284 MENTAL EVOLUTION IN ANIMALS. 



of its genial influence. As little by little the ocean encroached 

 on the land, the same advantages would remain, as in fact 

 they do to this day." 



To this ingenious theory dissent is expressed by another 

 gentleman who ]ias had a very large experience in observing 

 these migrations, namely Mr. Eobert CoUett, of the University 

 Museum, Christiania.* His view is that in years when re- 

 production is excessive, multitudes of individuals are led by 

 hunger, as well as by " the natural desire to wander possessed 

 by this species," to overflow the limits of their plateaux 

 home, and spread out " over an area that is considerably 

 larger than obtains in any other of the species under similar 

 circumstances." As breeding continues throughout the 

 wandering, in cases where in two or three succeeding years 

 the production of young has been excessive, " the masses 

 are incessantly pushed towards the sides of the fells ; and 

 the migration becomes an overrunning of the lower and far 

 remote portions of the country, as the individuals gradually 

 penetrate further in search of localities suitable to their 

 habits (and which are capable of giving them a permanent 

 subsistence), until they are stopped by the sea or destroyed 

 in some other manner." 



Looking to Mr. Collett's large experience on the subject, 

 as well as to tlie intrinsically probable nature of his views, 1 

 think we may most safely lend countenance to the latter. 

 The most important point of difference between Mr. Crotch 

 and Mr. CoUett has reference to a question of fact. For 

 while Mr. Crotch states that the migrations are made west- 

 wards without reference to the declivities of the country, 

 Mr. CoUett is emphatic in saying that " the wanderings take 

 place in the direction of the valleys, and therefore can branch 

 out from the plateaux in any direction." If this is so, there 

 is an end of Mr. Crotch's theory, and the only difficulty left 

 to explain would be why, when the lemmings reach the sea, 

 they still continue on their onward course to perish in their 

 multitudes by drowning. The answer to this, however, is 

 not far to seek. For their ordinary habits are such that when 

 in their wanderings they come upon a stream or lake, they 

 swim across it ; and therefore when they come upon the coast 

 line it is not surprising that they should behave in a similar 

 manner, and, mistaking the sea for a large lake, swim per- 



* Linn. Soc. Jour., voL xiii, p. 327, et seq. 



