ABSENCE OF GENERALIZED FORMS, 99 



little importance to our subject, the amount of 

 geological change being the important factor; for 

 rapid geological changes will cause rapid organic 

 developments. If, then, it is true, as embryology 

 teaches, that the first variations from the original 

 ancestral type produced directly the various sub- 

 kingdoms of animals, and if, further, the changes 

 were more rapid at this early period, it becomes 

 probable that the time required for the establish- 

 ment of the types of the Silurian period, was not so 

 long as would be required for what would seem to 

 be an equal amount of change in animals as they 

 exist to-day. 



The A bsence of Generalized Forms from the Silurian. 



This old Silurian fauna is puzzling in another re- 

 spect. It is undoubtedly a highly specialized fauna. 

 It has been and is still forcibly urged that, accord- 

 ing to evolution, the farther back we go, the greater 

 and greater should be the generic likeness of fos- 

 sils, and the less and less their specialization. For 

 if our classes, orders, and genera existed at any time 

 as simple species, the fossils ought all to be more 

 or less generalized forms ; i. e., forms possessing the 

 fundamental characters of all the classes that arise 

 from them. The oldest fossils ought to be almost 

 entirely of this character. The number of species 

 should become less and less, and it might be ex- 

 pected that in the earlier ages we should find almost 

 nothing corresponding to species, since no speciali- 

 zation would have taken place. But it is further 

 pointed out that this is not the teaching of paleon- 



