286 Heredity. 



easy to believe that less than two thousand million 3'ears 

 would be required for the totality of animal develop- 

 ment by no other means than minute, fortuitous, oc- 

 casional and intermitting variations in all considerable 

 structures. If this be even an approximation to the 

 truth, then there seem to be strong reasons for believ- 

 ing that geological time is not sufficient for such a pro- 

 cess 



"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 

 evolutionary work done during this deposition only 

 represents a liundredth part of the sum total, we shall 

 require 2,500,000,000 (two thousand five hundred mil- 

 lion) years for the complete develojmient of the whole 

 animal kingdom to its present state. Even one quarter 

 of this, however, would far exceed the time which phy- 

 sics and astronomy seem able to allow for the comple- 

 tion of the process. 



'^ . . Kow all these difficulties are avoided if we ad- 

 mit that new forms of animal life of all degrees of com- 

 plexity appear from time to time with comparative sud- 

 denness, being evolved according to laws in jiart depend- 

 ing on surrounding conditions, in jiart internal, similar 

 to the way in which crystals (and perhaps, from recent 

 researches, the lowest forms of life) build themselves up 

 according to the internal laws of their component sub- 

 stance, and in harmony and correspondence with all en- 

 vironing influences and conditions." 



Darwin himself seems to believe that in order to ex- 

 plain the harmonious co-ordination of all the inter-re- 

 lated parts of a complicated animal, we must believe that 

 natural selection is greatly aided by other influences, 

 such as the inherited effect of use and disease, the di- 



