THE STRUGGLE FOR EXISTENCE. 71 



elimination and inheritance ? It has been far too easily 

 assumed by Wallace, and other writers, that the result 

 of the continued selection of any character must neces- 

 sarily lead to its gradual increase. For instance it was 

 supposed that if, out of a number of birds varying in 

 wing-length from 5 to 7 inches, with a mean of 6 inches, 

 individuals with a wing-length of 7 inches were chosen 

 for breeding, the mean length of the wing of the offspring 

 would be raised. And that if for a number of generations 

 the parents were always selected from among the birds 

 with greatest wing-length, the average wing-length of 

 the progeny would be gradually raised from 6 to 7 inches, 

 from 7 to 8 inches, and so on indefinitely, so long as the 

 selection continued. But /this is not necessarily the 



case. 



In the first place, if the variations selected are modi- 

 fications induced by the environment in individuals 

 endowed with the same hereditary factors, no cumulation 

 at all will take place, however much the selection may be 

 prolonged. Thus the selection of individuals which have 

 become immune to a disease in the course of their life- 

 time will not increase that immunity, nor free the pro- 

 geny from the necessity of becoming immune, unless the 

 capacity to acquire immunity itself varies and is selected. 

 To take a simpler case that of the beans mentioned on 

 p. 45. Selection of the heaviest bean-seeds from any one 

 strain or pure line of uniform hereditary capacity will 

 not alter the mean weight of seeds within that strain ; 

 the offspring of such a selected bean is no more likely to 

 be heavier than is that of any other bean from the same 

 strain. But if heavier beans are selected from the whole 

 group of strains, differing in their hereditary capacities, 



* Whether the direction of variation is influenced by selection, whether 

 the continued selection of a character, and indirectly of its germinal 

 factors, encourages their further development, is indeed a most funda- 

 mental question. At present, however, it cannot fully be answered, 

 the evidence is quite uncertain. On the whole, current opinion is agai 

 the view. A, r>- x.0 



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