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, z.c. 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 
