1895.] MYODES LEMMUS. 51 

17. Diseases. 
Although many factors are thus unceasingly at work in the 
destruction of the wanderers, the termination of å migration 
would be prolonged for several years if it were dependent on 
such only. | 
Ås previously remarked, it can hardly be doubted but that 
the bulk of the lemmings suecumb to infectious complaints, 
which are invariably developed whenever individuals of a spe- 
cies of animal are crowded together within narrower limits than 
usual. 
This epizooty arises during migratory years, as surely on 
the mountain plateaux themselves, as in the lower regions, and 
therefore can not be solely due to the unusual conditions under 
which the emigrants exist: consequently the mountains themselves, 
immediately after the cessation of the migrations, appear to be 
as depopulated as the valleys. 
During all the great migrations that I have witnessed, I 
have noticed that, towards the commencement of the autumn, 
virtually all the old individuals have been attacked by a skin 
disease!. In Gausdal, 1891, I was enabled once more to ob- 
serve this disease, which manifested itself by leaving that por- 
tion of their bodies above the root of the tail, more or less 
bare of hair, and covered with numerous small, scaly protuberan- 
ces, the tail, at the same time, being thickened and swollen at 
the root. ; 
The old males were especially attacked by it, the females to 
a lesser extent. In young (full grown) individuals of the first 
litter it was traceable in å few males, but I did not notice it 
in any females, and never in the young of the subsequent sets*. 
1 Journ. Lin. Soe. Zoology, Vol. XIIT, p. 330 (1877); Nyt Mag. f. Na- 
turv. 27de B. 3 H., p. 226 (1881). 
2 Dr. 0. Johan-Olsen, to whom I sent a few of the affected animals, 
found the parts attacked filled with a bacteria, which he has de- 
A* 
