FERTILITY 593 



factors which control the mammary metabolism. Moreover, in 

 animals which are characterised by a periodic growth of the 

 secondary sexual characters, these sometimes fail to make their 

 appearance. The male linnet in captivity does not assume 

 its characteristic crimson breast, or the male bunting (Emberiza 

 passerina) the black colour on its head. Other birds, such as a 

 pyrrhula and an oriole, may acquire the appearance of the hen, 

 while a falcon (Falco albidus) has been observed to lose its adult 

 plumage. 1 These facts seem to show that the generative meta- 

 bolism may be so altered by changed conditions of existence as to 

 induce not merely a state of sterility, but also an interference with 

 the secretory activity of the essential organs of reproduction. 2 



Darwin says : " We feel at first naturally inclined to attribute 

 [such results] to loss of health, or at least to loss of vigour ; but 

 this view can hardly be admitted when we reflect how healthy, 

 long-lived, and vigorous many animals are under captivity, such 

 as parrots, and hawks when used for hawking, chetahs when 

 used for hunting, and elephants. The reproductive organs 

 themselves are not diseased ; 3 and the diseases from which 

 animals in menageries usually perish are not those which in any 

 way affect their fertility. The failure of animals to breed under 

 confinement has been sometimes attributed exclusively to a 

 failure in their sexual instincts. This may occasionally come 

 into play, but there is no obvious reason why this instinct should 

 be especially liable to be affected with perfectly tamed animals, 

 except, indeed, indirectly through the reproductive system 

 itself being disturbed. Moreover, numerous cases have been 

 given of animals which couple freely under confinement, but 

 never conceive ; or, if they conceive and produce young, these 

 are fewer in number than is natural to the species. . . . Change of 

 climate cannot be the cause of the loss of fertility, for whilst 



1 Darwin, loc. cit. 



2 The relation between the gonads and the secondary sexual characters, 

 and the apparent dependence of the latter upon the secretory activity of 

 the former, are discussed in Chapter IX. 



3 Few observations have been made upon the condition of the gonads 

 in animals in captivity, but Branca (" Recherches sur le Testicule et les Voies 

 spermatiques dans Lemuriens en captivite," Jour, de V Anat. et la Phys., 

 vol. xl., 1904) states that in captive lemurs he could find no spermatozoa in 

 the testicles. 



2p 



