FERTILITY 631 



domesticated animals also under new conditions occasionally show 

 signs of lessened fertility, and that animals such as the canary, 

 which now breed readily in a state of captivity, were formerly often 

 sterile. 



Bles's observations, 1 to which reference has already been made 

 (p. 18), seem to have a bearing on this question. This observer, 

 who has kept various kinds of Amphibia in captivity, has shown 

 that axolotls can only be induced to breed under certain special 

 environmental conditions. By feeding them copiously in summer 

 and allowing them to hibernate in winter, and then suddenly 

 transferring them to an aquarium stocked with growing plants and 

 provided with running water, these animals could be induced to 

 spawn within a few days. (Of. also Annandale's observations referred 

 to on p. 20.) Bles draws the conclusion that the difficulty so often 

 experienced in inducing Amphibians to breed in a state of captivity 

 is not due to toxic influence on the gonads resulting from the 

 confinement, but must rather be ascribed to the absence of the 

 necessary external stimuli without which the generative organs of 

 animals are incapable of properly discharging their functions. Bles 

 suggests that this view may help to explain why some animals (e.g. 

 insects) make their appearance in great numbers in one year, and 

 are comparatively scarce in another. 



In animals whicli as a general rule breed freely in a state of 

 domestication or under confinement, it is probable that nutrition 

 plays the chief part (though by no means the sole part) in regulating 

 the capacity to produce offspring. That an insufficient or markedly 

 abnormal diet must affect this power is almost self-evident, and 

 Chalmers Watson 2 has shown that sterility is a common condition 

 among caged rats when fed exclusively upon meat. 3 It is also 

 certain that an excessive quantity of nutriment is likewise prejudicial 

 to the proper discharge of the reproductive functions. No better 

 example could be given of the way in which overfeeding results in 

 a condition of sterility than that of the barren Shire mares, which 

 in some past years have been a striking feature at agricultural' shows 

 in England. Some foods are said to induce sterility more easily 

 than others. Sugar, molasses, and linseed are noted for having this 

 effect when given to cattle, but they are often used to prepare beasts 

 for show or sale, since they produce a good coat of hair and cause a 



1 Bles, " The Life-History of Xenopus Icevis" Trans. Roy. Soc. Edin., vol. xli., 

 1906. 



2 Campbell and Watson, " The Minute Structure of the Uterus of the Rat, 

 etc.," Proc. Phys. Soc.., Jour, of Physiol., vol. xxxiv., 1906. 



3 Cf. Pezard on "alimentary castration" as a result of a meat diet in fowls 

 (see above, p. 334). Moreover, Allen has shown that a deficiency of water- 

 soluble vitamins in the diet given to rats causes atrophy of the seminiferous 

 tubules of the testis (Anat. Record, vol. xvi., 1919). 



