112 Dr. IT. Gadow. Evolution of the Colour-pattern [May 22, 



diagrammatically. It should, moreover, be observed, that the number 

 of pores on the right and left thighs not unf requently differ by one 

 or two. 



Number of pores... 15 16 17 18 19 20 21 22 23 24 25 



: bocourti : 



: . mexicanus ; 



I deppei = 



: guttatus and striatus...... 



None of these characters, usually applied in " keys," are absolutely 

 reliable when considered singly and when used to establish categories, 

 but taken together they are sufficient to define the five kinds of 

 Cnemidophorus which occur in Southern Mexico. In addition, there 

 is the size of the adult, the colour-pattern and above all the mode of 

 its change. This brings us to a point of general importance. Two 

 given forms may structurally approach each other so closely, and the 

 differences still existing may be of such slight weight, that we can 

 separate them only by their pattern of colour, and if this happens 

 likewise to be in a changing mood, the two sets of creatures can no 

 longer (or not yet) be regarded as different, nor as geographical races 

 when they live in the same place. Where the physical conditions are 

 exactly the same, monotonous, there occurs only one species of the 

 same genus, for instance, in a desert, on a treeless prairie, in a dense 

 forest ; but if the terrain is varied, although for all practical purposes 

 remaining the same, there is as likely as not more than one species. 

 If these are closely allied we have a chance of correctly connecting 

 their differences with their surroundings. This applies to C. niexicanus 

 and C. bocourti. Again, close allies afford the same chance if they 

 inhabit markedly different localities, no matter whether these kinds 

 of terrain are geographically distant or close together. An instance 

 of this are C. guttatm and C. striatm. Now, C. bocourti and C. yuttatu* 

 are in most structural characters extremes of each other, but both are 

 the spotted representatives of their nearest allies in terrain which 

 produces an abundance of broken shade. Moreover, C. mexicanus arid 

 C. bocourti stand as larger sized modifications in the same relation to 

 C. sexlineatus of the Southern United States as C. guttatus and C. striaiu* 

 do to the small and strongly striated C. deypei, which is the southern 

 counterpart of C. sexlineatus. It is also significant that it is not 

 C. guttatus but C. sti-iatus with which C. deppei has more structural 

 affinity, and that both these forms occur partly together, at least in 

 the Tehuantepec district. 



