

EVOLUTION OF ANCESTRAL FORMS 27 



warrants the belief that the changes during these earlier 

 phases were at least as slow as those which occurred 

 later. 



If we take the history of another of the higher Phyla, 

 the Appendiculata, we find that the evidence points in 

 the same direction. The common ancestor of our Roti- 

 fera, earthworms, leeches, Peripatus, centipedes, insects, 

 Crustacea, spiders and scorpions, and forms allied to all 

 these, is generally admitted to have been Chaetopod- 

 like, and probably arose in relation to the beginnings 

 of certain other Coelomate Phyla, such as the Gephyrea 

 and perhaps Mollusca. At the origin of the Coelomate 

 sub-grade the common ancestor of all Coelomate Phyla 

 is reached, and its evolution has been already traced in 

 the case of the Vertebrata. 



What is likely to be the relation between the time 

 required for the evolution of the ancestor of a Coelomate 

 Phylum and that required for the evolution, which sub- 

 sequently occurred, within the Phylum itself? The only 

 indication of an answer to this question is to be found 

 in a study of the rate of evolution in the lower parts of 

 the animal kingdom as compared with that in the higher. 

 Contrary, perhaps, to anticipation, we find that all the 

 evidences of rapid evolution are confined to the most 

 advanced of the smaller groups within the highest Phyla, 

 and especially to the higher Classes of the Vertebrata. 

 Such evidence as we have strongly indicates the most 

 remarkable persistence of the lower animal types. Thus 

 in the Class Imperforata of the Reticularia (Foramini- 

 fera) one of our existing genera (Saccamina) occurs in 

 the Carboniferous strata, another (Trochammina) in the 

 Permian, while a single new genus (Receptaculites) occurs 

 in the Silurian and Devonian. The evidence from the 

 Class Perforata is much stronger, the existing genera 

 Nodosaria, Dentalina, Textularia, Grammostomum, Val- 

 vulina, and Nummulina all occurring in the Carboni- 

 ferous, together with the new genera Archaediscus (?) and 

 Fusilina. 



I omit reference to the much-disputed Eozoon from 

 the Laurentian rocks, far below the horizon which for 



