370 SCIENCE PROGRESS 



than most other carnivores, although it is difficult to assign 

 a reason for this fact. Furthermore, it was pointed out by- 

 Darwin that close confinement cannot be regarded as a sufficient 

 explanation of infertility in such cases, since the semi-domesti- 

 cated ferret, for example, breeds readily when kept in closely 

 cramped hutches. 



There can be no doubt that those environmented conditions 

 which are conducive to the occurrence of " heat" or oestrus are 

 generally also favourable to fertility, for although it is true 

 that many animals which have never succeeded in producing 

 offspring in captivity have been observed to experience oestrus 

 and indulge in sexual intercourse, there can be no question 

 that a prolonged or frequently recurring sexual season is 

 usually associated with an increased capacity to bear young. 

 The origin of the breeding season has been discussed in two 

 papers by Mr. Walter Heape, 1 who lays considerable stress on 

 the part played by nutritive influences, as shown especially by 

 the effects of certain stimulating foods upon the generative 

 functions of sheep. But it is none the less evident that 

 nutrition is not the sole factor in determining the time for 

 breeding, for, as Mr. Heape points out, its occurrence is to a 

 great extent seasonal, and is governed both by external forces 

 {e.g. climatic influences) consequent upon periodic change, and 

 by internal forces dependent on the inherited capacity of the 

 individual. 



Some interesting observations made by Mr. Bles on various 

 kinds of Amphibia seem to have a bearing upon this question. 2 

 This observer shows that axolotls, as well as certain species of 

 frogs and newts, can only be induced to breed in captivity in 

 the presence of particular environmented conditions., Mr. Bles 

 found that by feeding them copiously in summer and allowing 

 them to hibernate in winter, and afterwards suddenly trans- 

 ferring them to an aquarium stocked with growing plants and 

 provided with running water or with suitable pond water, 

 these animals could be induced to spawn within a few days. 

 He draws the conclusion that the difficulty so often met with in 

 inducing Amphibia to breed is not due to any toxic influence 

 on the gonads resulting from close confinement, but should 



1 Heape, Quart. Joum. Micr. Science, vol. xliv. 1900 ; Proc. Royal Soc. B, 

 vol. lxxvi. 1905. 



2 Bles, Trans. Royal Soc. Edinburgh, vol. xli. 1906. 



