28 The Problems 



of which we are as yet wholly unable to apprehend or 

 illustrate. 



To the new conceptions already enumerated we may 

 therefore add 



(4) Unit-characters of which some, when once arisen by 

 Variation, are alternative to each other in the consti- 

 tution of the gametes, according to a definite system. 



From the relations subsisting between these characters, 

 it follows that as each zygotic union of allelomorphs is re- 

 solved on the formation of the gametes, no zygote can give 

 rise to gametes collectively representing more than two cha- 

 racters allelomorphic to each other, apart from new variation. 



From the fact of the existence of the interchangeable 

 characters we must, for purposes of treatment, and to com- 

 plete the possibilities, necessarily form the conception of an 

 irresoluble base, though whether such a conception has any 

 objective reality we have no means as yet of determining. 



We have now seen that when the varieties A and B 

 are crossed together, the heterozygote, AB, produces 

 gametes bearing the pure A character and the pure B 

 character. In such a case we speak of such characters as 

 simple allelomorphs. In many cases however a more 

 complex phenomenon happens. The character brought in 

 on fertilisation by one or other parent may be of such a 

 nature that when the zygote, AB, forms its gametes, these 

 are not individually bearers merely of A and B, but of a 

 number of characters themselves again integral, which in, 

 say A, behaved as one character so long as its gametes 

 united in fertilisation with others like themselves, but on 

 cross-fertilisation are resolved and redistributed among the 

 gametes produced by the cross-bred zygote. 



In such a case we call the character A a compound 



