160 TEE GENESIS OF SPECIES. [CHAP. 



and arthropods 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. 



CUTTLE-FISH. 



A. Ventral aspect. B. Dorsal aspect. 



Now it will be a moderate computation to allow 

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

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

 tionary work done during this deposition only represents 

 a hundredth part of the sum total, w r e shall require 

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

 for the complete development of the whole animal king- 

 dom to its present state. Even one quarter of this, 

 however, would far exceed the time which physics and 



