92 Evolution and Adaptation 



idea is counter to all the most familiar facts of inheritance. 

 For instance, when two similar forms unite, we do not find 

 that the young show all the characters of the mother plus all 

 those of the father, i.e. each peculiarity that is the same in 

 both, increased twofold. On the contrary, the young are in 

 the vast majority of cases not essentially different from either 

 parent. 



A more thorough examination of the facts shows that the 

 problem is by no means so simple as the preceding general 

 statement might lead one to suppose, for our experience 

 shows that it is not always possible to increase all variations 

 by selection, and, furthermore, there is very soon found a 

 limit, even in favorable cases, to the extent to which the pro- 

 cess can be carried. The most important point appears to be 

 the nature of the variations themselves which may arise from 

 different causes, and which have different values in relation 

 to the possibility of their continuation. 



We may begin, therefore, by following Darwin in his analy- 

 sis of variation, as given in the opening chapter of the " Ori- 

 gin of Species." He thinks that the great amount of 

 variation shown by domesticated animals and plants is due, 

 in the first place, to the new conditions of life to which they 

 are exposed, and also to the lack of uniformity of these con- 

 ditions. Darwin thinks, also, that there is some probability 

 that this variability is due, in part, to an excess of food. " It 

 seems clear that organic beings must be exposed during sev- 

 eral generations to new conditions to cause any great amount 

 of variation, and that when the organization has once begun 

 to vary, it generally continues varying for many generations. 

 No case is on record of a variable organism ceasing to vary 

 under cultivation. Our oldest cultivated plants, such as 

 wheat, still yield new varieties ; our oldest domesticated ani- 

 mals are still capable of rapid improvement or modification." 

 In this statement of Darwin, full of significance, we must 

 be careful to notice that he does not mean to imply, when he 



