THE REPRODUCTIVE POWER IN ANIMALS. 49 



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 phys- 

 iological 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 con- 

 ditions without any necessary sequence in changes in the struct- 

 ure of the organs exercising the function. An illustration of this 

 is found in the well-known fact that warm-blooded animals (ex- 

 cepting those that hibernate) need more food in winter than in 

 summer to keep up their normal temperature, occasioning a con- 

 siderably increased activity of the nutritive functions, but without 

 any attendant structural changes whatever in the organs of ali- 

 mentation. 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 exer- 

 cising 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 succes- 

 sive generations could scarcely fail to have some effect on organic 

 structure. But the point to which special attention is here direct- 

 ed 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, 



VOL. XXXTI. 4 



