158 THE GENESIS OF SPECIES. [CHAP. 



brate sub-kingdom. Supposing 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 intermitting 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 sufficient 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, geology in this matter must rather 

 take its time from zoology than the reverse; for if Mr. 

 Darwin's theory be true, past time down to the deposi- 

 tion of the Upper Silurian strata can have been but 

 a very small fraction of the whole time 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 

 Vertebrata there were fishes not belonging to the lowest 

 but to the very highest groups which are known to have 

 ever been developed, namely, the Elasmobranchs (the 



