CHAPTER XII 

 SPECIES HYBRIDIZATION 



In the preceding chapters an attempt has been made to show how 

 character differences in a large number of plants and animals may be 

 interpreted on the basis of differences in the unit factors which are dis- 

 tributed to the germ cells during gametogenesis. The character differ- 

 ences, however, which were analyzed, although often seemingly complex, 

 were really rather simple, for rarely were more than four or five factor 

 differences taken into account. In a few species of plants and animals 

 the number of factors which have been investigated is considerable, but 

 when compared with the number of factors which must constitute the 

 entire hereditary material of a species it is an insignificant fraction of the 

 total. The analyses which have been presented, therefore, are for forms 

 which possess an enormous number of factors in common. The differ- 

 ences which they display are mostly unessential alterations in scattered 

 loci in these systems. 



With the taxonomic question as to what constitutes a species differ- 

 ence, we are not greatly concerned. It is clearly apparent that species 

 as they have been named represent widely divergent differences with 

 respect to the extent of separation from related species. It must be 

 clear to the geneticist, therefore, that specific difference is a variable 

 thing, sometimes meaning one thing, sometimes another. If we look 

 at the question from the standpoint of the number of factors involved, 

 we see clearly that races of plants and animals may differ in one or many 

 genetic factors. Just where the line should be drawn which distinguishes 

 varieties, forms, species, etc., would therefore appear to be almost wholly 

 an arbitrary matter, usually to be decided from considerations of con- 

 venience. Whatever it is, however, the distinction cannot well be viewed 

 from the genetic standpoint, for ordinarily the systematist works with 

 plants and animals which have not been investigated in such a fashion, 

 and, in the case of the more widely separated forms, with those which 

 cannot be so investigated. 



" A genetic investigation of the difference between two species depends 

 upon the possibility of crossing the species in question, and further upon 

 the possibility of securing offspring from the progeny of such a cross. 

 Not infrequently this latter condition is not fulfilled, for it often follows 

 as a result of species hybridization that the individuals thus produced, 



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