THE (ESTROUS CYCLE IN THE MAMMALIA 45 



enced by any sheep is that reached by certain Australian 

 Merinos which are described as being able to breed all the 

 year round, a fact which imphes, in the absence of gestation, 

 an unbroken series of dicestrous cycles. The report of the 

 Chief Inspector of Stock for New South Wales divides the 

 time of lambing into six periods which embrace the entire 

 year.^ 



That the great variability in sexual activity which the sheep 

 exhibits is dependent largely upon differences in food supply 

 and climate cannot be doubted, for the black-faced sheep in 

 Scotland, and the Merinos in Cape Colony afford direct evidence 

 that this is the case. Indeed, the effect of the environment on 

 the recurrence of breeding was noted long ago by Aristotle,^ who 

 observes that " in some places where the weather is warm and 

 fine, and food is abundant," sheep may have lambs twice a 

 year. The result of flushing (or the practice of stimulating 

 the generative system by supplying extra food or better pasture, 

 and thereby hastening the approach of the sexual season and 

 increasing the fertility) is further evidence of the effect of good 

 nourishment upon the sexual and reproductive powers. On 

 the other hand, there can be no question that the varying degrees 

 of breeding activity are in part racial characteristics, as is shown, 

 for example, by the Dorset Horn sheep in the south of England, 

 and still more evidently by the Limestone sheep of Westmor- 

 land and Derbyshire. But that an increase in the duration (or 

 more frequent recurrence) of the sexual season is not necessarily 

 a highly artificial condition or the result of special attention in 

 regard to food supply, &c., on the part of the flock-master, is 

 shown by such a condition occurring among the indigenous 

 sheep of India and the half -wild sheep of Patani. 



The duration of the dicestrous cycle in black-faced sheep, 

 as already mentioned, is from about thirteen to twenty-one 

 days, the variation appearing to depend partly upon the nature 

 of the country in which they live. In other breeds the cycle may 

 be said to vary within approximately the same Umits. EUen- 



' Wallace (R.), Farming Industries of Cape Colony, London, 1876 ; The Rural 

 Economy and Agriculture of Australia and New Zealand, London, 1891 ; 

 Argmitine Shotos and Live Stock, Edinburgh, 1904. 



^ Aristotle, History of Animals (Crosswell's Translation), Bohn's Library, 

 London, 1862. 



