THE INSTINCTS OF ANIMALS 153 



From -what I have already said at least so much must be clear, 

 that nothing, in principle, stands in the way of referring instincts to 

 selection, since their very essence is their adaptation to an end, and 

 such purposive changes are precisely those that are preserved in the 

 struggle for existence. It might, however, be supposed that in all 

 this the principle of use and disuse also had a share, and that without 

 it no changes in instincts could have come about. 



There are, however, numerous instincts in considering which this 

 can be entirely excluded. 



At an earlier stage we discussed in detail the protective colourings 

 which secure insects, and especially butterflies, from extermination by 

 their numerous enemies, and it was mentioned that this was always 

 accompanied by corresponding instincts, without which the protective 

 colouring and the deceptive form would have profited nothing, 

 or at any rate not nearly so much. If the caterpillar of the 

 Gatocala sponsa, which resembles the bark of an oak so deceptively, 

 did not possess at the same time the instinct to creep away from the 

 leaves and hide in the clefts of the bark on the trunk of the oak-tree, 

 its disguise would be of very little use to it ; and if the predatory and 

 grass-coloured praying mantis was not impelled by instinct to lie in 

 wait among the grass for its prey, instead of pursuing it, it would 

 rarely succeed in seizing any of its victims, because of its somewhat 

 leisurely mode of movement. This adaptation of the instincts to the 

 protective colouring is carried into the most minute and apparently 

 trifling details. Thus diflerent observers have established the fact 

 that the nauseous, sometimes even poisonous, butterflies, which are 

 distinguished by their glaring or sharply contrasted colour-pattern, 

 are all slow fliers. This is the case with the Danaides and Euploeides 

 of the Old World and the Heliconiides of the New ; many of their 

 mimetic imitators also fly slowly. 



If we inquire how this instinct of fluttering, careless flight has 

 come to be, we may leave habit as priTnum -movens out of the 

 question altogether, for there are no external conditions which could 

 have induced the butterfly to take to slower flight than its ancestors 

 exhibited. That it is now advantageous for it — since it acts as a 

 signal of its nauseousness — to be as clearly seen and recognized as 

 possible can exercise no direct influence on its manner of flight, since it 

 knows nothing about it. Even if we assume that individual varia- 

 tions cropped up which had an instinct for slower flight, there would 

 still, without selection, be no reason why this variation in particular 

 should multiply, still less why the originally slight slowing of the 

 flight should become more marked in the course of generations. On 



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