October 13, 1916] 



SCIENCE 



541 



be produced by double or triple crossing over, 

 and in other ways also. Moreover, granting 

 that the factors seem to be related to each 

 other in four systems of linkages, it must next 

 be proved that there is no linkage between 

 members of distinct systems. The evidence 

 of such independence is admittedly meager, 

 and indeed as to the behavior of the factors 

 comprised in the third system we have been 

 told very little at all. 



The machinery for dealing with uncon- 

 formable cases is extraordinarily complete. 

 Besides differential viability we hear of some 

 twelve lethal factors by whose operation certain 

 classes may be extinguished; changes in out- 

 put with age; a special phenomenon spoken of 

 as " interference " inside single chromosomes ; 

 some interaction between chromosomes; even 

 of a factor modifying the normal amount of 

 crossing over, and lastly of an altogether dis- 

 tinct kind of crossing over in the four-strand 

 stage. Can the action of all these processes be 

 severally traced? Can their consequences be 

 distinguished from each other, and especially 

 from those of multiple crossing over? There 

 remain, of course, also the various slips to 

 which all experimental work is liable, such as 

 in this case errors from the overlapping of 

 generations — several times alluded to as a real 

 danger — and others similar which no doubt 

 have been obviated more or less with the im- 

 provement in technique. Apart from obscur- 

 ities of this more superficial kind, is it clear 

 that the series of alternative hypotheses is 

 capable of ultimate analysis? As has been 

 already said, the authors may be able to make 

 such an analysis, but they have not yet offered 

 it to the reader in irrefragable form. Mean- 

 while the suspicion is unavoidable that, given 

 a conviction that the factors must be arranged 

 in rows along four chromosomes, the various 

 interpretations provide rather a method, or per- 

 haps we should say alternative methods, by 

 which the facts can be reconciled with the 

 hypothesis, than a proof that this hypothesis 

 is correct. 



Ever since the discovery of systems of link- 

 age it has not been in dispute that several fac- 

 tors, perhaps all, are arranged in some ordinal 



system or systems. We are dealing with phe- 

 nomena of linkage. The hypothesis of redupli- 

 cation was offered as one way in which the 

 processes could be logically represented, at 

 least in plants. It is admittedly a very crude 

 conjecture, but it has the merit of being non- 

 committal and applicable to units of various 

 magnitudes. So much may be remarked in 

 parenthesis; but the critical point now is 

 whether in the various forms of life the num- 

 ber of independent factors, or systems of fac- 

 tors, is or is not greater than the haploid num- 

 ber of the chromosomes. The determination of 

 this question all students of genetics will now 

 await with keen interest. 



In all the various parts of the subject ex- 

 plored, whether the main theory prove ulti- 

 mately to be truth or fallacy, there can be no 

 doubt as to the extraordinary value of the 

 Drosophila work as a whole. Of the discovery 

 that may perhaps come hereafter to be re- 

 garded as the most illuminating of all — the 

 phenomenon of " non-disjunction " — we have 

 still to speak. The exploration of this group 

 of facts has been made by Bridges, who, since 

 the brief note contained in the book, has pub- 

 lished in Genetics a detailed account of his 

 experiments. With this publication it must be 

 admitted we are lifted on to something like 

 solid ground. Hitherto amidst all that cytol- 

 ogy has contributed, in one respect only has it 

 been found possible to connect quite positively 

 cytological appearances with somatic char- 

 acters. That in certain forms of life sex is 

 connected with the X chromosome is the one 

 unambiguous fact. 



To this Bridges now adds evidence of a new 

 and very remarkable kind. In crosses between 

 females with recessive eye color and normal 

 " wild " males, the daughters normally resem- 

 ble the father and the sons the mother. As 

 exceptions, " matroklinous " daughters are pro- 

 duced, that is to say in this case with eyes of 

 the recessive color. It was argued a priori 

 that such a result might be reached if the two 

 X chromosomes of the female were by some 

 chance together passed into an ovum and that 

 ovum were fertilized by a T-bearing sperm. 

 Such a zygote would be female by virtue of the 



