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 
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