In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions.

The direct action of changed conditions leads to definite or indefinite results.

In the latter case the organisation seems to become plastic, and we have much fluctuating variability.

In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all, or nearly all, the individuals become modified in the same way.
It is very difficult to decide how far changed conditions, such as of climate, food, etc., have acted in a definite manner.

There is reason to believe that in the course of time the effects have been greater than can be proved by clear evidence.