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.

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, pro