Leonard Doncaster. 



xliii 



amenable to experiment had been studied. He at once saw the extra- 

 ordinary importance of the subject, and, as the result of correspondence 

 with Mr. Eaynor, matings were arranged and a critical investigation of the 

 case was begun. It was soon found that by mating lacticolor females with 

 Fi males (from lacticolor female x grossulariata male), the missing lacticolor 

 male could be produced, and the various possibilities tested. The facts 

 proved that the system of inheritance is exactly the converse of that 

 previously known to be followed by colour-blindness and certain other 

 conditions in man. Whereas the linkage of colour-blindness is with maleness, 

 that of lacticolor is with femaleness. The first clear proof, carried out by 

 the method of experimental breeding, that sex is determined in the gametes, 

 was then made. Incidentally, another very curious corollary followed, 

 namely, that ordinary wild grossulariata females are actually heterozygous 

 for lacticolor, though that variety is seen very rarely in nature. But the 

 cytological proof that in certain Hemiptera sex is determined by the gametes 

 of the male had recently been obtained by E. B. Wilson, and this great 

 discovery had naturally impressed Doncaster. Since the spermatozoa of these 

 Hemiptera were visibly dimorphic in respect of sex, and the eggs of Abraxas 

 were no less demonstrably proved by breeding methods to be dimorphic, 

 he dou.btless felt that this dimorphism must be a condition generally 

 obtaining among the germ-cells of loth sexes, and he therefore devised a 

 scheme of sex-determination (also proposed independently by Wilson) 

 representing both possibilities. This compromise involved the conception 

 that both sexes should be heterozygous in sex and the supposition that 

 dominance attached to the gamete received from the female. Subsequently, 

 he accepted the emendation by which the female only of Abraxas is 

 regarded as heterozygous, as the male is in the Hemiptera and Diptera, 

 the other sex in each case being taken simply to be homozygous, paradoxical 

 though that conclusion is. 



The next step was to look for a cytological proof of the dimorphism of 

 the eggs, but this was not to be had. The diploid number was large, 56, 

 and was only established with difficulty. But in the course of this further 

 work a remarkable new phenomenon was encountered — that a certain 

 strain of A. grossulariata had two kinds of females, of which one produced 

 females almost exclusively, the other giving the usual mixture of sexes. 

 Entomologists have met with great departures from the normal sex -ratios 

 in Lepidoptera, but none had ever been investigated systematically. 



Doncaster was still engaged on this inquiry when his final illness super- 

 vened. He found evidence of cytological distinctions between the two 

 kinds of females, the female-producers having 55 chromosomes instead of 

 the normal 56. He attempted an interpretation on the lines followed by 

 Bridges in his paper on " non-disjunction " in Drosophila, but, as Doncaster 

 pointed out, the suggestion was not consistently applicable, and much 

 remained to be done. 



In these latter years some further progress was made with the old 



