OCR Text |
Show 1873.] COLOURING IN INSECTS. 161 or for other purposes, would be certainly benefited by being able to assume the colour of any locality in which it might find itself. Here natural selection works, as before, in producing and maintaining a power to change colour, it being quite immaterial to this agency at what period of the insect's life the change of colour is induced, whether it only occurs in individuals born in the district, or in individuals that have roamed into the district in the perfect state and undergone subsequent change. So also is natural selection regardless, in these cases, whether the disguising colour is congenital or consequent on the emergence from the pupae, whether it is a colour exposed by ecdysis or one due to actual change of tint in the tissues-regardless also whether the change is voluntary or involuntary on the part of the insect. The examples adduced from classes of animals other than insects are capable of being reasoned upon in a precisely similar manner; but it is needless here to extend the argument. The results of this inquiry have thus led m e to conclude that the cases which I have grouped under Class V. are cases which differ from ordinary protective resemblance, inasmuch as the primary variations are indubitably produced by direct action, but controlled and accelerated by natural selection. As the particular manner in which these original variations are produced is, in nearly all these instances, quite unknown, the observation made at the outset, that the study of these cases " offers a wide and interesting field for observation," is, I think, fully justified. The part played by natural selection in the production of the class of cases I have here discussed is thus in some degree analogous to the function ascribed to this agency by Mr. Herbert Spencer in the formation of the woody vessels in trees* -that, viz., of an accelerator and controller of results primarily due to what this philosopher has, in mechanical terms, called " direct equilibration " f. In conclusion I beg leave to present in a tabular form the results of the classification above set forth, so as to include Class V., and thus embody all the known cases of protective resemblance. The classification refers, as before, to the object imitated ; but it is to be understood that the "object imitated" and the "disguising characters " of the species are convertible terms. A change in character witnessed in the object simulated in passing from one district to another, or in the same district, is termed for brevity a variation " in space," while a change occurring in the course of time is termed * " O n Circulation and Formation of Wood," Trans. Linn. Soc. vol. xxv. t The inquiry has suggested itself whether there might not occur conditions under which a case of ordinary protective resemblance might be mistaken for a case of Class V. It seems probable at first sight that natural selection would be able to produce local modifications in the disguising characters of protected species corresponding to local changes in the characters of the imitated object. Such local modifications might occur in districts so situated as to prevent interchange of species; but in contiguous districts presenting facilities for intercommunication it seems to m e that crossing would entirely prevent the formation of local varieties by natural selection, unless this agency were aided by "direct action," under which circumstances we should have an example of Class V. PROC. ZOOL. Soc-1873, No. XI. 11 |