VARIATION AND MENDELIAN INHERITANCE 397 



Passing now to the hybridization of races, I shall not discuss 

 this at any great length in the present paper, owing to the fact 

 that two other important series remain to be measured, and I 

 plan to publish a more complete account when these additional 

 data are available. I have not, therefore, included a table giving 

 the mean values of the various characters in the different crosses. 

 Regarding these mean values, I may say that, in respect to 

 characters in which the parent races differ, the figures for 

 both the Fi and F2 generations, are intermediate, though 

 not always midway between those of the parents. And with 

 one exception (foot length in the Eureka- Victorville cross), the 

 mean values agree pretty closely in the two hybrid generations. 

 In this exceptional instance there is a perceptible reduction in the 

 mean foot length in the second generation. 



It is important to know to what degree these hybrids have 

 been modified by captivity, and particularly whether any of the 

 relations to be discussed below are attributable to this cause. 

 In addition to frequent sterility and a greater tendency toward 

 adiposity, the chief modifications to be observed in many of the 

 cage-bred mice consist in: 1) reduction in general body size; 

 2) reduction in the relative length of the appendages and in the 

 width of the tail stripe, and, 3), for some characters, at least, an 

 increase in variability. 



Now, in both of the crosses to be considered, the mean body 

 length is somewhat less in the F2 generation than in the Fi, and 

 in one of these (Eureka-Victorville) there has occurred a small 

 reduction, both relative and absolute, in the mean length of the 

 foot. There seems to be evidence, therefore, that the F2 gen- 

 eration is somewhat less normal than the Fi. So far as this fact 

 has any bearing on the comparative variability of the two hy- 

 brid generations, it must be stated that the probable effect of an 

 increase of abnormality would be an increase of variability. As 

 regards tail length, at least, I have definite evidence of this in 

 comparing the standard deviations of wild and cage-bred mice. 22 

 The figures for the latter are much larger. Thus, if abnormality 



22 I have not yet computed standard deviations for most of the characters in 

 cage-bred animals of the pure races. 



THE JOURNAL OF EXPERIMENTAL ZOOiOGT, VOL. 30, NO. 3 



