SUMMARY 225 



one pair of allelomorphs has taken part in the cross, the 

 members of each pair are found, as a rule, to undergo 

 this process of segregation quite independently of all 

 the other pairs. 



The result of this phenomenon of segregation is 

 that we find our conception of what constitutes purity 

 in a strain of animals or plants to be completely 

 altered. We now know that purity does not depend 

 upon the length of time during which the race has 

 exhibited a constant character. A strain of absolute 

 purity may arise from the second generation of a cross. 

 Such a pure strain may show an entirely new com- 

 bination of the parental characters. But this is so 

 far the only kind of novelty which we can produce at 

 will. We know almost nothing as to the method by 

 which entirely new characters arise. We can only 

 take advantage of such characters when they happen 

 to make their appearance. 



I would draw special attention to the definiteness of 

 the characters with which we deal. We do not evoke 

 improved features by gradual selection ; the characters 

 are either there or they are not. Let it be further 

 remembered that every process of this kind which has 

 been worked out in the case of a plant can be paralleled 

 by similar phenomena taking place in some one or 

 other of the higher animals. 



On the mind of a biologist familiar with what 

 was known of heredity only about twenty years 

 since, these facts must fall with a sense of complete 

 novelty. The ideas current even so short a time ago 

 are not so much extended, or even altered, as replaced 



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