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,[17] Remak, and Koelliker;[18] but the clearest light was thrown upon it by the famous discoveries of Kowalevsky[19] 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 1860 to 1872, I myself had been studying the development of the Spongiæ, Medusæ, Siphonophora, and other Coelenterata. 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.
[17] Wilhelm Bischoff of Munich: works on the history of the development of the rabbit, dog, guinea-pig, roe-deer. 1840-1854.
[19] 'Ueber die Entwicklung der einfachen Ascidien,' Mém. Acad. St. Petersbourg, vii. ser., tome x. (1866). Other papers in 'Archiv f. Mikroskop. Anatomie,' vii. (1871); xiii. (1877).
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[20] repeats the stage of our Protozoan ancestors; the Blastula is equivalent to an ancestral coenobium of Magosphæra or Volvox; the Gastrula is the hereditary repetition of the Gastræa, the common ancestor of all the Metazoa.