First (and most important, as I believe), in early life both Darwin and myself became ardent beetle-hunters. Now there is certainly no group of organisms that so impresses the collector by the almost infinite number of its specific forms, the endless modifications of structure, shape, colour, and surface-markings that distinguish them from each other, and their innumerable adaptations to diverse environments. These interesting features are exhibited almost as strikingly in temperate as in tropical regions, our own comparatively limited island-fauna possessing more than 3,000 species of this one order of insects. Again, both Darwin and myself had what he terms "the mere passion for collecting," not that of studying the minutiæ of structure, either internal or external. I should describe it rather as an intense interest in the variety of living things--the variety that catches the eye of the observer even among those which are very much alike, but which are soon found to differ in several distinct characters. Now it is this superficial and almost child-like interest in the outward forms of living things which, though often despised as unscientific, happened to be _the only one_ which would lead us towards a solution of the problem of species. For Nature herself distinguishes her species by just such characters--often exclusively so, always in some degree--very small changes in outline, or in the proportions of appendages--as give a quite distinct and recognisable facies to each, often aided by slight peculiarities in motion or habit; while in a larger number of cases differences of surface-texture, of colour, or in the details of the same general scheme of colour-pattern or of shading, give an unmistakable individuality to closely allied species. It is the constant search for and detection of these often unexpected differences between very similar creatures that gives such an intellectual charm and fascination to the mere collection of these insects; and when, as in the case of Darwin and myself, the collectors were of a speculative turn of mind, they were constantly led to think upon the "why" and the "how" of all this wonderful variety in nature--this overwhelming and, at first sight, purposeless wealth of specific forms among the very humblest forms of life. Then, a little later (and with both of us almost accidentally) we became travellers, collectors, and observers, in some of the richest and most interesting portions of the earth; and we thus had forced upon our attention all the strange phenomena of local and geographical distribution, with the numerous problems to which they give rise. Thenceforward our interest in the great mystery of _how_ species came into existence was intensified, and--again to use Darwin's expression--"haunted" us.