SEX DETERMINATION IN CATTLE. 221 



strated. On the one hand the cytologists have shown for many 

 forms that certain chromosomes are definitely associated with 

 sex differentiation. On the other hand, the geneticists have 

 shown that in a considerable number of forms the phenomenon 

 of sex-linked inheritance is exhibited, a phenomenon which has 

 as yet received no more cogent explanation than the obvious one 

 that sex differentiation depends upon germinal factors, and that 

 these behave in inheritance in accordance with Mendelian 

 principles. 



No one could rate higher the evidence that sex is determined 

 primarily by inheritance than does the writer. But it is idle 

 to deny that there is also a large and increasing body of critical 

 evidence indicating that, in one way or another, sex ratios may 

 be modified experimentally, and to some degree indeed controlled. 

 Any adequate hypothesis of sex-determination must account 

 both for the facts indicating innate pre-determination in the 

 germ cells before fertilization, and also those indicating the 

 modifying influence of external factors. Because one of these 

 sets of facts is true does not mean that the other set is necessarily 

 false, and should be forthwith annihilated (if possible) by 

 destructive criticism. Such definite facts cannot be mutually 

 exclusive. It would seem that a logical view of the case is that 

 while sex is, in many cases at least, primarily determined by 

 innate hereditary causes, nevertheless external factors, acting at 

 the appropriate time, may in some instances modify the effect 

 of the innate factors. This is essentially the same conclusion as 

 is reached by Schleip 1 in his recent extensive and critical review 

 of the literature in this field. 



It is not difficult to conceive how such results could be brought 

 about physiologically. The evidence indicates that particular 

 chromosomes are concerned with the hereditary determination 

 of sex. Furthermore a broad survey of the experimental work 

 in the modification of sex-ratios by external conditions indicates 

 that the most effective agencies in bringing about such modifica- 

 tions are those which directly affect the metabolic condition of 

 the germ cells (e. g., staleness, extraction of water, etc.). Why 



'Schleip, W. t " Geschlechtsbestimmende Ursachen im Tierreich," Ergeb. u. 

 Fortschr. Zoo/., Bd. III., pp. 165-328, 1912. 



