NO. 1101. PROCEEDINGS OF THE NATIONAL MUSEUM. H 



in this way, the distribution of the genus is not exceptional, but we 

 shall still be obliged to inquire why it occupies only so small a part of 

 the cold temperate or boreal subregion. 



The genus Farascalops, as already stated, has a similar distribution 

 to that of Condylura, but, so far as known, extends neither so far north 

 nor so far west. It appears to remain entirely within the Alleghaniau 

 fauna (or transition zone),' but does not occupy the whole of the 

 area. 



The two western genera, Scapanus and N'eurotrichus, are found exclu- 

 sively in the Pacific Coast States, with the addition, as far as known, 

 of only a small area immediately northward, and in the case of one 

 specimen, the San Pedro Martir Mountains in Lower California. 



In the Pacific States these genera are confined to the area called 

 cold temperate by Dr. Allen, or the boreal and transition areas of Dr. 

 Merriam. In other words, they inhabit only the higher and better 

 watered regions in the south, siDreadiugout more and descending lower 

 in the north. So far as known, NeUrotriclius does not range south of 

 San Francisco Bay, and hence does not fill out the whole of the south- 

 ern extension of the cold temperate, as does Scapanus, 



Having discussed in general terms the distribution of the several gen- 

 era, and pointed out the peculiarities of their distribution, it remains 

 to consider the causes of these peculiarities. In other words, it is desir- 

 able to inquire into the causes which limit or favor the distribution of 

 the moles. 



It is generally agreed, as we have seen, that temperature is the most 

 potent agent in afl:ecting distribution, and that next to it stands 

 humidity. In the case of the moles, however, it would appear that 

 humidity is at least as eifective as temperature. This is evidenced in 

 the case of Scalops, which occupies the Alleghanies, but stops at the 

 plains. The case of Scapanus is probably also one in point, though the 

 conditions here are not so simple. How, we may inquire, do differ- 

 ences in humidity affect the moles'? Probably not to any great extent 

 directly, but rather in connection with food supply. The common mole 

 {Scalops), at least, lives almost exclusively on earthworms and burrow- 

 ing insects. It should therefore abound and flourish most where these 

 animals are most abundant and most easily obtainable. The latter 

 condition is equally as imi^ortant as the first, for while the worms may 

 be abundant they may not be readily obtainable, as for example in 

 rocky areas. If the ground is hard, the mole has great difficulty in 

 traversing it, and he may not be able to sustain himself even in ground 

 which contains more or less of worms and beetles. In regions having 

 a dry summer, during which the ground is parched and made hard, he 

 may subsist only with much difficulty. It is probably a debatable 

 question, however, whether the mole can not dig wherever the worm 

 can bore, but it is quite certain that worms could continue to exist in 

 small colonies where the difficulties of finding them would tax too 

 greatly the powers of the mole. 



