282 THE BIOLOGY OF BIRDS 



succumb to a sort of struggle for existence that goes on in 

 the ovary (Hansemann, 191 2), and it may be that many 

 mature ova have been already discharged, but Pearl and 

 Schoppe find that the number of readily countable ova is a 

 reliable index of the fecundity of the bird. 



Fecundity is a variable constitutional quality, and it is 

 also modifiable by changes in nurture. That is to say, it may 

 change for innate or intrinsic reasons, and for environmental 

 or extrinsic reasons. The egg-laying capacity of hens is 

 partly a question of breed and partly a question of food and 

 environment. The reproductive capacity is often delicately 

 poised. It is well known that while gallinaceous birds breed 

 readily in captivity, hawks will seldom do so. Canaries have 

 become fertile in captivity, while finches seldom lay. Darwin 

 remarked on the fact that whereas the female Jungle Fowl 

 {Gallus bankiva) lays only six to ten eggs, and the wild duck 

 about the same, the domestic descendants of these birds lay 

 huge numbers in the course of the year. The domestic 

 duck may lay a hundred eggs and the hen two hundred. On 

 the other hand, the guinea-fowl has retained her natural 

 number (14-16), and lays in a hidden nest. 



Fowls which lay a very large number of eggs, say 200 in 

 the year, have been obtained by selective breeding from 

 stocks which showed a persistent capacity for being prolific. 

 This illustrates innate fertility. On the other hand, a great 

 increase in the number of eggs laid has been secured for a 

 time by peculiar dieting. Thus Houssay (1903) has shown 

 that a carnivorous diet results in a great increase in the 

 annual yield of eggs, and the second carnivorous generation 

 may be more prolific than the first. This illustrates a 

 modificational increase in fertility, which does not last. 



Herbert Spencer advanced the generalisation that 

 Individuation and Genesis vary inversely. When it costs a 

 great deal of energy to keep the individual life going, there 

 is less to spare for producing new individuals. This con- 

 clusion was advanced as an induction from a survey of the 

 animal kingdom and mankind ; and it seems to be in a broad 

 way true that animals with a highly pitched life, with complex 



