20 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



encoimterecl. More tlum this on the eastern boundary the species 

 is replaced by a smaller form (butleri) which is not known to have 

 a higher formula than 19-17. I will not take the space here to 

 give further illustrations, as they will develop in the consideration 

 of the various forms. It is suflicient to say that the geographical 

 variation in fhe number of scale rows, shown to occur in radix, also 

 occurs in many other forms in the genus. Since the geographic 

 variation in the number of scale rows is brought about in the same 

 manner for the various forms as in the individuals of a given locality, 

 and it further develops that these forms at points in their range 

 where the number of scale rows is at a minimum or maximum often 

 grade into, or are replaced by, other evidently nearly related forms 

 with a smaller or larger number of rows, the conclusion seems to 

 follow that the racial differences in the number of dorsal scale rows 

 in the garter-snakes are the result of the loss of the ahridged rows in the 

 order in which they are dropped posteriorly in individuals that have the 

 maximum number of rows for the genus. 



At first sight it appears rather odd that the first abridged row 

 should end so constantly just beyond the middle of the body, and 

 that any further decrease should take place from the fore backward, 

 but an explanation presents itself. As has been previously stated, 

 the reduction in the number of rows posteriorly is correlated with 

 the taper of the body. If, therefore, we consider that there is also 

 a slight decrease in the size of the body anteriorly that culminates 

 in the constriction of the neck, the tendency for the rows to drop 

 out anteriorly may be considered analogous to their abridgment 

 posteriorly. This anterior taper is slight and not sufficient in many 

 specimens to make a difference in the number of rows between the 

 neck and body, in which case the first abridged row extends to the 

 head, but it is apparently such that a decrease in size would effect 

 the scale rows here before on the middle of the body. There 

 is no noticeable difference in the size of the scales between 

 individuals with a greater or fewer number of scale rows, 

 but it is very noticeable that those forms which possess the smallest 

 formula in the genus are distinctly the smaller in size, and the op- 

 posite. It seems safe to conclude, therefore, that the loss of the 

 rows is correlated with a general decrease in the size of the body, 

 and that in the decrease the number of rows is influenced by the 

 necessity of maintaining the symmetry of the body, so that the 

 decrease is noticeable first on the anterior and posterior parts of 

 the body, owing to the comparatively: smaller size of these parts. 

 Whether or not such dwarfing takes place we will be more able to 

 judge when the evidence of the other characters and the forms 

 themselves have been examined. 



