90 THE LAW OF BIRTHS AND DEATHS 



in the domestic state/ or at least in the half -domestic 

 state in which the females are generally kept, is of every 

 day occurrence." l This is attributed to the female 

 being allowed to " roam the forest with some degree of 

 freedom." Thus there is nothing very mysterious in 

 their failure to breed under the ordinary circumstances 

 of captivity. 



The same general phenomena are displayed by captive 

 birds. " Of the small graminivorous birds, many kinds 

 have been kept tame in their native countries, and have 

 lived long ; yet, as the highest authority on cage birds 

 remarks, their propagation is * uncommonly difficult.' " 2 

 The change in the habits of these birds is from an irregular 

 and uncertain diet upon seeds of poor quality to regular 

 feeding upon seeds of the highest feeding value : from a 

 life of incessant activity to a life of comparative inactivity. 

 They show the general law in that they are more fertile 

 when kept in large aviaries, where they can obtain a 

 fair amount of exercise, than when they are kept in small 

 cages. Canaries, which breed freely in captivity, have 

 had their potential degree of fertility increased and 

 adapted to the conditions of captivity by selection. 

 Nevertheless, they produce large numbers of infertile 

 eggs, particularly highly bred birds. 



The gratuitousness of the " race-suicide " theory is 

 admirably shown by several passages from Darwin. 

 " Monkeys, in the nine-year report from the Zoological 

 Gardens, are stated to unite most freely, but during this 

 period, though many individuals were kept, there were 

 only seven births." 3 We see that conception does not 

 necessarily follow union. The females, when coupling 

 took place, must have been impregnated with large 



1 Variation of Animals and Plants under Domestication, chap, xviii. 

 9 Ibid, 



