Xvi The Naturalist in Nicaragua 
so far as it provides concealment from snakes, which are 
particularly fond of these frogs, and aggressive in that it 
allows flies and other insects to approach without suspicion. 
There may be either General Resemblance to surrounding 
objects or Special Resemblance to definite objects. The 
plain sandy colour of desert animals, the snow white of the 
inhabitants of the arctic regions, the inconspicuous hues 
of nocturnal animals, the stripes of the tiger and the zebra, 
the spots of the leopard and the giraffe have all a cryptic 
effect which at a very short distance renders the creatures 
invisible amid their natural surroundings. Nor is it neces- 
sary in order to attain this invisibility that the colouring 
should be really dull and plain. It all depends upon the 
habitat. Mr. Wallace has described ‘“‘a South American 
goatsucker which rests in the bright sunshine on little bare 
rocky islets in the upper Rio Negro where its unusually 
light colours so closely resemble those of the rock and sand 
that it can scarcely be detected till trodden upon.” A 
little observation will supply large numbers of instances of 
such protective colouration. 
It is, however, in the insect world that this principle of 
adaptat on of animals to their environment is most fully 
and str.-ingly developed. ‘“‘ There are thousands of 
species of insects,’’ says Mr. Wallace again, ‘‘ which rest 
during the day clinging to the bark of dead or fallen trees; 
and the greater portion of these are delicately mottled 
with grey and brown tints, which though symmetrically 
disposed and infinitely varied, yet blend so completely 
with the usual colours of the bark, that at two or three 
feet distance they are quite undistinguishable.”’ 
In protective resemblances at their highest state of 
perfection the colouring is not constant but, as Professor 
Poulton puts it in his delightful book on The Colours of 
Animals, ‘“‘can be adjusted to harmonise with changes in 
the environment or to correspond with the differences 
between the environment of different individuals.”” The 
seasonal change of colour in northern animals is a well- 
known instance of the former, and the chameleon’s altera- 
tions of hue of the latter. 
Besides General Resemblance, in which the general 
effects of surrounding colours are reproduced, we have 
Special Resemblance, in which the appearance of a par- | 
