*3 167 



In cases of exact parallelism we reasonably suppose 

 the cause to be the same, since the conditions are iden- 

 tical, as has been shown ; that is, the higher conditions 

 have been produced by a crowding back of the earlier 

 characters and an acceleration of growth, so that a given 

 succession in order of advance has extended over a 

 longer range of growth than its predecessor in the same 

 allotted time. That allotted time is the period before 

 maturity and reproduction, and it is evident that as fast 

 as modifications or characters should be assumed suffi- 

 ciently in advance of that period, so certainly would 

 they be conferred upon the offspring by reproduction. 

 The acceleration in the assumption of a character, pro- 

 gressing more rapidly than the same in another charac- 

 ter, must soon produce, in a type whose stages were 

 once the exact parallel of a permanent lower form, the 

 condition of inexact parallelism. As all the more com- 

 prehensive groups present this relation to each other, 

 we are compelled to believe that acceleration has been 

 the principle of their successive evolution during the 

 long ages of geologic time. 



Each type has, however, its day of supremacy and 

 perfection of organism, and a retrogression in these re- 

 spects has succeeded. This has no doubt followed a law 

 the reverse of acceleration, which has been called re* 

 tardation. By the increasing slowness of the growth of 

 the individuals of a genus, and later and later assump- 

 tion of the characters of the latter, they would be suc- 

 cessively lost. 



To what power shall we ascribe this acceleration, by 

 which the first beginnings of structure have accumu- 

 lated to themselves through the long geologic ages 

 complication and power, till from the germ that was 



