MR. R. COLLETT ON MYODES LEMMUS IN NORWAY. 331 



years, and consequently favouring their pairing and bringing up 

 of their young ; the result of which is far greater numbers of 

 families and larger numbers of young in each set than in ordinary 

 years. Parallel instances of these circumstances are clearly exhi- 

 bited in the enormous increase in number of the larvae of Lepi- 

 doptera in certain years ; but I may add that it is apparently im- 

 possible to bring any direct proof of the true reason of that fact, 

 and one can only form theories about the point. It is one of the 

 many questions in the economy of nature that probably never 

 will be solved ; we only know that it is a fact. 



It is, however, worthy of remark that these circumstances 

 are always at the same time equally favourable to the increase of 

 nearly allied species which are quite independent of one another. 

 Thus in the years when M. lemmus migrates I have observed 

 there is certain to be an increase above the normal number of 

 one or more species of rats and mice (Muridae), — in the southern 

 parts of Norway chiefly the following Voles, Arvicola gregarius, 

 A. mnphibius, and A.\ratticeps ; in Einmark, A. rufocanus, A. rat- 

 ticeps, and A. rutilus ; further the Harvest-Mouse, Mus sylvaticus, 

 and possibly other small Rodents. The increase, however, in 

 these species never attains the magnitude that it does in Myodes 

 lemmus ; and none of these, as mentioned above, possess the mi- 

 gratory instinct in the same degree as the Lemming. 



Furthermore, almost every " year of migration " and " breed- 

 ing " that has taken place in the tracts below the Dovrefjeld and 

 bordering the Trondhjemsfjord the Shrews (Soridse), the Hare, 

 and most of the Grouse tribe (Tetraonidse) have at the same time 

 been unusually numerous. Besides there are different other 

 animals tlmt are generally considered to a certain extent to de- 

 pend upon M. lemmus, as they are found in larger numbers just 

 in the years when the Lemmings are migrating, as it is supposed, 

 in consequence of the superfluity of food ; for instance, the 

 two species of Mustela, Vulpes lagopus, Nyctea scandiaca, Asio 

 accipitrinus, Buteo lagopus, and others. In my opinion this 

 facility in obtaining superfluity of food is not the only or true 

 reason for the increase of numbers of these animals. 



I have elsewhere stated *, Nyctea scandiaca increased very nu- 

 merously during the summer of 1872 in many districts where there 



* "Remarks on the Ornithology of Northern Norway" (Forb. Vidensk. 

 Selsk. Christiania, 1872, p. 223). 



