Ill EXPEDITION OF THE "CHALLENGER" 105 



time. Hence, if the period represented by the 

 rocks is greater than, or co-extensive with, that 

 during which life has existed, we ought, some- 

 where among the ancient formations, to arrive at 

 the point to which all these series converge, or 

 from which, in other words, they have diverged 

 the primitive undifferentiated protoplasmic living 

 things, whence the two great series of plants and 

 animals have taken their departure. 



But, as a matter of fact, the amount of conver- 

 gence of series, in relation to the time occupied by 

 the deposition of geological formations, is extra- 

 ordinarily small. Of all animals the higher 

 Vertebrata are the most complex ; and among 

 these the carnivores and hoofed animals ( Ungulata) 

 are highly differentiated. Nevertheless, although 

 the different lines of modification of the Carnivora 

 and those of the Ungulata, respectively, approach 

 one another, and, although each group is repre- 

 sented by less differentiated forms in the older 

 tertiary rocks than at the present day, the oldest 

 tertiary rocks do not bring us near the primitive 

 form of either. If, in the same way, the conver- 

 gence of the varied forms of reptiles is measured 

 against the time during which their remains are 

 preserved which is represented by the whole of 

 the tertiary and mesozoic formations the amount 

 of that convergence is far smaller than that of the 

 lines of mammals, between the present time and 

 the beginning of the tertiary epoch. And it is a 



