2IO GEOLOGICAL BIOLOGY. 



known in fossil condition, the answer to the question is as 

 follows : Of the six branches of the Animal Kingdom all six 

 were differentiated in the Cambrian era; 13 classes of the 

 26 were differentiated in the Cambrian; of the 73 orders, 14 

 are known from the Cambrian, 14 more are first seen in Or- 

 dovician time, 4 more in^ the Silurian ; or before the close of 

 the Silurian out of a known 72 fossil orders 32 had alread)^ 

 appeared. 



Represented in the form of percentages between the num- 

 bers represented in the early ages and the number appearing 

 throughout all the geological ages, we find that, of the dif- 

 ferentiations of the primary and fundamental nature which 

 distinguish the branches of the Animal Kingdom one from 

 another, 80^ of all that has ever taken place was already ac- 

 complished before the close of the Cambrian. It may have 

 been still more complete, but this amount we know to have 

 been the fact. Of differences of only second rank in impor- 

 tance, i.e., those which mark the separate classes of the ani- 

 mal kingdom, 13 out of a known 23 fossil classes are already 

 known to have appeared in Cambrian time, or 56^ of the dif- 

 ferentiations of class rank had been already attained. In the 

 evolution of orders at least 32 of the 72 fossil orders appeared 

 before the close of the Silurian, and 14 orders are represented 

 in the Cambrian era, or 20^ in the Cambrian era and about 

 40^ of ordinal differentiation had been accomplished before 

 the close of the Silurian, 



It is probably well within the facts to say that six out of 

 the nine known branches were already differentiated in the 

 Cambrian, and that in all probability all the classes of these 

 six branches were already differentiated before the close of the 

 Silurian or third geological era, and probably four fifths of 

 them in the Cambrian era. In respect of ordinal differentia- 

 tions, it is probably true that, of the total ordinal differentia- 

 tion known in these six branches, one fourth, and probably 

 more, took place before the close of the Cambrian, and one 

 half before the close of the Silurian. If we recur to the time- 

 scale, described on page 54, bearing in mind that the rocks 

 of the Cambrian system may not and probably do not con- 

 tain records of the earliest organisms that appeared upon the 



