DEVELOPMENT OF VERTEBRATES FROM INVERTEBRATES. 95 



in no way the subject of our further investigations, which 

 are only concerned with the Vertebrates, 



The development of the Vertebrates from the Inverte- 

 brates most nearly related to them, the Chorda- Animals, 

 occurred millions of years ago, during the Archilithic Epoch. 

 (See Table XII., p. 11.) This is unmistakably shown by 

 the fact that the most recent sedimentary rock-strata 

 which were deposited during that immense period of time, 

 the higher layers of the Upper Silurian formation, contain 

 remains of fossil Fishes (Primitive Fishes, Selachii). As 

 these Fishes, although they belong to the lowest stage of 

 the Skulled Animals (Craniota), yet possess a compara- 

 tively high organization, and as they must necessarily have 

 been preceded by a long progressive series of lower Skull- 

 less Vertebrates, we must attribute the origin of the oldest 

 Skull-less Animals (Acrania) from the Chorda-animals to 

 a much earlier part of the Archilithic Epoch. Therefore, 

 not only all the invertebrate ancestors of our race, but also 

 the earliest form of our vertebrate progenitors must have 

 developed in that primordial time, which includes the 

 Laurentian, Cambrian, and Silurian Periods. (Cf. Tables 

 XIII., XIV., and XVI, pp. 12, 19, 44.) 



Unfortunately, Palaeontology can give us absolutely no 

 information with regard either to the structure of our oldest 

 vertebrate ancestors, or to the time of their appearance ; 

 for their bodies were as soft and as destitute of hard 

 parts capable of fossilization, as were the bodies of all 

 our preceding invertebrate ancestors. It is, therefore, not 

 surprising, but quite natural, that we find no fossil 

 remains of the former in the Archilithic formations. The 



Fishes in which the soft cartilaginous skeleton was partly 

 40 



