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THE AMERICAN NATURALIST 



[Vol. XLVI 



muted into the structures of the adult are evolutionary- 

 problems of the greatest importance. It is an amazing 

 fact that the great problems of ontogeny — viz. the under- 

 lying causes and mechanism of differentiation — are 

 to-day, after more than a century of scientific observa- 

 tion and experiment, almost as complete a mystery as 

 ever. If we are as yet unable to determine the precise 

 manner in which the structure of the germ evolves into 

 the structure of the adult in the common, ever-present 

 phenomena of reproduction, it is small wonder that we 

 have been unable to determine in detail the way in which 

 one race is transmuted into another. 



In conclusion I think it must be admitted that the ex- 

 perimental study of genetics has been a little disappoint- 

 ing. We had supposed that organisms would be more 

 tractable, more willing to evolve, than we find them. The 

 older view that organisms were plastic and could be 

 moulded "while you wait" now reminds one of the view 

 of certain childless theorists, that children are plastic 

 clay in the hands of parents or teachers; both of these 

 views neglect the fact that the living organism, delicate 

 and responsive beyond compare, is still wonderfully 

 strong, stable and stubborn. So far as the factors of evo- 

 lution are concerned experimental study has thus far 

 been a weeding-out process, and at times it seems that 

 nothing will be left. 



The problems of evolution are as much problems to- 

 day as they ever were, and though some of these prob- 

 lems may soon be solved, we may rest assured that there 

 will always be the evolution problem. The path which 

 we thought led straight to the goal has had to be retraced 

 with much labor; the hilltop from which we confidently 

 expected to see the spires of Eldorado has only served to 

 show us how great are the difficulties before us. But this 

 is the order of nature, the common experience of all 

 search for truth, and we would not have it otherwise. 

 ' 'For to travel hopefully is a better thing than to arrive, 

 and the true success is to labor." 



