Page:Popular Science Monthly Volume 85.djvu/37

Rh arms. Many responses of organisms are modified in a similar way, not only by artificial limitations, but also by natural ones.

(c) Responses which have become fixed and constant through natural selection or other means of limitation may become more varied and general when the compulsory limitation is relaxed. Behavior in the former case is fixed and instinctive, in the latter more varied and plastic. Thus Whitman found that the behavior of domesticated pigeons is more variable and their instincts less rigidly fixed than in wild species. If the eggs are removed to a little distance from the nest the wild passenger pigeon returns to the nest and sits down as if nothing had happened. She soon finds out, not by sight but by feeling, that something is missing, and she leaves the nest after a few minutes without heeding the eggs. The ring-neck pigeon also misses the eggs and sometimes rolls one of them back into the nest, but never attempts to recover more than one. The dove-cote pigeon generally tries to recover both eggs.

(d) Finally in all animals behavior is modified though previous experience, just as structure is also. Where several responses to a stimulus are possible and where experience has taught that one response is more satisfactory than another, action may be limited to this particular response, not by external compulsion, but by the internal impulse of experience and intelligence. This is what we know as conscious choice or will. Whitman says:

Freedom of action does not mean action without stimuli, but rather the introduction of the results of experience and intelligence as