CHAPTER XXXVIII 

 CROSS-BREEDING AND INBREEDING 



CROSS-BREEDING 



Cross-breeding is essentially hybridization, and we have already 

 studied various phases of hybridization in connection with MendeUan 

 heredity. There are, however, certain other aspects of cross-breeding 

 that have only a more or less remote connection with Mendelian 

 analysis. In this place we shall confine our attention to two questions : 

 (c) What is the role of hybridization as an evolutionary factor? (b) To 

 what extent is hybridization advantageous? 



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 occiurred 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 

 occurrence 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 



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