ON OUR PRESENT KNOWLEDGE OP THE ORIGIN OF MAN. 477 



easy to find. But this anatomical chain corresponds exactly with the 

 paleontological facts; selachians and ganoids are already found in the 

 Silurian formations, dipneusta in the Devonian, amphibia in the Car- 

 boniferous, reptiles in the Permian, mammalia in the Trias. 



These are historical facts of the first rank. They attest in the most 

 gratifying manner the successive steps of the development of verte- 

 brates, as they have been made out by the comparative researches of 

 Cuvier and Meckel, of Johannes Miiller and Gegenbaur, of Owen, Hux- 

 ley, and Flower. The historical succession of the principal steps in the 

 vertebrate stock is thereby definitely established, and this success is 

 much more important for an understanding of the human family tree 

 than if we had succeeded in placing, in a hundred fossil skeletons of 

 lemurs and apes, the entire series of our Tertiary primate ancestors in 

 coherent succession before our eyes. 



Much more difficult and dark is the oldest history of our stock, the 

 derivation of a vertebrate stem from an invertebrate ancestry. As 

 none of these possessed any hard and petrifiable parts of the skeleton 

 (resembling in this respect the lowest vertebrates, the cyclostomata and 

 acrania) the evidence of paleontology entirely fails us here; we must 

 rely alone upon the other two records of our family history, upon com- 

 parative anatomy and ontogeny. To be sure, their value is here so 

 great in many respects that for every expert and discriminating zoolo- 

 gist they throw the clearest light upon many great features of our older 

 phylogeny. Of the greatest value are these far-reaching inferences 

 which modern comparative ontogeny has drawn during the last thirty 

 years by the aid of the fundamental biogenetic law. Already the older 

 embryology has made clear the elements of vertebrate development by 

 the thorough work of Baer and of Bischoff, of Remak and Kolliker. 

 Then, in 1866, came the important discoveries of Kowalevsky, which 

 confirmed the suspicion of Goodsir and pointed to the close relation- 

 ship of vertebrates and tuuicates; the comparative anatomy of Amphi- 

 oxus and of the ascidiaus has since that time been the constant starting 

 point for all further investigations concerning our invertebrate prede- 

 cessors. 



Five years investigation of the structure and development of the 

 chalk-sponges (181)7-1872) had led me at that time to a reform of the 

 theory of the germinal layers and to advance the gastmea theory. It 

 first appeared in 1872 in my monograph on the chalk-sponges or Galci- 

 spongidce. These views obtained the most earnest support and the most 

 fruitful development by the excellent comparative researches of many 

 other embryologists, especially those of E. Ray-Lankester and Francis 

 Balfour, as well as those of the brothers Oscar and Richard Hertwig. I 

 had already then concluded from these comparative researches that the 

 first step of development in all Metazoa, or tissue-building animals, is 

 essentially the same, and that we may from this obtain definite insight 

 into the common origin and the older ancestral series of the same. The 



