658 THE PHYSIOLOGY OF REPRODUCTION 



among these problems is that which concerns itself with the factors 

 that control fertility. 



Despite its comparative prosperity, it is evident that the 1 >reeding 

 industry suffers annually from no inappreciable loss. Allusion has 

 been made to the losses sustained by breeders owing to the occurrence 

 of abortion in domestic animals. Sterility, whether persistent or 

 temporary, is responsible for a greater reduction of profit. The 

 prevalent barrenness among the better class of Shire mares has 

 been already referred to, while incapacity to breed is perhaps still 

 commoner among Thoroughbreds. As already mentioned, the Royal 

 Commission on Horse- Breeding found that no less than forty per 

 cent, of the mares chosen for breeding in any given year failed to 

 produce offspring. Recent statistical evidence for heavy horses shows 

 an even larger proportion of mares failing to breed. 1 Moreover, 

 there-is evidence that in certain districts of -India the percentage of 

 sterility is as high or higher than in this country.' 2 Among cattle 

 the average loss from sterility and abortion (together with mortality 

 of calves) is estimated by Heape 3 to be at least fifteen per cent., 

 while it is shown in the report (already referred to) issued by the 

 Royal Agricultural Society on fertility in English sheep for the year 

 1899, that the proportion of sterile ewes was 4*71 per cent, out of 

 a total number of 96,520, and this percentage does not include the 

 ewes which aborted (see p. 651). In view of these facts, it is obvious, 

 as Heape has pointed out, that any means by which sterility in 

 domestic animals can be checked and their capacity to bear young 

 increased, must be possessed of great commercial value. 



THE BIRTH-RATE IN MAN 



It is now more than a century ago since Malthus 4 advanced 

 his famous proposition that whereas population tends to increase 

 in geometrical ratio, the means of subsistence increase only in 

 arithmetical proportion. As a consequence of the acceptance of 

 that doctrine, the political economists of the early Victorian period 

 tended to see in over-population the most fruitful source of 

 pauperism, disease, and crime, and the cause of increasing congestion 

 in the future. That Malthus' predictions have not been verified is 

 a matter of common knowledge, and the problem before the modern 

 economist is not how to place a check on population generally, but 

 rather how to secure that future generations shall be sufficiently 

 recruited from that section of the population which is industrially 

 capable, while at the same time to prevent indiscriminate propagation. 



1 Marshall and Crosland, "Sterility in Mares, with Recommendations to 

 Breeders of Heavy Horses," Jour, of the Board of Agric., vol. xxiv., 1918. 



2 Ewart, loc. cit. 



3 Heape, loc. cit. 



4 Malthus, An Essay on the Principles of Population, 7th Edition, London, 1872. 



