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SCIENCE. 



[N. S. Vol. XXII. No. 573. 



ber of chroniosoines for the human species be 

 assumed to be 24; the child gets 12 from the 

 father and 12 from the mother. If amongst 

 the former there are 8 grandmother chromo- 

 somes and amongst the latter 7 grandmother 

 chromosomes the child will be a girl, for there 

 are at least 15 of the 24 derived from the 

 grandmothers' side. 



Ziegler admits that his grandmother theory 

 of sex will not apply to all cases. The ' pe- 

 culiar ' methods of reproduction of the honey 

 bee, the gall wasps, the daphnias, rotifers, and 

 Dinophilus can not, be says, be explained in 

 this way. This admission is in itself a serious 

 objection to the theory, for any satisfactory 

 theory of sex must be prepared to account for 

 this class of cases, that can not be put aside 

 by calling them ' peculiar.' But there are 

 other and more serious objections to be urged 

 against Ziegler's view. 



In the first place, Ziegler's theory is only a 

 special application of the differential chromo- 

 some theory, which Sutton first suggested 

 might account for the Mendelian ratio. 

 Boveri has recently followed Sutton's inter- 

 pretation, and Ziegler also, it appears now, 

 adopts this point of view. Let us look for a 

 moment more closely at this hypothesis, since 

 it has an important bearing on Ziegler's as- 

 sumption in regard to sex. Two views, both 

 purely hypothetical, may be held as to the way 

 in which the chromosomes represent the he- 

 redity qualities. Either, each chromosome 

 contains only one set of characters, i. e., the 

 chromosomes are all different in regard to 

 their hereditary material, or, they are all alike 

 in this respect. Mendel's law is sometimes 

 worked out on the former supposition, and 

 appears to give a satisfactory explanation of 

 how the assumed purity of the germ cells of 

 hybrids may arise. On the other supposition, 

 viz., that the chromosomes are all alike, it is 

 difficult to explain the supposed purity of the 

 germ cells of hybrids. In fact, on this sup- 

 position it can rarely happen that the germ- 

 cells are pure in respect to any one character. 

 If we reject the assumed purity of the germ- 

 celTs in Mendelian cases, and still attempt to 

 explain the Mendelian ratio on our second 

 assumption, viz., that the maternal or the 



paternal chromosomes are all alike, we can 

 give a formal solution for some cases pro- 

 vided we assume that the reduced number of 

 chromosomes is an unequal one; for, the re- 

 sults may then depend on whether more of 

 the grandfather chromosomes or more of the 

 grandmother chromosomes happen to get into 

 a particular cell. But if we examine the list 

 of cases given by Ziegler himself we find that 

 the reduced number of the chromosomes is 

 an even one in 29 species, and odd only in 10. 



On Ziegler's theory of sex it is evident that 

 whenever the reduced number of chromosomes 

 is even there must often occur an exact bal- 

 ance of grandmother and grandfather chro- 

 mosomes, hence the child can have no sex at 

 all ! For a small number of chromosomes 

 this would often occur. 



There is also a serious difficulty in the case 

 of the other assumption that chromosomes are 

 individually different. The peculiar inherit- 

 ance of the Mendelian extracted recessives is 

 difficult to understand from this point of view. 

 For exam.ple, if a white mouse is bred to a 

 gray mouse gray offspring will be obtained. 

 If these gray offspring are inbred they give 

 some gray and some white according to the 

 Mendelian ratio. These white mice (extract- 

 ed) are assumed to have been formed by pure 

 white-bearing germ cells meeting white-bear- 

 ing germ cells, but that this explanation will 

 not account for their origin is shown by 

 crossing these extracted white mice with black 

 mice. The offspring will be gray according 

 to Cuenot. This must mean that the ex- 

 tracted whites must contain gray in a latent 

 condition, and moreover in sufficient amount 

 to dominate the black color of the black mouse. 

 Cuenot who has discovered this and similar 

 facts offers an hypothesis to account for them, 

 but it is an hypothesis far removed from the 

 chromosome theory as applied to Mendelian 

 cases, at least in the form maintained by 

 Ziegler. Since neither assumption in regard 

 to the chromosomes is capable of explaining 

 certain results of the Mendelian cases the 

 most obvious conclusion is, that the germ cells 

 are not ' pure,' and that the Mendelian ratio 

 is not due to this sort of purity but to domi- 

 nance and recessiveness of contrasted charac- 



