!54 THE GENESIS OF SPECIES. [Cp. 



primary placental and implacental mammals, and perhaps 

 also for the divergence of the most ancient stock of these 

 and of the monotremes, for in all these cases modifications 

 of structure appear to increase in complexity in at least 

 that ratio. Finally, a vast period must be granted for the 

 development of the lowest mammalian type from the prim- 

 itive stock of the whole vertebrate sub-kingdom. Sup- 

 posing this primitive stock to have arisen directly from a 

 very lowly-organized animal indeed (such as a nematoid 

 worm, or an ascidian, or a jelly-fish), yet it is not easy to 

 believe that less than two thousand million years would 

 be required for the totality of animal development by no 

 other means than minute, fortuitous, occasional, and inter- 

 mitting variations in all conceivable directions. If this be 

 even an approximation to the truth, then there seem to be 

 strong reasons for believing that geological time is not suffi- 

 cient for such a process. 



The second question is, whether there has been time 

 enough for the deposition of the strata which must have 

 been deposited, if all organic forms have been evolved 

 according to the Darwinian theory ? 



Now this may at first seem a question for geologists 

 only, but, in fact, in this matter geology must in some re- 

 spects rather take its time from zoology than the reverse ; 

 for if Mr. Darwin's theory be true, past time, down to the 

 deposition of the Upper Silurian strata, can have been but a 

 very small fraction of that during which strata have been 

 deposited. For when those Upper Silurian strata were 

 formed, organic evolution had already run a great part of 

 its course, perhaps the longest, slowest, and most difficult 

 part of that course. 



At that ancient epoch, not only were the vertebrate, 

 molluscous, and arthropod types distinctly and clearly 

 differentiated, but highly-developed forms had been pro- 

 duced in each of these sub-kingdoms. Thus in the Verte- 



