CHAPTER VII 



THE LOCATION OF GENES IN RELATED 



SPECIES 



DE VRIES' mutation theory quite apart from its 

 I special interpretation discussed in an earlier 

 chapter postulates that '' elementary " species 

 are made up of a large number of identical genes ; and that 

 their differences are due to different recombinations of 

 these genes. The more recent work on hybridizing re- 

 lated species has furnished evidence bearing on this 

 theory. 



The most obvious way to study the problem would be 

 to cross species and determine in this way, if possible, 

 whether they are made up of the same number of ho- 

 mologous genes, but several difficulties stand in the way. 

 Many species cannot be crossed, and some of those that 

 can be crossed produce sterile hybrids. Nevertheless, a 

 few species are fertile inter se, and some of them also 

 give fertile hybrids. Even then, another difficulty arises, 

 namely, the identification in the two species of the char- 

 acters that behave as Mendelian pairs; for the differ- 

 ences that serve to distinguish one species from another 

 species are dependent on a multitude of factors in each 

 case. In other words, it is rare to find two well-marked 

 species in which any single difference is due to one dif- 

 ferential factor. Mutant differences of recent origin in 

 one or in both species must be resorted to for the neces- 

 sary evidence. 



There are several cases in plants and two at least in 

 animals where species having mutant types have been 



