304 THE SIGNIFICANCE OF SEXUAL BEPRODUCTION 



result of the creeping of insects ? How can the suggestion of such 

 a cause offer any interpretation of the fact that flowers which open 

 by day are tinted with various colours, or of the fact that there is 

 often a bright or highly coloured spot which shows the way to the 

 hidden nectary ? 



There are, moreover, a large number of very striking adaptations 

 in form and colour, for which no stimulus acting directly upon the 

 organism can be found. Can we imagine that the green caterpillar 1 , 

 plant-bug, or grasshopper, sitting among green surroundings, 

 is thus exposed to a stimulus which directly produces the green 

 colour in the skin ? Can the walking-stick insect, which resembles 

 a brown twig, be subject to a transforming stimulus by sitting on 

 such branches or by looking at them ? Or again, if we consider 

 the phenomena of mimicry, how can one species of butterfly, by 

 flying about with another species, exercise upon the latter such an 

 influence as to render it similar to the first in appearance ? In 

 many cases of mimicry, the mimicked and the mimicking species 

 do not even live in the same place, as we see in the moths, flies, 

 and beetles which resemble in appearance the much-dreaded wasps. 



The interpretation of adaptation is the weak part of Nageli's 

 theory, and it is somewhat remarkable that so acute a thinker 

 should not have perceived this himself. One very nearly gains 

 the impression that Nageli does not wish to understand the theory 

 of natural selection. He says, for instance, in speaking of the 

 mutual adaptation observable between the proboscis, the so-called 

 ' tongue ' of butterflies, and flowers with tubular corolla 2 : ' Among 

 the most remarkable and commonest adaptations observable in the 

 forms of flowers, are the corollas with long tubes considered in re- 

 lation to the long " tongues " of insects, which sack the nectar from 

 the bottom of the long narrow tubes, and at the same time effect 

 the cross-fertilization of the plant. Both these arrangements have 

 been gradually developed to their present degree of complexity 

 the long-tubed corollas from those without tubes, and from those 



[ l It is now known that many such caterpillars are actually modified in colour by 

 their surroundings, but the process appears to be indirect and secondarily acquired by 

 the operation of natural selection, like that of the change of colour in the chamaeleon, 

 frogs, fish, etc. ; although the stimulus of light acts upon the eyes of the latter animals 

 and upon the skin of the caterpillar. See the seventh Essay (pp. 394-397) for a more 

 detailed account. E. B. P.] 



3 I. c., p. 150. 



