280 Heredity. 



can any one believe that the Avhole island will gradually 

 acquire a white or even a yellow population? 



"Darwin says that in the struggle for life a grain 

 may tarn the balance in favor of a given structure, 

 which will then be preserved. But one of the weights 

 in the scale of nature is due to the number of a given 

 tribe. Let there be 7000 A's and 7000 L's, representing 

 two varieties of a given animal, and let all the B's, in 

 virtue of a slioht difference of structure, have the bet- 

 ter chance of life by a yqV^ P^i't. We must allow that 

 there is a slight probability that the descendants of B will 

 supplant the descendants of A; but let there be only 

 7001 A's asfainst 7000 B's at first, and the chances are 

 once more equal, while if there be 7002 A's to start, the 

 odds would be laid on the A's. True, they stand a 

 greater chance of being killed, but then they can better 

 afford to be killed. The grain will only turn the scales 

 when these are very nicely balanced, and an advantage 

 in numbers counts for weight, even as an advantage in 

 structure. As the numbers of the favored varictv di- 

 minish, so must its relative advantages increase, if the 

 chance of its existence is to surpass the chance of its ex- 

 tinction, until hurdlv any conceivable advanta^^e would 

 enable the descendants of a single pair to extermi- 

 nate the descendants of many thousands, if they and 

 their descendants are supposed to breed freely with the 

 inferior variety, and so gradually lose their ascend- 

 ancy." 



Darwin acknowledges that the justice of these re- 

 marks cannot be disputed, and there is no escape from 

 the conclusion that if variations do not appear simulta- 

 neously in a great number of individuals, the theory of 

 natural selection fails to explain the origin of species. 

 But the theory itself is so firmly estabished by other 



