1895.] MYODES LEMMUS. 19 

to å sudden combination of circumstances which are favourable 
to pairing and rearing tlie young (the thriving of the offspring), 
but the nature of these circumstances is still inexplicable to us. 
These temporary augmentations may be traced, amongst 
many species of small mammals and various kinds of birds 
especially, but are, above all, conspicuous in many sorts of in- 
sects. Several of these belong to the very same regions as those 
inhabited by M. lemmus. 
In M. lemmus, it would appear that the circumstances which 
give rise to the increase, act in various ways. On the one hand 
a greater number of connections are formed than in ordinary 
years, whilst the reproductory powers are simultaneously in- 
ereased to such an extent, that litter follows upon litter through- 
out the entire summer; on the other hand the young possess an 
exceptional power of resisting the diseases to which they (and 
other animals) are usually exposed, and thus, each set arrives 
at maturity, comparatively speaking in its entirety. 
It is probable that the abnormal increase which takes place 
during occasional years, will prove to be due to the activity of 
certain bacteria whose characteristics we do not know, and 
whose presence even, we cannot establish at present). 
That the climatic conditions are, to å great extent, condueive 
to the increase, is probable, although it is not always easy to 
indicate the connection. It can thus be proved that Myodes 
have had prolific years just after cold and wet summers*. 
I Provided one could assume that all life was dependent on the 
mutual relations of certain bacteria, of which those destructive 
to existence are, under normal conditions, kept in the balance 
by those that support vitality, it is likewise presumable that, under 
conditions which are unknown to us, the latter group might tem- 
porarily gain an ascendency, which would result in an excess of 
population. 
2 During the migration of 1862, in the Gudbrandsdal mountains, Mr. 
Barth (Inspector of Woods and Forests), who there observed the 
phenomena of the increase and migration from their commence- 
ment, thus states that, the weather, from the middle of June to the 
middle of August was incessantly cold and rainy; yet, nevertheless, 
all the mountains in that district swarmed with their myriads, and, 
23 
