CHAPTER VII 
MENDELISM 
WE have already had occasion to point out how im- 
portant it is, when engaged upon questions of heredity, 
not to treat whole animals or plants as units, but to 
deal with their separate characters one at atime. In 
the course of the present chapter the reason for pro- 
ceeding in this way will appear more clearly, and we 
shall find that the adoption of this method is fully 
justified by the results which it enables us to obtain, 
and which could not have been arrived at in any other 
way. We shall also find reasons for believing that 
this method is the correct one from a theoretical point 
of view. 
Naturally, considerable care is necessary in deter- 
mining what are and what are not separable characters. 
At the outset it is not always possible to make this 
discrimination with certainty, but during the course 
of the experiments which follow it is almost always 
possible to arrive at a clear definition of each character, 
and in many cases the distinction of characters is quite 
obvious from the beginning. 
Up to the present time the experimental study of 
heredity by the methods of definite breeding has yielded 
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