Hyatt.] 162 [March 5, 



of forms in the Cambrian or perhaps earlier as claimed by Pro- 

 fessor Marcou. 



We have applied this specific statement as a generalization to the 

 history of smaller groups of fossils in several branches of the ani- 

 mal kingdom, and in many formations, and have found that the 

 sudden appearance of the smaller groups occurs according to the 

 same law. 



There is an obvious plasticity in the animals which first make 

 their appearance in any unoccupied field, or at the beginning of an}' 

 new formation, which reminds one of the plastic nature, of the 

 most generalized type of Metazoa, the existing Porifera. The 

 generalized types, which always occur first in time, exhibit excep- 

 tional capacity for adaptation to the most varied requirements of 

 the surroundings, and meet the conditions of the new period 

 or habitat by the rapid development of numbers of suitable and 

 more highly specialized forms, species and genera. 



The whole picture as presented by morphology, embryology and 

 palaeontology favors the hypothesis we have previously advanced 

 in papers cited above, namely, that the early geologic history of 

 animal life, like the early stages of development in the embryo 

 was a more highly concentrated and accelerated process in evolu- 

 tion, than that which occurred at any subsequent period of the 

 earth's history. 



The history of the Porifera and higher Protozoa suggests also that 

 the evolution of the Metazoa may have occurred more rapidly than 

 we can now calculate. One of the great errors of the present day 

 is the assumption, that such changes and transitions occurred 

 slowly and gradually ; and it is evident, that this assumption is 

 based almost wholly upon investigations of the more highly spe- 

 cialized animals, in which the capacity for change may be reasona- 

 bly considered as very much less than in their more generalized 

 and embryonic ancestral forms. 



The history of every embryo is a progress from a more general- 

 ized to a more specialized form. We may make partial exceptions 

 in favor of those forms in which the surroundings have been such 

 as to destroy progressive characteristics, and have replaced them 

 with retrogressive characteristics, but even these exceptions are 

 the very best examples of a high specialization, as in the case of 

 parasites, the Cirripedia, and hosts of other forms. All special- 



