54 A TREATISE ON HORSE-BREEDING. 



Nor are we without a cause for the irregular manifestation 

 of these general tendencies. The mixed physiological units 

 composing any organism being, as we have seen, more or less 

 segregated in the reproductive centers it throws off, there 

 may arise various results, according to the degrees of differ- 

 ence among the units and the degrees in which the units are 

 segregated. Of two cousins who have married the common 

 grandparents may have had either similar or dissimilar con- 

 stitutions; and if their constitutions were dissimilar the 

 probability that their married grandchildren will have off- 

 spring will be greater than if their constitutions were 

 similar. Or the brothers and sisters from whom these 

 cousins descended, instead of severally inheriting the con- 

 stitutions of their parents in tolerably equal degrees, may 

 have severally inherited them in very different degrees; in 

 which last case intermarriages among the grandchildren 

 will be less likely to prove infertile. Or the brothers and 

 sisters from whom these cousins descended may severally 

 have married persons very like or very unlike themselves, 

 and from this cause there may have resulted either an undue 

 likeness or a due unlikeness between the married cousins. 

 These several causes, conspiring and conflicting in endless 

 ways and degrees, will work multiform effects. * * * 

 Hence it may happen that among offspring of nearly-relaled 

 parents there may be some in which the want of vigor is not 

 marked, and others in which there is decided want of vigor. 

 So that we are alike shown why in-and-in breeding tends to 

 diminish both fertility and vigor, and why the effect can not 

 be a uniform effect, but only an average effect.* 



It follows, then, as a practical deduction from 

 the foregoing, that the more purely bred and 

 uniform in type our stock becomes the greater 

 is the danger from breeding in-and-in. That 

 while, as before remarked, it is a powerful 



* "Principles of Biology," Vol. I, pp. 283 and 284. 



