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.

Finally, both Darwin and myself, at the critical period when our minds were freshly stored with a considerable body of personal observation and reflection bearing upon the problem to be solved, had our attention directed to the system of positive checks as expounded by Malthus in his “Principles of Population.”  The effect of that was analogous to that of friction upon the specially prepared match, producing that flash of insight which led us immediately to the simple but universal law of the “survival of the fittest,” as the long-sought effective cause of the continuous modification and adaptations of living things.

It is an unimportant detail that Darwin read this book two years after his return from his voyage, while I read it before I went abroad, and it was a sudden recollection of its teachings that caused the solution to flash upon me.  I attach much importance, however, to the large amount of solitude we both enjoyed during our travels, which, at the most impressionable period