126 PRINCIPLES OF STOCK-BREEDING. 



may produce an unusual activity of the functions of 

 reproduction. 



The unusually rapid increase of population, after 

 a country has been scourged by a famine or pestilence, 

 has often been remarked. 



After the plague of 1348 in England, the " flocks 

 and herds wandered about at will, without herdsmen, 

 shepherd, or owner," and labor was so scarce that 

 landlords were glad to have their lands cultivated by 

 their tenants without payment of rent. Population, 

 however, speedily righted itself. " We are told that 

 after the plague double and triple births were fre- 

 quent, that most marriages were fertile, and that no 

 serious effects were produced, in a short time, on the 

 numbers of the people." 1 



In examining the various causes of impaired fe- 

 cundity, we must not lose sight of the influence of 

 the transmission of ancestral tendencies and peculi- 

 arities. 



If the ancestors of an animal are not prolific, it 

 will inherit a bias of the organization that is favorable 

 to the action of the various causes of sterility and bar- 

 renness ; that is, the natural tendency or predisposi- 

 tion of the organization will, as it were, add to the 

 intensity of the forces that interfere with the normal 

 performance of the function of reproduction, and thus 

 aid in its suppression. 



The production of twins will be found to depend, 

 not only upon the supply of food, as already noticed, 

 but on peculiarities of the system that have been in- 

 herited. 



1 Rogers, " History of Agriculture and Prices," vol. i., pp. 299-301. 



