CHAPTER XIII 



VARIATION AND EVOLUTION 



Through the facts of heredity we have reached a 

 new conception of the individual. Hitherto we have 

 been accustomed to distinguish between the members 

 of a family of rabbits like that illustrated on Plate 

 I. by assigning to each an individuality, and by 

 making use of certain external features, such as the 

 coat colour or the markings, as convenient outward 

 signs to express our idea that the individuality of these 

 different animals is different. Apart from this, our 

 notions as to what constituted the individuality in 

 each case were at best but vague. Mendelian analysis 

 has placed in our hands a more precise method of 

 estimating and expressing the variations that are 

 to be found between one individual and another. 

 Instead of looking at the individual as a whole, 

 which is in some vague way endowed with an indi- 

 viduality marking it off from its fellows, we now 

 regard it as an organism built up of definite char- 

 acters superimposed on a basis beyond which for the 

 moment our analysis will not take us. We have 

 begun to realise that each individual has a definite 

 architecture, and that this architecture depends 



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