SEX-RATIO OF THE DEER-MOUSE. 163 



Other biologists, while granting this absolute predetermination 

 of sex for some organisms, believe that in other cases the condi- 

 tion of the nucleus may be neutral in this regard, and that which 

 of two potentialities shall finally prevail may be determined by 

 chemical or physical influences acting upon the developing organ- 

 ism. There is, indeed, evidence which seems irreconcilable with 

 the view that sex is always predetermined in the nucleus of the 

 fertilized egg. We think, accordingly, that this second position 

 is the safer one in the present state of our knowledge. 2 



As regards the data here reported for Peromyscus, we believe 

 that (granting the reality of certain of the differences found) 

 they lend themselves perhaps equally well to either of the above 

 conflicting viewpoints. There may, for all we know, be seasonal 

 differences in the relative production of male-producing and fe- 

 male-producing spermatozoa. Or, one kind may be more active 

 at one time of the year than another. Or, the ova may so vary 

 in their chemical composition that their attractive influence upon 

 one or another type of spermatozoon varies with the season. Any 

 or all of these things may be true, though there is not a scintilla 

 of evidence that such is the case. 



As regards the effect of crossing upon the sex ratio, this too, 

 is reconcilable with either viewpoint. There might, in one way 

 or another, be a preponderant selection of male-producing sperma- 

 tozoa in hybrid fertilizations. On the other hand, the increased 

 vigor which frequently accompanies hybridization might well 

 influence the metabolism of the parents, and thus affect that of 

 the developing fetuses. 



When we come to those fluctuations in the sex ratio which are 

 met with from one year to another, we must admit that their 

 causes are utterly obscure. In the case of Peromyscus, it is dif- 

 ficult to conceive of any scientific explanation whatever for the 

 fluctuations found. Even when we admit the likelihood that 

 environmental conditions may play a direct or indirect part in sex 

 determination, it is hard to see how the relatively slight differences 

 which distinguish one year from another could bring about a 

 greater effect than the vastly larger differences which distinguish 

 the seasons of a single year. The calendar year, as such, would 



2 This is the viewpoint adopted by Doncaster (1914, Chapter X.). 



