156 THE GENESIS OF SPECIES. [CHAP. 



incipient worm-like group, but are distinctly differentiated 

 Crustacea of no low form. 



We have in all these animal types nervous systems 

 differentiated on distinctly different patterns, fully-formed 

 organs of circulation, digestion, excretion, and generation, 

 complexly-constructed eyes and other sense organs ; in fact, 

 all the most elaborate and complete animal structures built 

 up, and not only once, for in the fishes and mollusca we 

 have (as described in the third chapter of this work) the 

 coincidence of the independently-developed organs of 

 sense attaining a nearly similar complexity in two quite 

 distinct forms. If, then, so small an advance has been 

 made in fishes, mollusks, and anthropods, since the Upper 

 Silurian deposits, it will probably be within the mark to 

 consider that the period before those deposits (during which 

 all these organs would, on the Darwinian theory, have 

 slowly built up their different perfections and complexities) 

 occupied time at least a hundredfold greater. 



Now it will be a moderate computation to allow 

 25,000,000 years for the deposition of the strata down to 

 and including the Upper Silurian. If, then, the evolution- 

 ary work done during this deposition only represents a 

 hundredth part of the sum total, we shall require 

 2,500,000,000 (two thousand five hundred million) years 

 for the complete development of the whole animal kingdom 

 to its present state. Even one-quarter of this, however, 

 would far exceed the time which physics and astronomy 

 seem able to allow for the completion of the process. 



Finally, a difficulty exists as to the reason of the ab- 

 sence of rich fossiliferous deposits in the oldest strata 

 if life was then as abundant and varied as, on the Darwinian 

 theory, it must have been. Mr. Darwin himself admits 1T 

 " the case at present must remain inexplicable ; and may 

 be truly urged as a valid argument against the views" 

 entertained in his book. 



17 " Origin of Species," 5th edit., p. 381. 



