446 SCIENCE PROGRESS 



But we neeo not multiply instances. It must be sufficiently- 

 evident that the value of the tool which has been put into the 

 hand of the breeder of animals and plants is such as can hardly 

 yet be estimated. 



Having now given examples of some of the more salient 

 hereditary phenomena with which the Mendelian has been 

 able to deal, and having made a brief reference to the 

 economical importance of by-products of Mendelian investi- 

 gation, we must resume the thread of the story and see 

 whither it leads. 



The culmination of Mendelian speculation is reached in the 

 suggestion — for it is no more than that at present — that the in- 

 heritance of sex is Mendelian ; that " male " and " female " repre- 

 sent two allelomorphs of a pair, like yellowness and greenness 

 in peas. Sex certainly is a character which is inherited 

 alternatively — that is to say, in organisms in which bi-sexual 

 reproduction is the rule, an individual exhibits either the 

 character of one parent or that of the other, but never a 

 mixture of the two. It is a male or a female. But there is 

 one profound difference between the hereditary relationship 

 of the two characters, male and female, and the two characters, 

 yellow and green ; in the latter, one character (yellow) is always 

 dominant over the other (green), in the former this is manifestly 

 not the case. Let us see how the matter is dealt with by Castle, 

 who was the first to formulate definitely the theory of the 

 Mendelian inheritance of sex in a paper entitled " The Heredity 

 of Sex." J He supposes that half of the germ-cells of every 

 male individual contain the unit character " male," and half the 

 unit character " female " ; and that the germ-cells of females 

 are similarly constituted. Now the result of the random 

 union of a number of such spermatozoa and ova would be : 

 25 per cent. S S zygotes, 50 per cent. ? 6*, and 25 per cent. ? ?. 

 We see at once that all the homozygotes — the dds and 

 the ? ? s— if they mated with their like (i.e. ? ? with ? ? , or 

 S3 with 6 6) would be capable of producing members of 

 one sex only. This difficulty is got over by saying that 

 there is no reason to suppose that such sexually pure — 

 i.e. homozygous — individuals exist; and Prof. Castle quotes 

 cases which tend to show that in reality every organism, 

 whether male or female, is potentially a hermaphrodite, and 



1 Mus. Comp. Zool. Harvard, vol. xl. No. 4. 



