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 znstinct 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-butterfliés; 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. polychloros. 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 
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