552 GENETICS IN RELATION TO AGRICULTURE 



closely related species. Although it is impossible, therefore, to general- 

 ize as to what particular factor of the environment is responsible for the 

 condition of lowered fertility among wild animals in captivity, there 

 can be no question as to the strikingly adverse effect of confinement in 

 certain cases. 



Unfavorable conditions of the accessory reproductive organs occasion- 

 ally cause sterility. Thus in cattle barrenness is sometimes the result 

 of an acid condition of the secretions of the vagina. Simply injecting 

 a weakly alkaline solution into the vagina has been found effectively 

 to overcome this difficulty. The practice of artificial insemination has, 

 also, been used in cases where the mucous secretions are unfavorable 

 for conception, and in cases where structural bars to conception exist 

 in the accessory reproductive organs. Among cattle especially conta- 

 gious abortion is a serious cause of barrenness. This disease is bacterial 

 in etiology, transmissible from animal to animal, perhaps usually by 

 the agency of the herd bull, although possibly at times through food, 

 and experimentally by intravenous injection. Not only does the disease 

 cause abortion in animals in which it has not developed until after con- 

 ception, but in animals previously infected it leads to barrenness. The 

 disease is characteristic in its lesions and effects and may be controlled 

 by the adoption of proper antiseptic measures. 



In general domestic animals are much more prolific than their wild 

 progenitors. Several reasons for this fact may be pointed out. Those 

 species which can adapt themselves to conditions of domestication 

 usually find such surroundings more favorable to development and to 

 the production of offspring. Moreover, there is a natural tendency for 

 selection to favor the survival of those strains or races, which reproduce 

 most rapidly, and man has augmented this tendency by choosing the 

 more prolific members of the race for breeding stock. 



But even the long-continued selective processes of domestication have 

 not sufficed to attain to the maximum of fertility for the species. Few 

 realize how great is the field for improvement in this respect. In England 

 horse breeding, according to Marshall, suffers an enormous loss each 

 year because of the failure of no less than 40 per cent, of mares selected 

 for breeding purposes to produce offspring. Cattle, sheep, and swine 

 appear to suffer somewhat less in this respect but the loss is far from 

 inconsiderable. Heape estimates the average loss among cattle to 

 amount to over 15 per cent. Among sheep the loss from actual sterility 

 alone amounts to nearly 5 per cent. In view of such statistics the in- 

 crease of fertility in domestic animals becomes a problem of prime 

 economic importance. 



The Darwinian Theory of Fertility. The results of Darwin's extensive 

 investigations of problems of vigor and fertility in plants and animals 



