12 REVISION OF AMERICAN MOLES— TRUE. vol.xix. 



I have been unable as yet to ascertain anything of a definite charac- 

 ter in regard to the distribution of earth worms in the United States, 

 but it is a matter of common experience that such worms, together 

 with soft grubs, etc., are found in abundance only in comparatively 

 moist and light ground. Hence, we may conclude that if moles are 

 not as a rule found in arid regions, it is chietiy, and perhaps solely, 

 because suihcient food is unobtainable in such places. If moles are not 

 found in the hot Sacramento Valley, as appears to be the case, and in 

 the arid portions of southern California, it is not on account of the high 

 temperature, but of the aridity, which causes the lack of an abundant 

 food supi)ly.^ The western limitation of Scalops and the southern lim- 

 itation of Scapanus, as well as the peculiar features of the distribution 

 of the latter genus, are illustrations, in my opinion, of this proposition. 



As regards the limitation of the several genera toward the north, 

 the case is different. The western genera Scapanus and Xeiirotrichiis, 

 so far as known, stop at the Fraser River, British Columbia. In the 

 east, t^calops stops at the northern boundary of the warm temperate 

 subregion (except in the Alleghanies, as already explained), while 

 Condylura reaches to Hudson Bay. Parasealops, as far as known, goes 

 northward oidy to Maine. 



In the case of Parascalo])S^ it is perhaps useless to attempt any 

 explanation, as the amount of material at command has been small, 

 and the ajiparently limited range may be much extended by subse- 

 quent researches. At all events, as the genus Condylura reaches much 

 farther north, it will not be allowable to hold that Parasealops is limited 

 northward by lack of food supply, and there is certainly no lack of 

 humidity.^ 



In the case of the western genera, likewise, there are no known pecul- 

 iarities of food supply or humidity at the northern limit, as for example 

 at the Fraser River, which can be evoked to explain the termination 

 of the range at this point. 



In explanation of the northern limitation of the several genera, there- 

 fore, we ])robably can not do better than to make use of JVIerriam's 

 theory, which is that "the northward distribution of animals and plants 

 is determined by the total quantity of heat, the sum total of effective 

 temperatures."^ This implies that the several species of moles can not 

 reproduce their kind at more northerly points for want of a sufficient 

 amount of heat during the year. This theory does not, of course, 

 explain why different species require a varying total of heat, some more 

 and some less. Furthermore, it does not take into consideration the 

 direct effect of cold on animals, nor the effects of accumulated snow, 



'Eisen has described earthworms from Fresno, but whether from garden spots or 

 the open fields I do not know. 



-I have taken an earthworm out of the stomach of a star-nosed mole from Moose 

 Factory, Hudson Bay territory, so that there can be no question that these Avornia 

 occur there. 



3Nat. Geog. Mag., VI, p. 236. 



