and other Vermalia, which we unite in one large class—Frontonia.

No doubt these pre-Cambrian Vermalia, and the common root of all Metazoa, the Gastræades, were connected during the Laurentian period by a long chain of intermediate forms, and probably among these were some older forms of Rotatoria and Turbellaria; but at present it is not possible to fill this wide gap with hypotheses that are satisfactory, and we have to admit that here indeed are many missing links in the older history of the Invertebrata. Still, every zoologist who is convinced of the truth of transformism, and is accustomed to phylogenetic speculations, knows very well that their results are most unequal, often incomplete.