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1895.] MYODES LEMMUS. | 31 
building, probably belonged to these young individuals who 
were breeding for the first time. 
Exactly therefore as is the case with M. lemmus, the old 
individuals (which had wintered) produce, during å prolific year, 
up to four litters in one summer, and the young of the first set 
at least one litter. 
Furthermore, the young of the parent stock numbered 5 to 6, 
occasionally 7 to 8. One female contained 10 embryos (of the 
third, or possibly the fourth litter). The yearlings (of the first 
litter) generally contained only 3 or 4, occasionally 5 embryos. 
But notwithstanding that Å. ratticeps on the whole exhi- 
bited åa productiveness not inferior to that of M. lemmus, their 
numbers were nevertheless considerably less. Partly were the 
localities, inhabited by them (the grass grown spots), far more 
confined in extent than those frequented by M. lemmus; partly 
does Å. ratticeps not possess the latters” faculty of bringing 
itself to notice, besides which its desire to emigrate is far less 
prominent. 
8. The Causes of the Migrations. 
During prolific (not in ordinary years) å wandering: tendency 
arises in many of the lesser rodents and other animals, and as 
Soon as the multitudes become palpably greater than the rule, 
the emigration commences. 
This phenomenon (the so-called *migratory instinct*) is, 
however, quite unequally developed amongst the various species. 
Thus, as previously stated, Arvicola ratticeps lives approximately 
under similar conditions as M. lemmus; it inhabits an adjoining 
tract, has prolific years simultaneously with it, and both species 
meet at such times; but *the migratory instinct” is, in it, but 
slightly developed. 
