16. Pithecanthropi.
17. Homines.
It will be noticed that except for the hypothetical forms (marked with an asterisk), which are themselves generalised classificatory groups, the ancestral forms belong to long-recognised classes. The whole course of the evolution follows well-worn systematic lines. This is typical of Haeckel's phylogenetic speculations.
A more abstractly morphological scheme of the evolution of Vertebrates is given in the Systematic Phylogeny of 1895.[441] The ontogenetic and ancestral stages are arranged in parallel columns thus:—
| Cytula. | Cytæa (Protozoa). | ||
| Morula. | Moræa (Cœnobium ofProtozoa). | ||
| Blastula. | Blastæa (Volvocina,etc.). | ||
| Depula (invaginated blastula). | Depæa. | ||
| Gastrula. | Gastræa (cf. Olynthus,Hydra, and primitive Coelentera). | ||
| Cœlomula (with one pair ofcœlom-pockets). | Cœlomæa (cf. Sagitta,Ascidia, and primitive Helminthes). | ||
| Chordula (with medullary tube andchorda). | Chordæa (cf. Ascidian larvaand larva of Amphioxus). | ||
| Spondula (with segmented mesoderm). | Prospondylus (Primitive Vertebrate). | ||
This scheme differs from the earlier one chiefly in taking into account certain advances, notably as regards the cytology of the fertilised ovum and the true nature of the cœlom, which had been made in the interval of some twenty years.
Haeckel's Gastræa theory, though it exercised a great influence upon the subsequent trend of phylogenetic speculation, was by no means universally accepted telle quelle. Opinions differed considerably as to the primitive mode of origin of the two-layered sac which was very generally admitted to be of constant occurrence in early embryogeny. Ray Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a "Planula" theory, according to which the ancestral form of the Metazoa was a two-layered closed sac formed typically by delamination, less often by invagination. He denied that the invagination opening (which he named the blastopore) represented the primitive mouth,[443] holding that this was typically formed by an "inruptive" process at the anterior end of the planula, which led to the formation of a "stomodæum." A similar process at the posterior end gave rise to the anus and the "proctodæum."
The question as to whether delamination or invagination was to be considered the more primitive process was discussed in detail by Balfour,[444] without, however, any very definite conclusion being reached. He held that both processes could be proved in certain cases to be purely secondary or adaptive, and that accordingly there was nothing to show that either of them reproduced the original mode of transition from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by no means rejected the theory that the Gastræa, "however evolved, was a primitive form of the Metazoa," but, having regard to the great variations shown in the relation of the blastopore to mouth and anus (pp. 340-1), he was inclined to think that if the gastrula had any ancestral characters at all, these could only be of the most general kind. Balfour's attitude perhaps best represents the general consensus of opinion with regard to the Gastræa theory.
From the same origins as the Gastræa theory arose the theory of the cœlom. The term dates back to Haeckel in 1872, and the observations which first led up to the theory were made by the men who supplied the foundations of the Gastræa theory—A. Agassiz, Metschnikoff and Kowalevsky. But it was not Haeckel himself who enunciated the cœlom theory.
It will be remembered that Remak introduced in 1855 the conception of the mesoderm as an independent layer derived from the endoderm. The pleuro-peritoneal or body-cavity was formed as a split in the "ventral plates" of the mesoderm. Haeckel's "cœlom" corresponded to the "pleuro-peritoneal cavity" of Remak, but his view of the origin of the mesoderm brought him much closer to von Baer's conception of the origin of two secondary layers from ectoderm and endoderm respectively than to Remak's conception of the mesoderm as a single independent layer.