CHAPTER VIII 



FERTILITY 



We have seen that among the individuals of the higher animals 

 whereas the number of ripe ova released is small — in many species 

 usually only one being discharged at any single oestrus— -the 

 number of spermatozoa produced by the male and ejaculated at 

 insemination is extremely large. It follows that in a certain sense 

 the female is a more important factor in fecundity than the male — 

 that is to say, that the number of young in a litter is mainly 

 controlled, if not determined, by the mother rather than by the 

 father. There is another sense, however, in which the male may 

 be said to be the more important factor in fecundity. With 

 the domestic animals it is customary for one male to mate with 

 a large number of females ; thus in ordinary practice, a stallion 

 \^dll serve some eighty mares in a season, a bull will mate with 

 80 to 120 cows, and a ram will cover fifty ewes. If for any 

 reason the stud animal is lacking, or partially lacking, in repro- 

 ductive capacity, so that some or all of the females served are 

 sterile, the practical results are far more serious than if any one 

 individual female is wanting in fertility. Moreover, it some- 

 times happens with human beings that the woman is held account- 

 able for failure to have children when in reality it is the male 

 partner who is at fault. Such sterility may be caused by disease, 

 or it may be due to some abnormal condition, such as excessive 

 poverty of nutrition or adiposity, the latter being a commoner 

 cause. 



Effects of Environment and Nutrition 



That the generative system is peculiarly susceptible to 

 changed conditions of existence has been recognised from early 

 days. Thus Aristotle commented on the increased fertility of 

 sheep in a favourable environment. It is well known that the 

 domestic animals not only breed oftener, but also produce larger 



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