218 THE PHYSIOLOGY OF REPRODUCTION 



" Sterility in the form of structural degeneration when it occurs 

 gradually increases upon in-breeding until homozygosity is attained, 

 but for the most part it does not show any clear-cut segregation. 

 Yet reduction in fertility is noticeable only so long as there is a 

 change in other characters, constancy in visible characters being 

 accompanied by constancy in the matter of fertility. In other 

 words, there is no more an accumulation of sterility on continued 

 in-breeding than there is an accumulation of any other effect. Any 

 reduction in fertility ceases when homozygosity is reached, but the 

 end result may be decidedly different in various lines coining 

 originally from the same stock." In another passage East writes, 

 " While we are not justified in concluding . . . that in-breeding 

 accompanied by rigid selection will be beneficial [the experiments] 

 certainly show close mating is not invariably injurious." 



The diminished fertility of in-bred animals may be due partly 

 to a decrease in the supply of mature ova perhaps correlated with 

 a general want of vigour. It seems possible, however, that it also 

 results from failure on the part of the gametes to conjugate, since 

 the productiveness of in-bred animals can often be increased by 

 cross-breeding with other varieties (see p. 639). 



Heape l states that Dorset Horn sheep, when served by rams of 

 their own breed, show a greater tendency towards barrenness than 

 when served by Hampshire Down rams. It is possible that what 

 in this case appears to be barrenness is in reality very early abortion, 

 the in-bred embryos tending to die at an early stage and to be 

 absorbed in- utero, thus escaping observation. 2 It seems not unlikely, 

 however, that, in the absence of cross-breeding, there is sometimes 

 an insufficiency of vitality at the very outset, the elective affinity 

 of the gametes being too feeble to induce conjugation. 



Some years ago the writer carried out an experiment upon a 

 bitch belonging to the Dandie Dinmont variety, which is known to 

 be very in-bred. Seminal fluid was obtained from a pure-bred Dandie 

 Dinmont dog, and also from an obviously mongrel terrier of unknown 

 ancestry. The semen from the two dogs was examined micro- 

 scopically, and in each case was found to be rich in sperms, which 

 so far as seen were all moving and in a vigorous condition. Approxi- 



1 Heape, "Abortion, Barrenness, and Fertility in Sheep," Jour. Royal A(//-i<: 

 Soc., vol. x., 1899. 



2 Hammond ("On Some Factors Controlling Fertility in Domestic Animals," 

 Jour, of Agric. Science, vol. vi., 1914), who has described the common occurrence 

 of degenerative foetuses in the uterus of the sow, suggests that this may be the 

 result of " lethal factors " intensified by in-breeding in the manner postulated 

 by East. Kirkham (" Embryology of Yellow Mouse," Proc. Amer. Soc. ZooL, 

 Anat. Record, vol. xi., 1917) and Ibsen and Steigleder ("Evidence of the Death 

 in utero of the Homozygous Yellow Mouse," Amer. Nat., vol. li., 1917) have 

 supposed that homozygous yellow mice, which are never born, but seem 

 invariably to die in the uterus, carry a " lethal factor." 



