I 
90 R. COLLETT. i [No. 3 

Such *prolific” years amongst mammals are most conspicu- 
ous in the lesser species, especially amongst those who, likewise, 
under normal cireumstances, belong to the more productive groups. 
Thus (in our regions) these occur in several species of rodents, 
occasionally also amongst the Sortcidae, but are least perceptible in 
species with scant powers of reproduction (such as the Chiroptera). 
Such prolific years are especially well known in the sub- 
family Arvicolimi, and there are none of the species of this group 
which appear in Norway, who have not, at irregular intervals 
of years, had abnormal increases. In the sub-family Murimi the 
same is the case with Mus sylvaticus. In none of the species is 
this abnormal power of reproduction more qualified to arouse 
attention than in M. lemmus, as it never assumes such vast 
proportions in others as it does in the lemming, whilst at the 
same time, the swarms occupy far greater areas than any of 
the other species, the latter being, on the whole, tied to more 
circumscribed localities. 
6. Contemporary Prolific Years in Other Species. 
Such prolific years commonly occur simultaneously in several 
of the kindred species. In all those years in which over- 
production and migration takes place amongst the M. lemmus, 
an abnormal increase, likewise, almost invariably occurs in the 
numbers of one or more species of Arvicola which inhabit the 
same, or adjoining regions. OQccasionally also, å similar increase 
takes place amongst species in tracts that are widely separated. 
Thus, during a *lemming year,” in our southern mountain 
regions Å. ratticeps as å rule likewise appears in multitudes; 
and occasionally, in such a year, Å. gregarius or Myodes scmisti- 
when autumn set in, their emigration cowmmenced on a large scale. 
In the great migratory year of 1891, the spring, too, was exception- 
ally late and cold. 


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