3 o THE AGE OF THE EARTH 



with the lower Phyla of the Coelomate sub-grade, when 

 further back it passed through a Coelenterate, a higher 

 Protozoan, and finally a lower Protozoan phase, we are 

 led to believe that its evolution was probably very slow 

 as compared with the rate which it subsequently attained. 

 But this conclusion is of the utmost importance ; for the 

 history contained in the stratified rocks nowhere reveals 

 to us the origin of a Phylum. And this is not mere 

 negative evidence, but positive evidence of the most 

 unmistakable character. All the five Coelomate Phyla 

 which occur fossil appear low down in the Palaeozoic 

 rocks, in the Silurian or Cambrian strata, and they are 

 represented by forms which are very far from being 

 primitive, or, if primitive, are persistent types, such as 

 Chiton, which are now living. Thus Vertebrata are 

 represented by fishes, both sharks and ganoids ; the 

 Appendiculata by cockroaches, scorpions, Limulids, Tri- 

 lobites, and many Crustacea ; the Mollusca by Nautilus and 

 numerous allied genera, by Dentalium, Chiton, Pteropods, 

 and many Gastropods and Lamellibranchs; the Gephyrea 

 by very numerous Brachiopods, and many Polyzoa ; the 

 Echinoderma by Crinoids, Cystoids, Blastoids, Asteroids, 

 Ophiuroids, and Echinoids. It is just conceivable, al- 

 though, as I believe, most improbable, that the Vertebrate 

 Phylum originated at the time when the earliest known 

 fossiliferous rocks were laid down. It must be remem- 

 bered, however, that an enormous morphological interval 

 separates the fishes which appear in the Silurian strata 

 from the lower branches, grades, and classes of the Phylum 

 in which Balanoglossus, the Ascidians, Amphioxus, and 

 the Lampreys are placed. The earliest Vertebrates to 

 appear are, in fact, very advanced members of the Phylum, 

 and, from the point of view of anatomy, much nearer to 

 man than to Amphioxus. If, however, we grant the im- 

 probable contention that so highly organized an animal 

 as a shark could be evolved from the ancestral vertebrate 

 in the period which intervened between the earliest Cam- 

 brian strata and the Upper Silurian, it is quite impossible 

 to urge the same with regard to the other Phyla. It has 

 been shown above that when these appear in the Cam- 



