is to say, in long periods of time species of animals are affected by changes in external conditions according to the same principle of natural law by which individual animals are affected in short periods. It is to be understood, of course, that species are affected by other causes than a changed environment—causes such as are included in Darwin's phrase of natural selection—but the fact of the modification of species becomes evident when it is seen that familiar observations made upon individual animals have an application to species also.
It is clear that the modifications thus undergone are primarily functional rather than structural, since no part of the animal body can be altered in its anatomical characters except through physiological action. But it is also true that functional modifications occur not merely as subordinate to structural changes but as ends in themselves. That is to say, functional activity may be increased or diminished in response to changes in external conditions without any necessary sequence in changes in the structure of the organs exercising the function. An illustration of this is found in the well-known fact that warm-blooded animals (excepting those that hibernate) need more food in winter than in summer to keep up their normal temperature, occasioning a considerably increased activity of the nutritive functions, but without any attendant structural changes whatever in the organs of alimentation. The same holds true with hibernating animals, which, on the other hand, take no food for a long period, the nutritive function being greatly reduced in activity, yet the organs exercising this function undergoing no structural changes. In respect to species of animals, we should not expect to find the principle hold true so strictly as in individual animals, since increased or diminished functional activity extending through many successive generations could scarcely fail to have some effect on organic structure. But the point to which special attention is here directed is that function as well as structure responds to changes of environment, and that variations in functional activity occur without any closely correlated changes in structure.
The present object is to show that the reproductive function in animals is profoundly affected by conditions of environment. It will be sufficient to state the law or principle according to which the activity of the reproductive power appears to be regulated, and then to adduce instances exemplifying the law.
When circumstances are such that most of the ova produced are likely to develop, and the young to reach maturity, then the reproductive function is least active; on the other hand, when by reason of lack of food-supply or danger of destruction by adverse physical conditions, or by natural enemies, it is probable that only a small proportion of the ova will give rise to mature animals,