Evidences of Theory of Natural Selection. 321 



landscape. In all such cases, of course, there has 

 been a deviation from the normal type in respect 

 of colour, with the result that the inherited instinct 

 is no longer in tune with the other endowments of 

 the animal. Such a variation of colour, therefore, 

 will tend to be suppressed by natural selection ; while 

 any variations which may bring the animal still more 

 closely to resemble its habitual surroundings will be 

 preserved. Thus we can understand the truly 

 wonderful extent to which this principle of protective 

 colouring has been carried in many cases where the 

 need of it has been most urgent. 



Not only colour, but structure, may be profoundly 

 modified for the purposes of protective concealment. 

 Thus, caterpillars which resemble twigs do so not 

 only in respect of colour, but also of shape ; and this 

 even down to the most minute details in cases where 

 the adaptation is most complete : certain butterflies 

 and leaf-insects so precisely resemble the leaves upon 

 which, or among which, they live, that it is almost 

 impossible to detect them in the foliage not only 

 the colour, the shape, and the venation being all 

 exactly imitated, but in some cases even the defects 

 to which the leaves are liable, in the way of fungoid 

 growths, &c. There are other insects which with 

 similar exactness resemble moss, lichens, and so forth. 

 A species of fish secures a complete resemblance to 

 bunches of sea-weed by a frond-like modification 

 of all its appendages, and so on through many other 

 instances. Now, in all such cases where there is so 

 precise an imitation, both in colour and structure, 

 it seems impossible to suggest any other explana- 

 tion of the facts than the one which is supplied by 



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