MENDELIAN CHARACTERS 217 



have some hesitation in accepting them without further 

 study. 



It must be admitted that the suggestion that Men- 

 delian segregation may be a universal phenomenon 

 accompanying the formation of the germ-cells of all 

 animals and plants is one which will be disputed by 

 many biologists. There is ample evidence, they will 

 say, of first crosses breeding true in the case of 

 numerous plant and animal hybrids. But there are 

 several points to be considered before we can accept 

 the mere fact of a uniform progeny as sufficient evidence 

 that there was nothing in the nature of segregation 

 amongst the gametes of the hybrid. 



To prove a negative is proverbially difficult, and 

 the attempt to show that hybrid characters exist 

 which do not segregate in the germ cells of the hybrid 

 is no exception to this rule. In favour of the opposite 

 contention the following considerations may be 

 alleged : 



The characters for which non-segregation has been 

 asserted are generally complex and difficult to define. 

 Such characters, for instance, are the general habit 

 of a plant or the general shape of a flower. Characters 

 such as these may reasonably be supposed to owe 

 their appearance to the interaction of a considerable 

 number of independent allelomorphs. 



Where two such factors are concerned, in cases 

 where the heterozygote form is intermediate, we 

 should expect one out of sixteen F 2 individuals to 

 resemble each parent form, the remaining fourteen 

 plants being more or less intermediate. 



