ABSENCE OF GENERALIZED FORMS. IOI 



It must not be understood, however, that no syn- 

 thetic types have been found at this early period, 

 for many are well known ; but it is a fact that their 

 number is very few compared with the highly 

 specialized types which possessed no synthetic fea- 

 tures. Upon the theory of evolution, great numbers 

 of these synthetic types must have then existed, 

 and it can be well asked why we get no more traces 

 of them. It is a significant question to ask where 

 are any traces of the simple generalized vertebrate 

 which must have lived in the Silurian age, possess- 

 ing the characteristics of all classes of vertebrates. 

 But at the same time we can readily understand 

 how other groups of simple animals should have 

 met with such favorable conditions as to give rise 

 to rapid divergence and specific distinction. The 

 high specialization present in some of the Silurian 

 fauna, the great numbers of highly developed forms 

 of Brachiopods, Echinoderms, Crustacea, etc.; the 

 great profusion of reptiles in the Jurassic, the abun- 

 dance of plants in the Dakota group, require no 

 special explanation. But the absence of cotempo- 

 raneous forms, with generalized characteristics, is for 

 evolution unfortunate. 



Two reasons for this can be given. Firstly, there 

 are certain definite reasons for believing that at 

 these very points the imperfection of the record 

 would be great. As a rule, the generalized forms 

 which we would expect, would be those which from 

 absence of hard parts could not be preserved. The 

 primitive vertebrate, for instance, could have pos- 

 sessed no skeleton, at least nothing more than a 



