CHAPTER XXIV 



CROSS-BREEDING AND INBREEDING 



Cross-breeding is essentially the same as hybridization, and we 

 have already studied various phases of hybridization in connection 

 with Mendelian heredity. There are, however, certain other aspects 

 of cross-breeding that have only a more or less remote connection with 

 Mendelian analysis. 



The role of hybridization in evolution. — "This," says R. R. Gates, 

 "is a thorny subject, on which different investigators have taken quite 

 different attitudes to the same facts. The extreme view that all flower- 

 ing plants, or even all sexual organisms, are hybrids has been held. 

 This has been accompanied in some cases (Lotsy) by the denial of any 

 true germinal change, though why such labile substance as protoplasm 

 should be incapable of undergoing a permanent or germinal change is 

 difficult to understand. Jeffries and others appear to hold that poly- 

 ploidy and other changes in which hybridization may be a factor, have 

 nothing to do with evolution. A more reasonable view would appear 

 to be that crossing has occurred in various groups from time to time, 

 with more or less frequency and between sometimes more and some- 

 times less closely related forms. Crossing is therefore a condition 

 under which much evolution has taken place. It by no means follows 

 that crossing, any more than gravitation, is a vera causa, still less the 

 vera causa, of evolution, but it is a contributing condition. Polyploidy, 

 frequently accompanied by hybridization, appears to be a common oc- 

 currence among flowering plants, but it would be futile to deny on this 

 account that flowering plants have had an evolution; nor would it be 

 safe to assume at present that the evolution of this group differs very 

 essentially from that of any other." 



The exact role of hybridization in the formation of new species is 

 at present merely a matter for speculation, but that many new races 

 have been the result of favorable combinations of the genetic factors 

 of different strains can scarcely be doubted. In a sense, it may be said 

 that hybridization is the rule in all organisms that reproduce sexually, 

 for no completely homozygous individuals exist in such groups, and 

 therefore there will always be a certain amount of factor segregation 

 in gamete formation and of recombination in the process of zygote for- 

 mation. Also, it must be admitted that there are all grades of hetero- 



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