VARIATIONS OF GARTER-SNAKES. 



51 



SO that if one sex greatly predominates in a locality it may throw the 

 average either above or below that of another locality in which the 

 opposite sex prevails, antl thus obscure any geograpiiical variation that 

 may exist unless very pronounced. Even when each sex is plotted by 

 itself geographical variation may l)e obscured by the individual varia- 

 tion owing to the small number of specimens. If, however, it can be 

 shown that there is a similar geographic variation in both sexes and 

 this cause of fluctuation is removetl by combining the two sexes in 

 approximately equal proportions, the increased amount of material 

 should reduce the individual variation and also tend to decrease both 

 the sexual and individual variations below the geographic. 



174 



166 



158 



150 



23 



n~- 7 



u- - _ 



9 



^5 



10 



"^ 7- 



Fig. n 



-Diagram showing the variation in the number of ventral scutes in Thamnophis 



megalops. 



When the range of variation in the two sexes is examined separately 

 it is found that in Chihuahua the average number of gastrosteges in 

 the males is 170 and in the females 165, a condition that seems to 

 prevail southward nearly to Patzcuaro; at the City of Mexico, how- 

 ever, it is 165 in the males and 160 in the females, which indicates a 

 reduction in gastrosteges in both sexes very similar to that of the scale 

 rows. In diagram, fig. 17, I have plotted the locality variations in 

 the number of these plates, and, although the sexes are only approxi- 

 mately equally represented in each series, I believe that the mean 

 represents nearly the average number for each locality. If I am 

 right in this it will be seen that there is a gradual decrease in the number 

 of ventrals from Chihuahua and Patzcuaro toward Mexico City and 



