142 INTRODUCTION TO SEXUAL PHYSIOLOGY 



Darwin set out to unravel, but which in spite of long-continued 

 experimental investigation has so far resisted solution. 



Multiple Births 



The number of offspring born at a time, that is, tjie number 

 of young in a litter, is approximately constant for each sort of 

 animal, but there are often differences in this respect between 

 varieties and even between families. In man, single births are 

 the rule, twins occur in rather more than 1 per cent, of births. 

 and triplets once in six or seven thousand (the estimates varying 

 rather widely). Quadruplets and quintuplets are very rare, 

 especially the latter. Among animals, the mare and the cow 

 usually produce only one young at a time, but occasionally two, 

 and the sheep either one or two (sometimes three, but very rarely 

 more). The sow and bitch, on the other hand, may have much 

 larger litters, the sow sometimes producing as many as seventeen 

 or even twenty piglings. 



It occasionally happens in multiple pregnancy that the off- 

 spring are by two different sires. Thus it has been known that 

 a woman has had intercourse with two different men within a 

 short time of one another, one white and one coloured, and nine 

 months afterwards has given birth to twins of different colours. 

 Among dogs, mixed litters got by more than one sire are not very 

 uncommon. Such a phenomenon is known as Superfecundation. 

 It more rarely happens that as a consequence of ovulation and 

 coition during pregnancy — both of which are abnormal — the 

 uterus may contain embryos of two different ages and sizes, but 

 no authentic case of this has been recorded for man. This 

 condition is called Superfoetation. 



Inheritancpj of Fertility 



Like other racial characteristics, fertility is inherited. This 

 has been proved for horses, sheep, and pigs, as well as for poultry, 

 and it has also been established that an increased fertility can 

 be transmitted through the male to the next generation of females, 

 in just the same kind of way as the deep milking properties of 

 cattle are transmitted through the bull to the next generation 

 of cows. In certain breeds of fowls Pearl has found further that 

 one of the factors for increased egg-production is sex-linked, 



