202 ANIMAL LIFE 
advantage. Such an advantage may be swiftness of move- 
ment, or unusual strength or capacity to withstand unfa- 
vorable meteorological conditions, or the possession of such 
color and markings or peculiar shape as tend to conceal the 
animal from its enemies or from its prey. Resemblances 
may serve the purpose of aggression as well as protection. 
In the case of the polar bears and other predaceous ani- 
mals that show color likenesses to their surroundings, the 
resemblance can better be called aggressive than protective. 
The concealment afforded by the resemblance allows them 
to steal unperceived on their prey. This, of course, is an 
advantage to them as truly as escape from enemies would be. 
We have already seen that by the action of natural 
selection and heredity those variations or conditions that 
give animals advantages in the struggle for life are pre- 
served and emphasized. And so it has come about that 
advantageous protective resemblances are very widespread 
among animals, and assume in many cases extraordinarily 
striking and interesting forms. In fact, the explanation 
of much of the coloring and patterning of animals depends 
on this principle of protective resemblance. 
Before considering further the general conditions of 
protective resemblances, it will be advisable to refer to 
specific examples classified roughly into groups or special 
kinds of advantageous colorings and markings. 
108. General protective or aggressive resemblance.—As 
examples of general protective resemblance—that is, a gen- 
eral color effect harmonizing with the usual surroundings 
and tending to hide or render indistinguishable the animal 
—may be mentioned the hue of the green parrots of the 
evergreen tropical forests; of the green tree-frogs and tree- 
snakes which live habitually in the green foliage; of the 
mottled gray and tawny lizards, birds, and small mam- 
mals of the deserts; and of the white hares and foxes 
and snowy owls and ptarmigans of the snow-covered arc- 
tic regions. Of the same nature is the slaty blue of the 
