312 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



constitute stimuli for the production of an increased rate of 

 growth ? And how is it possible in any way to explain, by 

 mere increase in growth, the origin of a structure in which each 

 part has its own distinct meaning and pla3^s a peculiar part in 

 attracting insects and in the process of cross-fertilization 

 effected by them ? Even if the manifold peculiarities of form 

 could be explained in this way, how can such an explanation 

 possibly hold for the colours of flowers ? How could the white 

 colour of flowers which open at night be explained as the 

 direct result of the creeping of insects ? How can the sugges- 

 tion 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 adapta- 

 tions in form and colour, for which no stimulus acting directly 

 upon the organism can be found. Can we imagine that the 

 green caterpillar ^, 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 mimicr}^, 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 

 theor}^, 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 



^ [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 for a more detailed ac- 

 count.— E. B. P.] 



