I IO III I EN Dl-.AN KING. 



If from the above table we omit the records for tin- 27 indi- 

 viduals belonging to tin- Kj generation of hybrid:- \\hirh formed 

 only a very small percentage of tin- total number of offspring 

 produced in tin- experiment to which they belonged and also 

 the records for the selected individuals belonging to tin- F- and 

 to the F 3 generation, there remain the data for 277 individuals 

 which comprise the very great majority of tin- hybrid offspring 

 obtained in several series of experiments. It would seem as if 

 the sex ratio in these individuals might justly be taken to repre- 

 sent the probable sex ratio in any large lot of hybrid rats. Of 

 these individuals 150 were males and 127 \\ere females; this 

 gives a sex ratio of nS.i i males to 100 females. This sex ratio 

 is but very little lower than that found in the total number of 

 hybrids for which records are at hand, and it is not very much 

 higher than that in the 356 hybrid mice bred by von Guaita 

 (Table I.). 



The excess of males among these hybrid rats is seemingly 

 beyond the limits of normal variation in the proportion of the 

 sexes in the pure stock, and it is too uniform in the various series 

 of experiments to be attributed to chance. It appears, therefore, 

 that hybridizing alters the sex ratio by producing a marked 

 increase in the relative proportion of males. This conclusion is 

 in essential agreement with that reached by Buffon, by R. and M. 

 Pearl and by Guyer. 



Guyer ('09) has offered an explanation for tin- excess of males 

 among hybrid offspring which accords with the theory, advocated 

 by a number of investigators, that sex is determined in the ovary 

 chiefly by nutritive conditions. Guyer suggests that in the /y- 

 gote produced by cross fertilization there would probably be 

 "more or less default in the metabolic processes because of the 

 incompatibilities which must necessarily exist bet wren t\\o gcrm- 

 plasms so dissimilar." An interference with the metabolic 

 processes would naturally retard the constructive phases of 

 metabolism in the fertilized ovum, and therefore tend to tin- 

 production of relatively more males, since tin- theory assumes 

 that females are produced only when the conditions are most 

 favorable for constructive metabolism. 



To explain the sex ratio in hybrids according to the ciirreni 



