NO. 1101. PROCEEDINGS OF THE NATIONAL MUSEUM. 17 



soiitli, as is the case with so many other Xorth American mammals. 

 West of the Alleghanies, however, Scalops maintains large size over a 

 wide area, without regard to latitude. On tlie west coast, Scapanus 

 ealifornicus decreases gradually in size from north to south, as does 

 Scalops on the east coast. 



In cases where the conditions are simple, size may be supposed to 

 depend upon the amount of energy expended in obtaining a given 

 amount of food.^ This relation would at first afi'ect the individual, and 

 finally the race. 



A light, moist, fertile soil would contain the greatest number of earth- 

 worms and beetles and be most easily worked; hence the moles would 

 be large. 



Where the soil was dry and hard, worms and insects would be fewer 

 and the exertion of finding them great; here we should expect to find 

 small moles.^ 



As we have shown above, these anticipations are fulfilled in the dif- 

 ferent parts of the range of Scalops and Scapanus. 



One feature of the moles as regards size remains to be mentioned. 

 The largest moles are found at the northern boundary of the range in 

 the case of Scalops and Scapanus. There is no gradual diminution as 

 the limit is reached.^ 



It is not entirely clear what is the cause of this phenomenon. We 

 might suppose, in accordance with the terms of Br. Merrianrs theory, 

 that although food was most plentiful near the northern limit, and hence 

 size large, a point was suddenly reached where the total amount of heat 

 was insufficient for reproduction, and the moles, though abundantly 

 nourished, ceased to propagate. This at first would seem to be a satis- 

 factory explanation; but that it does not really explain the matter will 

 appear from a consideration of the effect of cold on plants. The wil- 

 lows, which are goodly trees in the temperate zone, grow smaller and 

 smaller northward, till in the iV^rctic they are dwarfed to such an extent 

 that they will rise only a few inches above the ground, or even trail 

 along it like a vine; yet they flower profusely and their catkins are of 

 a very large size. We might expect to find something anah»gous in 

 connnection with the moles, but, as we have seen, quite the reverse 

 occurs. They cease when their size is at a maximum. 



'"The available supply of assimilable matter being the same, and other conditions 

 not dissimilar, the degree of growth varies according to the surplus of nutrition over 

 expenditure — a generalization which is illustrated in some of the. broader contrasts 

 between dift'erent divisions of organisms, and is a direct corollary from the resistance 

 of force." Spencer, Principles of Biology, I, 18>!!1, p. 131. 



■In this connection it may be interesting to note the fact that in the hard clay 

 soil of the District of Columbia I find that the earthworms are collected in summer 

 in colonies under stones or cow dung, leaving large areas unoccupied, through which 

 a mole might tunnel in vain, expending a large amount of energy, with no return. 



^Leaving out of consideration the small Scapanus orarius which lives with the large 

 Scapanus townsendi at the northern part of its range, but does not appear to intergrade 

 with it. 



Proc. N. M. vol. xix 2 



