Both of these have, in reality, been already replied to in the preceding chapter. Nevertheless, we may briefly repeat, that, so far as the first is concerned, it is in islands that we detect the maximum of instability to which the wings of the Insecta are liable, and that it is in seasons of extraordinary heat that their development is everywhere inclined (if at all) to be especially stimulated: whilst, as regards the second, it will be sufficient to state, that in _continents_, when any decided alteration takes place in the organs of flight, it for the most part comes to pass that an _increased_ (rather than diminished) action is the result; whereas in _islands_, provided that the species are not absolutely dependent on aerial progression for their food (in which case, in order to prepare for the contingency of being blown out to sea, the capacity of the wings is commonly augmented), the _reverse_ is nearer the truth. So that the _second_ problem,--the _reason why_ appendages thus apparently essential should be subject to inconstancy,--is at once rendered intelligible from the consideration, that it is only under circumstances in which the indiscriminate employment of those organs would be apt to bring the creatures into trouble that (when not an actual _sine qua non_ to their existence) they are liable to be taken away; whilst, even in that case, it generally happens that some partial equivalent for the privation incurred is granted, as a recompense. Mr. Westwood, in his admirable _Introduction to the Modern Classification of Insects_, has recorded many instances of alary variation; which, however, as he does not appear to have noticed the peculiarity of island faunas, are principally in corroboration of what I have just insisted upon as the usual tendency in continents,--namely, an _enlargement_ of the erratic powers. Speaking of the _Aphelocheirus aestivalis_ (a member of the Hemiptera), he observes: "My British specimens have but short, rudimental, oval hemelytra, like those of the bed-bug; but I possess one of Bosc's original examples, described by Fabricius, not quite so large as the others, in which _the wings are fully developed_. I do not, however, on that account, regard the former either as pupae or distinct species, but as undeveloped specimens in the imago state[47]." And whilst discussing the _Hydrometridae_, he expresses himself thus: "It appears to me, that, from causes of which we are ignorant, numerous individuals of many of the species of these tribes are subjected to an inferior kind of development in the imago state, which does not allow the acquirement of wings,--which, however, in certain cases, _acquire their full size_. Hence, I consider that the apterous specimens of _Hydrometra stagnorum_, those with very short elytra, and those with the full-sized wings and wing-covers, are all in the imago state, although some are more perfect than others[48]." And, again, in his reflections on the Hemiptera, Mr.