THE COLOEATIOX OF ANIMALS 89 



out irregularly at different angles to the body, and usually remain 

 motionless when the insect is resting. These creatures are vegetarian, 

 and generally keep so still, that even the naturalist who is on 

 the look-out for them may easily overlook them. Even such an 

 experienced student of insects as Alfred Russel Wallace was deceived, 

 for a native of the Phillipines once brought him a specimen as a 

 ' walking-stick ' insect, which he rejected, saying that this time it was 

 no animal but really a twig, until the native showed him that it was 

 an insect whose likeness to a twig was increased by the fact that it 

 bore on its back a ragged green growth, which looked exactly like 

 a liverwort (Jungermannia), which occurs on the twigs of the trees 

 in that region. 



We must also notice here the thorn-bugs, which are numerous on 

 the prickly shrubs of tropical deserts and plateaux, especially in 

 Mexico. These bear on the relatively very small body two or three 

 large spines, which make them look like a part of the thorny bush on 

 which they sit. But this masking by mimicry of thorns is not 

 confined to insects, it is seen in lizards as well, notably in Moloch 

 horridus, a lizard that lives in the Australian bush, and is covered all 

 over with thorn-like scales. 



These examples should be enough to show that mimicry of 

 the usual surroundings on the part of animals which are in need 

 of protection, or are wont to lurk on the watch for their prey, are not 

 isolated exceptions, chance resemblances, or, as they used to be called, 

 ' freaks of nature/ but that, on the contrary, they are the rule, depend- 

 ing on natural causes, and always occurring when these causes are 

 operative. That such protective resemblances seem to be much more 

 frequent in warmer climates than with us is probably a fallacy due 

 to the fact that the number of species (especially of insects) is very 

 much greater there, and that many insect types have their repre- 

 sentatives of considerable size of body, which not only makes them 

 more conspicuous to us, but makes some protective device in relation 

 to their enemies or victims much more necessary. 



But we must here take account of one more example which 

 occurs in our fauna in many modifications: the caterpillars of 

 Geometridaj. Many of these soft and easily injured caterpillars 

 resemble closely, in colour and shade, the bark of the tree or shrub on 

 which they live (Fig. 17). At the same time they have the habit, 

 when at rest, of stretching themselves out straight and stifi", so that 

 they stand out free, at an acute angle from the branch, thus seeming 

 like one of its lateral twigs. In many species the resemblance is 

 heightened by the extraordinary pose of the head (A^) and of the 



