Mendel's Discovery 93 



species contains. The further investigation of the species-problem 

 must thus proceed by the analytical method which breeding experi- 

 ments provide. 



In the nine years which have elapsed since Mendel's clue became 

 generally known, progress has been rapid. We now understand the 

 process by which a polymorphic race maintains its polymorphism. 

 When a family consists of dissimilar members, given the numerical 

 proportions in which these members are occurring, we can represent 

 their composition symbolically and state what types can be trans- 

 mitted by the various members. The difficulty of the "swamping 

 effects of intercrossing " is practically at an end. Even the famous 

 puzzle of sex-limited inheritance is solved, at all events in its more 

 regular manifestations, and we know now how it is brought about 

 that the normal sisters of a colour-blind man can transmit the 

 colour-blindness while his normal brothers cannot transmit it. 



We are still only on the fringe of the inquiry. It can be seen 

 extending and ramifying in many directions. To enumerate these 

 here would be impossible. A whole new range of possibilities is 

 being brought into view by study of the interrelations between the 

 simple factors. By following up the evidence as to segregation, 

 indications have been obtained which can only be interpreted as 

 meaning that when many factors are being simultaneously redis- 

 tributed among the germ-cells, certain of them exert what must be 

 described as a repulsion upon other factors. We cannot surmise 

 whither this discovery may lead. 



In the new light all the old problems wear a fresh aspect. Upon 

 the question of the nature of Sex, for example, the bearing of 

 Mendelian evidence is close. Elsewhere I have shown that from 

 several sets of parallel experiments the conclusion is almost forced 

 upon us that, in the types investigated, of the two sexes the female 

 is to be regarded as heterozygous in sex, containing one unpaired 

 dominant element, while the male is similarly homozygous in the 

 absence of that element 1 . It is not a little remarkable that on this 

 point — which is the only one where observations of the nuclear pro- 

 cesses of gameto-genesis have yet been brought into relation with the 

 visible characteristics of the organisms themselves — there should be 

 diametrical opposition between the results of breeding experiments 

 and those derived from cytology. 



Those who have followed the researches of the American school 

 will be aware that, after it had been found in certain insects that the 

 spermatozoa were of two kinds according as they contained or did 

 not contain the accessory chromosome, E. B. Wilson succeeded in 



1 In other words, the ova are each either female, or male (i.e. non-female), but the 

 sperms are all non-female. 



