THE CROSSING, &C, OF PIGS. 28"? 



proportion, it will be sterile, with others, of a like 

 structure ; because, from them, the characters wanting, 

 obviously cannot be supplied. But, when such indi- 

 vidual is bred with another, dissimilarly developed, he 

 will be fertile ; because this other has positive differ- 

 ences which go to make up, in the reproductive ele- 

 ments, the requisite number of characters required for 

 any degree of fertility. Thus, it is clear, that two indi- 

 viduals of different breeds, each of which is perfectly 

 sterile with others of its own breed, may be highly 

 fertile with each other. 



The keeping, of stock, at different localities, has been 

 noticed, by Darwin, to have a tendency to eradicate 

 the evils of interbreeding. The reason is, at different 

 localities, there is different air, different food, there may 

 be different occasions for exercise of the legs and of 

 other parts ; and the effect on the development of any 

 one part, by means of such agencies, involves a cer- 

 tain degree of effect upon the development of other 

 parts, by correlation. This is the mystery of the less- 

 ened evil, effects, attendant upon the same degree of 

 interbreeding, when the individuals have been kept at 

 different stations. The mere difference, of the ground, 

 in one yard, where fowls, or Pigs are kept, being undu- 

 lating, to a degree, just perceptible to the eye ; and of 

 the ground, in the other yard, being of a dead level, 

 would occasion some decrease in the evil effects of 

 breeding two relations, kept at the different places. 

 The little increase, in exercise, required by the nature 

 of the ground, in the first instance, would strengthen 

 different parts and muscles of the legs and feet ; thence, 



