88 THE EVOLUTION THEORY 



their explanation in the slow and gradually cumulative effects of 

 natural selection cannot be disputed ; it is beyond doubt that they 

 cannot be explained, so far as we know, in any other way. 



If, however, it were possible for a species of butterfly living in 

 the forest and among leaves to become, through natural selection, 

 in any degree, and in a continually increasing degree, like a leaf, 

 surely many insects living in the woods, and especially in the tropical 

 woods, would also have followed such an advantageous path of 

 variation — at least, so we should be inclined to think. And this is 

 indeed the case; numerous insects, of different orders, if they are 

 as large as a leaf, have taken on the colour, form, and usually also 

 the markings, of a leaf. Thus green and also decaying and dead 

 leaves are most realistically imitated by many tropical Locustidse. 

 Besides Tropidoderus, figured on p. 79, a Pterochroa of South Brazil 

 affords a particularly fine illustration of this, for not only does the 

 ground-colour, brown or green, harmonize with that of a dead or 

 fresh leaf, but, at the same time, all sorts of details are marked on the 

 insect, which help to heighten the deceptive impression. Even the 

 outline of the wings is leaf -like, and leaf- veins are marked on the wing- 

 covers with the most beautiful distinctness, and finally there is, 

 especially in the light-green individuals, a spot at the wing tip which, 

 by means of a mixture of brown, yellow, reddish, and violet colour- 

 tones, mimics a decaying spot with astonishing fidelity. Here, again, 

 the origin of this special adaptation can be clearly recognized, for the 

 vaguely concentric arrangement of the colours indicates that, in 

 the ancestors of the species, an eye-spot had occurred on this area, 

 of the same kind as we still see on the posterior wing, which is 

 covered in the resting position. Thus we can again look back on the 

 history of the species and conclude that the dissolution and degenera- 

 tion of the eye-spot began at the time when the leaf resemblance was 

 evolved, and this was probably caused by some change of habitat, 

 which we can now no longer guess at. 



Many species of leaf -like Orthoptera, both in the Old and New 

 World, have tough, green, parchment-like wing-covers which bear a 

 remarkable resemblance to the thick Magnolia-like leaves of tropical 

 plants. Along with these we must also mention the 'walking leaf/ 

 which has been well known for centuries. In its case, not the wing- 

 covers alone, but the head and thorax, and even the legs, are of the 

 colour and shape of a leaf. 



The stick-insects, too, must not remain unnoticed ; those quaint 

 inhabitants of warm countries, whose elongated brown body looks like 

 a knotted twig, and whose long legs, likewise stick-like, are stretched 



