CHAPTEE XIX, 
RODENTS. 
The rodents, psychologically considered, are, of all orders 
in the animal kingdom, most remarkable for the differ- 
ences presented by constituent species. For while the 
group contains many animals, such as the guinea-pig, 
whose instincts and intelligence cannot be said to rise 
above the lowest level that obtains among mammalian 
forms, it also contains other animals with instincts as 
remarkable as those of the squirrel, intelligence as con- 
siderable as that of the rat, and a psychological develop- 
ment as unique as that of the beaver. In no other group 
of animals do we meet with nearly so striking an exempli- 
fication of the truth that zoological or structural affinity 
is only related in a most loose and general way to psycho- 
logical or mental similarity. Up to a certain point, 
however, even here we meet with an exemplification of 
what I may call a complementary truth, namely, that 
similarity of organisation and environment is in a general 
way related to similarity of instincts (though not neces- 
sarily of intelligence). This is obviously the case with 
the habit from which the order takes its name ; for 
whether the instinct of gnawing is here the cause or the 
result of peculiar organisation, the instinct is unquestion- 
ably correlated with the peculiarity. And similarly, though 
less obviously, is this the case with the instinct of storing 
food for winter consumption, which is more prevalent 
among the rodents than in any other order of mammals — 
tats, mice, squirrels, harvesters, beavers, &c., all mani- 
festing it with remarkable vigour and persistency. Here 
we probably have a case of similar organisation and en- 
vironment determining the same instinct; for the latter 
