5IO EVOLUTION, GENETICS, AND EUGENICS 



mule, nothing but good would apparently come out of it; but in man 

 and among other animals and plants where mating is more or less 

 indiscriminate, cross-breeding is sure to continue into the F2, Fj, and 

 subsequent generations, entailing the production of all sorts of unfortu- 

 nate combinations and the outcropping of all sorts of unbalanced 

 recessive weaknesses. In view of these considerations it is practically 

 certain that hybridization, unless accompanied by rigid selection and 

 the elimination from parentage of the less desirable combinations, is 

 on the whole disadvantageous. In nature, natural selection serves to 

 eliminate sooner or later the worst combinations resulting from con- 

 tinuous cross-breeding, but in man, little is done to prevent the worst 

 racial admixtures from predominating, the result being that the popu- 

 lations of some parts of the New World are made up mainly of a rather 

 homogeneous hybrid type possessing little more than the worst traits 

 of the various races that have contributed to the melting-pot. 



By way of a general summary let us quote the following paragraphs 

 from D. F. Jones,' by whose book Genetics in Plant and Animal Im- 

 provement this discussion has been largely suggested: 



"Summary. — From the foregoing it will be realized that if any 

 individual is deficient or handicapped in its hereditary make-up, there 

 is a good chance that its needs will be supplied when it is crossed with 

 other individuals, because all are not apt to be wanting in the same 

 things. What one lacks is furnished by the other, and conversely. In 

 other words, there is a pooling of hereditary resources, with the result 

 that the combined effect is better than either could produce alone. 



"It should now be clear that the beneficial effects of crossing follow 

 from the workings of the laws of heredity and not from any mysterious 

 stimulus from the act of crossing itself. If good qualities exist in the 

 parents, but not in sufficient amount or not in their proper association, 

 then there is a good opportunity for the offspring to bring together 

 the favorable factors from both and surpass their parents in develop- 

 ment. This is a temporary and transitory effect, however. The in- 

 creased vigor is shown at its best only in the first generation following 

 the cross, and is quickly lost in later generations unless it can be 

 perpetuated by some form of asexual reproduction." 



INBREEDING 



There is a widespread and deep-seated feeling among men that the 

 mating of close relatives is unnatural and harmful. In most civilized 

 countries there are laws both religious and civic forbidding the mar- 



' Loc. cil. 



