What is Heredity f 13 



least a partial solution of the problem; that we have every 

 reason to hope that in time its hidden causes will all be 

 made clear, and that its only mystery is that which it 

 shares with all the phenomena of the universe. 



In this introductory statement we have presented one 

 side of the problem of heredity: the transmission from pa- 

 rent to child of the established congenital hereditary 

 characteristics of the race. We must not forget, though, 

 that there is another aspect which is fully equal to this 

 in importance. We know that each characteristic has 

 been gradually acquired through a long series of modifi- 

 cations; that all the wonderful adaptations which, fit 

 animals to their surroundings, and meet their particular 

 needs, has been evolved step by step by the natural se- 

 lection of the fittest congenital variations. Each race- 

 characteristic has at one time been a new variation, and 

 the process of modification is still going on and perfect- 

 ing the harmony between the structure of each organism 

 and its needs. No theory of heredity has any value un- 

 less it explains the way in which new features, which may 

 become hereditary, continually make their appearance 

 as congenital variations, at the same time that it accounts 

 for the way in which established peculiarities are handed 

 down from generation to generation. 



The problem is two-sided; what is now hereditary was 

 at one time variation, and each new variation may soon 

 be hereditary. Heredity and variation are opposite as- 

 pects of the same thing, and an explanation must be 

 examined and tested on the one side, as well as on the 

 other, before it can be accepted. 



There is still another consideration which remains to 

 be noticed. 



Darwin has never failed to perceive, and he has fre- 

 quently pointed out, that the law of natural selection is not 



