62 The Selection Theory 



be eliminated by selection, and that no mammals would be able to 

 live there at all. But in most cases a certain percentage of animals 

 resists these strong influences, and thus selection secures a foothold 

 on which to work, eliminating the unfavourable variation, and estab- 

 lishing a useful colouring, consistent with what is required for the 

 maintenance of the species. 



Everything depends upon adaptation ! We have spoken much 

 of adaptation in colouring, in connection with the examples brought 

 into prominence by Darwin, because these are conspicuous, easily 

 verified, and at the same time convincing for the theory of selection. 

 But is it only desert and polar animals whose colouring is determined 

 through adaptation ? Or the leaf-butterflies, and the mimetic species, 

 or the terrifying markings, and "warning-colours" and a thousand 

 other kinds of sympathetic colouring ? It is, indeed, never the colour- 

 ing alone which makes up the adaptation ; the structure of the animal 

 plays a part, often a very essential part, in the protective disguise, 

 and thus many variations may cooperate towards one common end. 

 And it is to be noted that it is by no means only external parts that 

 are changed ; internal parts are always modified at the same time — 

 for instance, the delicate elements of the nervous system on which 

 depend the instinct of the insect to hold its wings, when at rest, in 

 a perfectly definite position, which, in the leaf-butterfly, has the 

 effect of bringing the two pieces on which the marking occurs on 

 the anterior and posterior wing into the same direction, and thus 

 displaying as a whole the fine curve of the midrib on the seeming- 

 leaf. But the wing-holding instinct is not regulated in the same way 

 in all leaf-butterflies; even our indigenous species of Vanessa, with 

 their protective ground-colouring, have quite a distinctive way of 

 holding their wings so that the greater part of the anterior wing- 

 is covered by the posterior when the butterfly is at rest. But the 

 protective colouring appears on the posterior wing and on the tip 

 of the anterior, to precisely the distance to which it is left uncovered. 

 This occurs, as Standfuss has shown, in different degree in our two 

 most nearly allied species, the uncovered portion being smaller in 

 V. urticae than in V. polycMoros. In this case, as in most leaf-butter- 

 flies, the holding of the wing was probably the primary character ; 

 only after that was thoroughly established did the protective mark- 

 ing develop. In any case, the instinctive manner of holding the 

 wings is associated with the protective colouring, and must remain as 

 it is if the latter is to be effective. How greatly instincts may change, 

 that is to say, may be adapted, is shown by the case of the Noctuid 

 " shark " moth, Xylina vetusta. This form bears a most deceptive 

 resemblance to a piece of rotten wood, and the appearance is greatly 

 increased by the modification of the innate impulse to flight common 

 to so many animals, which has here been transformed into an almost 



