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 Ivuthven A. G. “ Variations of the Garter Snakes.” 
