FERKIS — CONCERNING LICE 
17 
tic, although these seals have not had the advantage of any modern 
Panama Canal. 
Another aspect of the same phenomenon appears when we consider 
the distribution of related species of lice. The nearest relative of the 
louse of the domestic pig is found upon the wart hogs and bush pigs 
of Africa. The Sciuridse may boast of the possession of three genera 
of lice of which two are known only from squirrels, upon which they 
may be found wherever squirrels are found. The other genus is found 
also upon the Murids. The lice of the monkeys find their nearest 
relatives in the lice of man. 
It is in fact a general rule — to which there are by all means excep- 
tions — that a single species of louse will be found only on closely re- 
lated hosts and that related species of lice will be found on related 
hosts. 
I have said that there are exceptions to this rule, and some of them 
are glaring enough. One such exception is the occurrence upon Sorex 
araneus (the only Soricid that is known to possess lice) of a species that 
is identical with or at least very closely related to a species that occurs 
also on the domestic rat. Another is the occurrence upon the domestic 
dog and also upon foxes of a species the nearest relative of which is 
another species found upon the domestic sheep. And what are we to 
say of the louse which appears to be normal to kangaroos but which 
has once been taken from a dog in Africa, twice from dogs in, and once 
from a coyote near, San Francisco and once from a man in the Federated 
Malay States? 
Aside, however, from these and some other disturbing exceptions the 
facts are fairly harmonious and their broader implications are fairly 
evident. It would seem that the normal occurrence of a common para- 
site species upon two or more geographically separated but genetically 
related host species may adequately be accounted for only by the as- 
sumption that the common ancestor of these hosts had upon it this 
same parasite species. In a manner of speaking the occurrence of the 
parasite upon these different host species is a part of their racial in- 
heritance. It requires only an extension of this theory to account as 
well for the occurrence of related parasites upon related hosts. In 
fact it appears that in general the problem of the geographical distri- 
bution of the parasite species is to a very considerable degree bound up 
with the problem of the relationships of their hosts. 
Now if we attempt to push this theory to its apparently logical con- 
clusion we shall be led to the assumption that the original ancestor of 
all the mammals had upon it the original ancestor of all the lice. I 
JOURNAL OP MAMMALOGY, VOL. 3 , NO. 1 
