CLASS/F/CAT/OXS IX XATUKAL HISTORY. 211 



earth, but only the earHest records we have of distinct orsi^an- 

 isms, it will be seen that the statistics given above mean that 

 at least three quarters of the total evolution of the i^^randcr 

 distiui^uishing characters of organisms are known to have been 

 completed before the close of the first quarter of their re- 

 corded history. The percentage would be much smaller if 

 the generic and, particularly, if the specific characters of all 

 known organisms were to be considered ; but to form a cor- 

 rect idea of what the statement means it is necessary to con- 

 sider that these latter characters are, both from the point of 

 view of importance of the characters in the economy of indi- 

 vidual life and from the point of view of the degree of spe- 

 cialization to particular conditions of environment, far less 

 important than those whose differentiation was so rapidly 

 culminated. 



Nature and Extent of the Elaborations. — In order to form a 

 definite notion of the extent of the differentiation thus early 

 attained in the evolutionary history of organisms, we may 

 next consider what structures and functions had been elabo- 

 rated in each of the several branches of the Animal Kingdom 

 in the Cambrian era. 



In the Cambrian system are found traces of six, at least, 

 of the nine branches of the Animal Kingdom, and when we 

 are looking at organic form, of either the morphology or 

 physiology of organisms, this means that the characters by 

 which these various branches are distinguished were differen- 

 tiated before the close of the Cambrian era ; and in most 

 cases there is evidence to show that it was before the close 

 of the lower division of the Cambrian. As has been noted, 

 this statement applies also to a remarkably large proportion 

 of those characters by which the different classes and even 

 orders of these branches are distinguished. 



Recurrence of Characters accounted for by Descent. — There 

 follows as correlative t(^ the fact that these characters have 

 appeared in the Cambrian, that their reappearance in succes- 

 sive organisms up to the present time is to be explained by 

 the ordinary laws of heredity. Regarding them no evolution 

 is observed. Whatever evolution is necessary to explain 

 their appearance in the world took place i)rior to the Cam.- 



