340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



Mississippi River. (The genus Citellus of ground squirrels furnishes 

 the exception; two species of that genus go as far east as Indiana.) 

 From a possibly economic bearing, with respect to digging, moles 

 and earthworms, though ecologically not at all homologous, seem to 

 take the place in the far eastern States that the burrowing rodents 

 take here. 



The line of demarcation, eastward of which the burroAving type of 

 rodent begins to disappear, is, approximately, the one-hundredth 

 meridian. In other words, there is a north and south line of transi- 

 tion between two major faunal regions which roughly coincides with 

 this meridian. The limitations of the animals in question undoubt- 

 edly have to do with the physical peculiarities of the regions east 

 and west of the one-hundredth meridian. These peculiarities involve 

 differences in atmospheric humidity, in rainfall, and, of seeming 

 major importance, the sharp alternation of dry and wet season which 

 occurs to the westward. Linked up with these conditions, there is, 

 probably, in the West a relatively greater abundance of plants with 

 nutritious roots or thickened underground stems (corms, rootstocks). 



With regard to abundance of mammals in California by indi- 

 viduals, I have made numerous estimates. It proves to be highly 

 variable, all the way from zero per acre, as on parts of the floor of 

 Death Valley, up to 120 per acre, as in certain parts of the San 

 Diegan district. I have figured a conservative average throughout 

 the entire State to be 20 mammals of all sorts per acre, so that the 

 total mammal population in California, at the period of the year 

 just before the breeding season, when the population is at its lowest 

 ebb, is 2,000,000,000. Estimating further, on the basis of the results 

 of trapping and of field observation in different parts of the State, 

 I find that the population of burrowing rodents is at the very mini- 

 mum one-half that of all the mammals, which would thus be in the 

 aggregate about 1,000,000,000. 



Even cursory observation suffices to establish to one's satisfaction the 

 relative abundance of such burrowing types of mammals as ground 

 squirrels, pocket gophers, and kangaroo rats. Along any of the 

 railroads or highways, interminable stretches of the right of way, 

 or of the adjacent plains or mountain slopes, show a profusion of 

 the so-called " workings " of these animals — mounds, trails, mouths 

 of burrows (open or closed), caved-in burrows, winter earth cores, 

 and the like. If a person starts out on foot, he will inevitably " fall 

 into" subterranean runways: every little while he steps through 

 into some tunnel or cavern. The surface of the ground is seen to 

 be nearly covered with disturbed soil showing footprints of these 

 animals, especially if the season be the dry one. The vegetation 

 will show abundant evidence of having been foraged upon by ro- 

 dents. 



