But much more complicated cases than this might be cited, if we were in a position to estimate exactly the functional value of the individual parts of the wing-venation in the different insects, for it is well known that this venation serves the systematist as a basis for the definition of genera, especially in Lepidoptera and Hymenoptera. 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 species 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 demonstrate, 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.