ARTICLES 443 



the male : " femaleness is due to maleness plus something 

 else." 



The first objection to the hypothesis is the inheritance of 

 certain characters in Lepidoptera and Birds, when it is necessary 

 to assume a reversal of the above order of things. As mentioned 

 above, however, there is experimental evidence that cytologically 

 such a reversal is justified for at any rate the Lepidoptera. 



This is undoubtedly an unexpected conclusion, but, as 

 Doncaster has said, " It can hardly be coincidence that the 

 spermatozoa should be dimorphic in respect of a chromosome 

 in the forms in which sex-limited inheritance by the male takes 

 place, and the eggs dimorphic in the same way iri those in which 

 sex-limited transmission is by the female." 



Although it is impossible to give any large part of the 

 evidence in support of this quantitative theory of sex-determi- 

 nation, yet it is sufficiently great to warrant its general 

 acceptance. 



I say the general acceptance, because none of the theories 

 outlined above are completely satisfactory. The quantitative 

 one is certainly the most fitting at the present ; but there are 

 objections to it, as, for example, the two kinds of parthenogenetic 

 eggs laid by the gall-fly Neuroterus lenticularis ,^ one of which 

 produces males only and the other females. Obviously in this 

 case the sex is determined before maturation. 



Further, the experiments of Geofi^rey Smith ' and others have 

 shown that in certain instances the sex of an animal can be 

 modified or even reversed by changed metabolic conditions. 

 Such cases do not appear to accord with the theory that the 

 quantity of sex-determiner, carried by the X chromosomes, is 

 the cause of sex-determination. Cause is, however, a word that 

 should not be used in this connection, for it is highly probable 

 that the X chromosomes do not cause a particular sex to develop, 

 but are rather links in a long chain of events. This has already 

 been pointed out by Doncaster, who makes use of the following 

 analogy to illustrate the point : 



" A locomotive engine has a reversing lever which deter- 

 mines whether the engine shall go forwards or backwards, 

 but the lever is not the cause of the motion of the engine ; and 

 by altering other parts of the machinery it could be arranged 

 that when the lever was reversed the engine should go forwards. 

 In the same way it is possible that, other things being equal, 

 the presence of a certain chromosome may lead to the develop- 

 ment of a particular sex, but it is not impossible that other 



1 Doncaster, L., Proc. Roy. Soc. B., 82, igio, p. 88; 83, 1911, p. 476; 

 89, 1916. p. 183. 



2 Smith, Geoffrey, Quart. Journ. Micros. Sci., 55, 1910, p. 225 ; 57, 191 1, 

 p. 251 ; 59, 1913. P- 267. 



