
99 BR. COLLETT, 
diaca even from 8 to 10, whilst the normal number is 5 or 6. 
That this is not exclusively due to the facility of obtaining food 
would appear from the cirecumstance that, in some lemming 
years Nyctea scandiaca, or other birds of prey (especially Asio 
brachyotus and Falco tinmunculus), have oceasionally appeared in 
numbers on stretches of mountains, although no multitudinous in- 
crease of Myodes has taken place in those parts. 
It is all the more remarkable that the years may be especi- 
ally favourable to the development of various forms of animals, 
of which many are, apparently, quite independent of each other. 
During many lemming years the game-birds (Tetraomdae) 
and the hare, have increased in unusual numbers; and at the 
same time Sorex araneus has appeared in multitudes, while 
large tracts of the mountain-birch woods (Betula odorata, var. 
alpigena) were denuded of leaves by the larvæ of å geometer 
(Cidaria dilutata)!. 
This simultaneous abnormal increase of various species has 
been observed, and also oceasionally referred to, by older authors. 
Thus, in one of the written descriptions of the parish of Ranen, 
in Nordland, by Parson Heltzen, which, in manuscript, is pre- 
served in the Museum at Bergen (written 1842), it is mentioned 
that, during the migratory year 1839, about 300 lemmings, 
80 Sorices, and about 100 field or wood mice (possibly Arvi- 
cola glareolus or Å. ratticeps) were killed on one occasion in 
a small field at Hemnæs parsonage. Subsequently Wolley* 
mentions that, during a migration of lemmings which he wit- 
nessed in Finmarken i 1853, Arv. amphibius and Å. ratticeps 
simultaneously appeared in great numbers, besides two or three 
other arvicoles (probably Å. rufocamus and Å. gregarwus). They 
all disappeared at the same time in the spring of 1854. 

1 The said Geometer's larva is one of the greatest enemies of our al- 
pine regions. When the foliage of the birch is consumed, it seizes 
upon almost all the other species of plants within that region, such 
as Beluta nana, Vaccinium uliginosum, Myrtillus myrtillus, and even 
Aconitum septemtrionale, besides the various kinds of grass. 
? Forh. Skand. Naturforskeres 7de Møde. Christiania 1856, p. 216. 
