TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 87 



it is well known that this venation serves the systematist as a basis 

 for the definition of genera, especially in Lepidoptera and Hymenop- 

 tera. That is to say, it varies from genus to genus in a characteristic 

 manner, obviously corresponding to the differences in the wing-form, 

 and in the flight itself. But, unfortunately, we are still far from 

 being able to make more than quite general hypotheses as to the 

 meaning of the lengthening and strengthening, or conversely, the 

 degeneration or elimination, of this or that vein. From extreme cases, 

 however, as for instance the rich venation in good fliers with large 

 wings, and the scanty venation in poor fliers with small wings, we 

 learn at least so much, that the degree and even the manner of 

 venation bears a definite relation to the function of the wing, and 

 this we might have assumed. But these wing-veins, in as far as 

 they serve as a support for the weak wing-membrane, are purely 

 chitinous structures, skeletal parts which are not even renewed 

 from time to time like the skeletal parts of the leg and many 

 other parts of the insect. As they are laid down at first in the 

 pupa as soft strings of cells, so they remain, and they only begin 

 to be used when they are completely hardened. They can therefore 

 never have been caused to vary through use in the course of the 

 phyletic development of S23ecies and genera, and the Lamarckian 

 principle can have no part in their transformations. But if they 

 follow the most subtle changes, which we cannot precisely demon- 

 strate, of the whole wing-surface and in the mode of flight, as a man 

 is followed by his shadow, there must be some other principle which 

 adapts the organ to its function, and which is able continually to 

 adapt the large number of individual wing- veins in the manner 

 most advantageous for the general function. Here, therefore, we 

 have a state of matters exactly corresponding to that obtaining in 

 the transformation of actively functioning parts which form a system 

 with common co-operative action, as, for instance, in the case we first 

 discussed, that of the stag's antlers. 



Other even more complicated examples of harmonious adaptation 

 of passively functioning parts are afforded by the markings of animals, 

 such as those of the butterfly's wing. The colours have only a passive 

 role, whether they be due to pigments alone, or to structure, or to 

 both combined. When the coloration of a surface undergoes adaptive 

 variation, this cannot be due to any action of the colour, but must 

 depend on adaptation through selection. Yet it is well known that 

 there are many butterfly-wings whose surfaces exhibit different 

 colours and different shades of colour on their different parts, and 

 that in such a way that together they form a picture, that of a leaf. 



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