128 ORIGIN OF LIFE IN AMERICA 



from a systematic point of view. Several of the species are 

 exceedingly variable, and all are difficult to discriminate from 

 one another. 



The genetic relationships of the various species and the 

 causes which have given rise to the differentiation of the 

 garter-snakes are most attractive subjects for study. Dr. 

 A. E. Brown* has discussed the connection between moisture 

 and variability, especially in the direction of colour intensity 

 in this group of snakes. More recently an ingenious and 

 novel method of carefully estimating the value of the cha- 

 racters commonly held to be specific in snakes has been 

 adopted by Dr. Ruthven. He shows that the reductions in the 

 number of rows of dorsal scales as the girth of the body 

 decreases in the individual snake, are brought about by the 

 dropping of certain definite rows. This leads him to the con- 

 clusion that specific variation in the scale rows follows the 

 same sequence and is also correlated with the circumference 

 of the body. Similarly, presence, absence or fusion of the 

 labial scuta are dependent on the length of the head. Dr. 

 Ruthven's f assumption is that the garter-snakes started in 

 America with the maximum number of dorsal rows of scales 

 known in the genus, and that the forms resulting from geo- 

 graphical extension are mostly due to dwarfing in consequence 

 of unfavourable environment. He then traces four lines of 

 descent, which all emanate from northern Mexico as the centre 

 of origin of the genus. 'The area inhabited by the nineteen 

 species of garter-snakes includes all North America and south- 

 ward as far as the southern boundary of Guatemala. The 

 genus is evidently a geologically recent immigrant to Central 

 America. 



It is of the greatest interest to the student of zoogeography 

 that Thamnophis differs from its nearest American relative, 

 Tropidonotus, by the absence of scale pits, and by the pre- 

 sence of an undivided anal plate, for it seems almost certain 

 that Thamnophis has originated in North America from some 

 ancestral form of Tropidonotus (Natrix), the latter being 

 clearly a much older genus. Tropidonotus has a vast range 



* Brown, A. E. Variations of Eutaenia." 



t Euthven A. G. "Variations of the Garter Snakes." 



