96 HUMAN FECUNDITY 



interval between births. That this is the result of an improved 

 environment in the case of the sheep has been shown by Marshall. 

 He found, for instance, that in * Scotch Blackfoot, Cheviot, and 

 other Scottish sheep the normal percentage of ova discharged at 

 any single oestrous is not appreciably in excess of the usual 

 percentage of births at the lambing season '.* This, it may be 

 noted, is an example of what was said in the second chapter 

 regarding, in the first place, the measurement of fecundity by the 

 number of ripe ova produced, and, in the second place, regarding 

 the fertilization on the average of all, or nearly all, ripe ova. 

 Marshall also found that * there was every reason for supposing 

 that the processes of growth and maturation can be very largely 

 influenced both by insufficiency of food supply on the one hand 

 and by artificial stimulation on the other '. 2 What apparently 

 happens is that insufficiency of food retards the development of 

 the ova and may cause the degeneration of some of them. There- 

 fore if the food-supply is good, there will be more ripe ova at the 

 sexual season, fewer sterile females, and a greater number of twin 

 births. * There is overwhelming evidence/ says Heape, * that 

 flocks in good condition at tupping time have a higher subsequent 

 development of fertility than flocks in poor condition at tupping 

 time.' 3 By * good ' condition is meant not a ' f at ' but a strong, 

 healthy, and vigorous animal. 



It may be asked whether there is any evidence as to the increase 

 in numbers at a birth in the human female. Among civilized races 

 about one birth in eighty to ninety is on the average a twin birth. 

 Our knowledge of primitive races is not sufficiently precise to 

 enable any estimate to be made regarding the frequency of twin 

 births among them. There are, however, very numerous references 

 in the accounts of these races to the superstitions attaching to 

 twin births. The nature of these superstitions, and in general the 

 mystery felt to surround twin births, very strongly suggests 

 that the phenomenon is rare. It can only be said that it is 

 probable that twin births are rarer among primitive races than 

 among civilized races. 



So far, therefore, as we have gone, the evidence points to the 

 conclusion that two of the three factors which determine fecundity 

 have varied in the direction of increasing fecundity, and that 



1 Marshall, loc. cit., p. 596. * Ibid., p. 597. 3 Heape, 



Jow. Roy. Agric. Soc., vol. x, p. 236. 



