1915. ] 



IXTEODUCTION. 



29 



The more nearly related forms generally intergrade or coalesce 

 where their ranges join, some gradually, others abruptly, according 

 to the change of physiographic or chmatic conditions. 



In Dr. Merriam's phylogenetic tree of the Geomyidae,^ TJiomomys is 

 given as one of the lower branches. The great numbers of genera 

 and species of the family represented in southern Mexico and Central 

 America are spoken of as of the utmost interest in view of the time 

 and place of origin of the family to which they belong. ^ If, as seems 

 probable, the family originated in tropical Mexico and Central Amer- 

 ica, TJiomomys has pushed out to the greatest distance of any genus 

 in the family and now reaches from Boca del Monte (near Orizaba), 

 Vera Cruz, Mexico, north to Edmonton, Alberta, and from eastern 

 North Dakota to the Pacific coast. 



Over tliis vast region the species occupy every life zone from the 

 Tropical to the Arctic Alpine, and the greatest extremes of heat and 

 cold, dryness and humidity, hght and shade to which the area is sub- 

 ject. Although Uving underground and being partially nocturnal in 

 habits, the animals respond perfectly to environmental conditions and 

 vary m color according to light and shade, in claws and incisor teeth 

 according to soil texture, in length of fur according to temperature, 

 and in size and form of ears in accordance with length of fur. Thus 

 the great number of recognizable forms is a direct result of the wide 

 distribution and adaptation of a plastic group. The temptation to 

 theorize beyond the Umits of actual knowledge in treating such a 

 group is difiicult to resist. 



The boundaries of ranges of the recognized forms follow closely 

 the border lines of known physiographic or climatic areas. Many 

 forms occupy definite divisions of a life zone, or, in some cases, 

 as on a mountain slope or on several different mountain slopes, 

 extend regularly through a section of two or rarely three life zones, 

 maintaining their boundary fines with surprising accuracy. On 

 some steep and narrow mountain peaks or ranges, however, a valley 

 species will extend up and down the sides through several cfimatic 

 zones without undergoing noteworthy change. 



Permanent residence in a given region seems necessary for notice- 

 able change of characters, and even very restricted but very strongly 

 characterized areas have developed weU-marked forms. Species with 

 the widest range are T. talpoides and T. rvfescens of the great plains 

 and prairies of the Saskatchewan and Dakota regions, or T.fuscus and 

 T. fossor of the Canadian Zone areas of the Rocky Mountains. Some 

 interior vafiey forms also have extensive ranges, but to the south 

 and west, as the country becomes more broken and irregular, the 

 recognizable forms of TJiomomys become more numerous and local. 

 California leads in the number of species, partly from its size and 



1 N. Am. Fauna Xo. 8, 24, 1895. 



2 Ibid., 28. 



