52 A TREATISE ON HORSE-BREEDING. 



The closest interbreeding does not seem to induce varia- 

 bility, or a departure from the typical form of the race or 

 family, but it causes loss of size, of constitutional vigor in re- 

 sisting unfavorable influences, and often of fertility. On the 

 other hand, a cross between plants of the same sub-variety, 

 which have been grown during some generations under dif- 

 ferent conditions, increases to an extraordinary degree the 

 size and vigor of the offspring. 



Some kinds of plants bear self-fertilization much better 

 than others; nevertheless it has been proved that these 

 profit greatly by a cross with a fresh stock. So it appears to 

 be with animals, for Short-horn cattle perhaps all cattle 

 can withstand close interbreeding with very little injury; but 

 if they could be crossed with a distinct stock without any loss 

 of their excellent qualities it would be a most surprising fact 

 if the offspring did not also profit in a very high degree in 

 constitutional vigor. If, therefore, any one chose to risk 

 breeding from an animal which suffered from some inherita- 

 ble disease or weakness, he would act wisely to look out not 

 merely for a perfectly sound animal of the other sex, but for 

 one belonging to another strain, which had been bred during 

 several generations at a distant place, under as different 

 conditions, as to soil, climate, etc., as possible, for in this 

 case he might hope that the offspring, by having gained in 

 constitutional vigor, would be enabled to throw off the taint 

 in their blood. 



The view of the case presented by Darwin 

 and Herbert Spencer in the foregoing extracts 

 affords an explanation of many apparent con- 

 tradictions which result from breeding in-and- 

 in. The farmer who permits his stock to pair 

 miscellaneously, without infusing fresh blood 

 for many generations as is the case with some 

 must necessarily practice breeding in-and-in; 

 but, as in such cases the stock is almost inva- 

 riably, at the beginning, of a heterogeneous 



