252 BULLETIN 114, UNITED STATES NATIONAL MUSEUM. 



8. Continuity and directness of individual variations or modifications 

 radiating from the center of origin along the highways of dispersal. — 

 Lengthy demonstration of this criterion is not necessary here, since 

 the descriptions and summaries demonstrate conclusively its direct 

 applicability to the Southwest. 



It will be evident that although these criteria are not of equal 

 value, all point to the Southwest, and that the most dependable of 

 these, the third, sixth, and eighth, are especially definite in this 

 respect. The argument for the Southwest is not expected to rest 

 upon any single piece or line of evidence, but it is believed that the 

 weight of positive and the lack of negative evidence, as brought out 

 in the summaries of the separate groups and in the discussion of the 

 preceding criteria, amount to a demonstration that the center of 

 dispersal of the genus Lampropeltis is in the southwestern portion of 

 the North American continent. 



Before concluding, a few remarks on the relation between the 

 environment and the distribution of species and subspecies, as 

 exhibited by the genus Lampropeltis, may not be out of place. It is 

 noticeable that each form inhabits a region of rather definite environ- 

 mental conditions, and that, within any such region, a group of 

 directly related forms has but a single representative. Thus, re- 

 stricted to the southeastern States, we have three forms, each repre- 

 senting a different line of descent. Each of these forms is replaced 

 west of the Alabama-Mississippi region by a closely allied form, and 

 the different environmental conditions of southern Florida are re- 

 flected in these forms as follows: One apparently does not extend 

 south of the northern portion of the State, one is replaced by a closely 

 allied form derived from it, and the third expresses an extreme re- 

 duction that may yet lead to subspecific differentiation. The prairie 

 region is characterized by two forms (calligaster and syspila), the 

 lower Mississippi Valley by one (amaura), and another (holhrooJci) 

 ranges over both of these areas, but does not extend to any adjacent 

 region. Two characterize the northeastern deciduous forest province ; 

 one of these (niger) is confined to the southern section, west of the 

 Alleghenies, the other (triangulum) inhabits the whole province, but 

 has developed most successfully in the north. At the western limit 

 of forests in Texas, both hoTbrooTci and amaura are replaced by their 

 close relatives, splendida and gentilis, respectively. In Mexico the 

 major environments are represented by polyzona in the lowlands of 

 the east and south, nelsoni on the west coast, and annulata in the 

 plateau region. Whatever may be the cause of speciation, or the 

 breaking up into subspecies, it is a fact that when a form has mi- 

 grated into a region of decidedly different environment it has become 

 altered, and the alteration has remained constant in its main features 



