GENERATION AND DEVELOPMENT 689 



ancestrum until the next year. The number of annual sexual 

 cycles which any given species passes through is vastly influenced 

 by domestication. Probably in all primitive species one sexual 

 season yearly was the rule. Domestication alters this. The 

 abundant food supply renders the struggle for existence no 

 longer acute, the dread of being preyed upon by the enemies 

 peculiar to each species is removed, and one of the responses 

 to these altered conditions is a greater desire to multiply, for 

 the reason that the energies previously expended in the struggle 

 for life are turned into a fresh channel. The cat in a wild state 

 has one sexual period a year ; the domestic variety has three 

 or four. The wild dog and wolf breed once annually, in captivity 

 twice annually. The lioness in a wild state has probably but a 

 single breeding season ; in captivity the cestrous period may be 

 three or four times a year. Bears in a wild state have a single 

 breeding season, in captivity more than one. The wild otter 

 has a single season, but in a state of captivity she comes ' in 

 season ' every month (Marshall and Jolly). So far, in fact, as 

 evidence is available, a single sexual season for animals in a state 

 of freedom appears to be the natural condition, polycestrum being 

 an acquired character. The frequency of oestrus under domes- 

 tication is essentially influenced by food, temperature, and 

 environment. 



The complete cestrous cycle in the dog* under domestication 

 is six months. Every six months, in spring and autumn, the 

 majority of dogs come ' on heat,' though there are many excep- 

 tions to this rule, some of the smaller breeds of dogs having a 

 three and four heat period in the year. The period of procestrum 

 lasts from seven to ten days, and cestrus lasts another week. 



In the mare the complete cestrous cycle, with its dices trous 

 intervals, may last for months, in the majority of mares from 

 February to June or July ; and unless rendered pregnant, the 

 dicestrous periods last twenty-one days, and are followed by 

 procestrum, cestrus, etc., as previously described, though the 

 time-duration of these is irregular, generally brief, and always 

 uncertain. For instance, the exact period at which the mare is 

 ripe to receive the male may only be a matter of a few hours, 

 whereas she may be several days in a highly unsettled sexual 

 condition. The mare is in a condition of cestrus on the seventh 

 to tenth day after foaling ; with thoroughbred mares it may be 

 the sixth. At this period, though still nursing, she desires inter- 

 course, and in this respect differs from the nursing cow and sow. 

 If she does not conceive, the period of dicestrum is twenty-one 



* ' Contribution to the Physiology of Mammalian Reproduction.' 

 Part I. : ' The GEstrous Cycle in the Dog.' By F. H. A. Marshall and 

 W. A. Jolly, Phil. Trans., B., vol. cxcviii., 1905. 



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