Principles of Heredity 1 33 



of something new. Our AB's may have one or more 

 characters peculiar to themselves. We may in fact have 

 made a distinct " mule " or heterozygote form. Where this 

 is the case, there are several subordinate possibilities we 

 need not at present pursue. 



4. There may be definite variation (distinct from that 

 proper to the "mule") consequent on causes we cannot 

 yet surmise (see pp. 125 and 128). 



The above possibilities are I believe at the present time 

 the only ones that need to be considered in connexion with 

 these exceptional cases*. They are all of them capable 

 of experimental test and in certain instances we are 

 beginning to expect the conclusion. 



The " mule " or heterozygote. 



There can be little doubt that in many cases it is to 

 the third category that the phenomena belong. An indication 

 of the applicability of this reasoning will generally be found 

 in the fact that in such " mule " forms the colour or the 

 shape of the seeds will be recognizably peculiar and proper 

 to the specimens themselves, as distinct from their parents, 

 and we may safely anticipate that when those seeds are 

 grown the plants will show some character which is 

 recognizable as novel. The proof that the reasoning may 

 apply can as yet only be got by rinding that the forms in 



* I have not here considered the case in which male and female 

 elements of a pure variety are not homologous and the variety is a 

 permanent monomorphic " mule." Such a phenomenon, when present, 

 will prove itself in reciprocal crossing. I know no such case in 

 peas for certain. 



