210 THE CROSSING, &C, OF PIGEONS AND FOWLS. 



ing. I thus secure sufficient crossing to prevent dete- 

 rioration.' " 



It is plain, that, when Mr. Bollance says, that "breed- 

 ing in-and-in does not necessarily cause deterioration," 

 he shows, that he knows whereof he speaks, better than 

 does Darwin. It is clear, that Mr. Bollance would not 

 have formulated any "general law of nature," for a 

 factor so infinitely variable in the quantity of its re- 

 sults, as is close-interbreeding. 



The reason, why the evils of interbreeding, are 

 lessened by keeping the animals at different places, 

 is, because the different conditions, prevailing, at such 

 different places, entail slight differences in the growth 

 of the individuals. Such slight structural differences 

 suffice to stave off the evils attendant upon close-inter- 

 breeding. Different organs of the body, are differently 

 affected, — either favorably or unfavorably, — in different 

 places. This may be due to the differences in the food, 

 in the air, in the water, or in the amount of exercise 

 required for the legs and wings. These conditions act 

 directly, upon the organs or parts obviously involved. 

 But, differences of growth, are entailed in other parts 

 of the organization, by correlation with the parts, 

 immediately affected. 



Thus, two individuals, derived from the same strain 

 or family, will have their likeness to each other less- 

 ened, by being subjected to different conditions. When 

 they interbreed, therefore, there is not the same amount 

 of increased evil, that there would have been, had they 

 been reared under exactly similar circumstances; for, 

 there is not close similarity of defects; but each, proba- 



