1909] CURRENT LITERATURE 471 



quate treatment of such a subject. Wilson's discussion would have been rendered 

 simpler and more cogent, if he had grasped the logical homologies between plants 

 and animals now generally accepted by students of genetics. He does not seem 

 to appreciate the fact that it is the gametophyte of plants which finds no clear 

 homologue in animals, and so fails to assign a proper degree of importance to the 

 parallelism betw r een the sporophyte and the animal body or soma. Again, if he 

 had been more familiar with the most recent developments of Mendelian theory 

 he would have found that the discoveries in these insects are in perfect accord 

 with Mendelian heredity. Instead of this, he presents as a " naive assumption" 

 what is now generally held by the students of Mendelism, and known as the " pres- 

 ence and absence hypothesis,'' the assumption being that the heterozygote and 

 the positive homozygote differ from each other in that the former has- an unpaired 

 unit, X, and the latter a pair of units of the same kind, XX. 



Castle 10 takes up this question and shows the perfect agreement between 

 the results of these cytologists, and the requirements of the presence and absence 

 hypothesis in Mendelian heredity. In Castle's exceedingly clever discussion an 

 attempt is made to harmonize the apparently antagonistic results with Bryonia, 

 the Hemiptera, and Coleoptera on the one hand, and those with Abraxas and the 

 cinnamon canaries on the other, by assuming that in all cases the female possesses 

 one more unit than the male, this unit being called by Wilson the "X-element." 

 Bryonia, and all of the insects whose male germ-cells have been found to be of 

 two kinds, represent a condition in which the male is a heterozygote, and the female 

 is a positive homozygote. Castle calls this a "dominant female," but this is 

 obviously a misleading terminology, for if the female were really dominant the 

 heterozygote would also be a female and there could be no males. In Abraxas, 

 and the cinnamon canaries, and, as suggested by Castle, perhaps also in the 

 pheasant, the female is heterozygous and the male is assumed to be a negative 

 homozygote, i. e., wholly lacking the X-element. This is the most promising 

 attempt yet made to bring all the recently discovered facts of sex-heredity in dioe- 

 cious animals and plants under a single hypothesis. 



Castle attempts further, by an extension of the same hypothesis, to account 

 for the fact that male animals usually possess more characters than the female. 

 He supposes that these added male characteristics are associated with or produced 

 by a Y-element, the "synaptic mate" of the X-element. He also suggests that 

 progressive evolution may have taken place by the appearance and development 

 of such a "synaptic mate" for the X-element, but this, and also the attempt 

 to explain orthogensis on the same basis, is carrying hypothesis rather far from 



empirical knowledge. 



There can be no question that the problems of sex possess many intricacies 



and difficulties yet to be solved, but the results of these investigations both from 



the experimental and the cytological side have placed these problems on a new 



"> Castle, W. E., A Mendelian view of sex-heredity. Science N. S. 29:395-400 



1909 



