a bird, and so devoured, is wiped out of existence for ever, with all its possible progeny. Every butterfly which escapes, by however slight a peculiarity, is enabled to lay

its eggs in peace, and to hand on its peculiar

ities to its posterity. This sort of selection is going on every day around us, and no difference is too slight for it to select, no resemblance is too clumsy provided it once for a moment aids the insect in avoiding destruction.

Now, we all know that the eyes of birds are very sharp and keen indeed. A hawk soaring so high in the sky that human sight fails to perceive it, will yet discriminate and pounce down upon a lark in the fields below

-a small brown bird seated upon a brown clod of earth exactly like itself in colour. In just the same way the insectivorous birds keep a sharp look-out for moths and butterflies, upon which they swoop at once whenever they distinguish them upon the ground beneath. Every day those insects whose

are

colour betrays them get thinned out by their watchful enemies, while those whose colour protects them manage to lay their eggs in peace, and hand on their own peculiar spots and lines to their descendants. The consequence in the long run is that the butterflies get better protected from generation to generation, as the chances of interbreeding with badly protected individuals eliminated by the action of the birds, while only the most imitatively coloured individuals are left to mate with one another and to become the parents of future swarms. Thus the hostile birds are themselves the instruments through which the insects have been armed defensively against their depredations. For the various individuals tend always to vary a little in marking-no two plants or animals of the same species are ever exactly alike-but the picking off of the brightly coloured individuals by the birds helps to preserve the protected specific type intact. And of course the same causes which now

preserve it originally produced it. Ever since birds and butterflies have existed, the process must constantly have been at work; and the birds and butterflies themselves, in the forms that we know, are the final outcome of its perpetual interaction.

This sufficiently accounts for the imitative colouring of the under surface, but it does not sufficiently account for the brilliant and attractive hues of the upper side. Those hues were probably produced in a very different manner. At this exact moment I see two red admirals above the hedge yonder, engaged in their pretty rhythmical courtship, flying round and round one another, now on top and now beneath, chasing each other in graceful curves, and seeming to be engaged for ten minutes at a time in a sort of aërial quadrille. These two butterflies are helping in their small degree to keep up and intensify the beautiful colours of their race. They are coquetting and flirting together, each eager to display all its charms to the best advan

tage, and to attract the other by its own beauty. If a third and prettier butterfly happens to sail up, the belle will bestow her affections upon the new-comer, and the vanquished beau will slink away disgraced, leaving her to her chosen mate. This second sort of selection is going on for ever, side by side with the first; the prettiest, freshest, and most daintily-marked insects being always preferred in the pairing over duller, dingier, or more battered rivals.

It is interesting also to note how the two kinds of selection run parallel with one another. While the butterflies are poised motionless upon twigs or flowers they are in the greatest danger from birds; but in such positions they close their wings and display only the outer surface, which is imitatively and protectively coloured. The constant picking off of all those which can be distinguished when at rest suffices to keep the protective colours always true. On the other hand, when the insects are on the wing,

hovering about flowers or rising in the air to pirouette and gambol with their mates, they run comparatively little risk from the birds. They are too nimble for their pursuers, and they seem fairly secure by their power of doubling as they flit rapidly along from spray to spray. The birds are bad marksmen at a moving target; they cannot double like their prey, and they prefer to aim at their butterflies sitting, as French sportsmen are said to do at partridges. On the other hand, with butterflies as with men, faint heart never won fair lady. If the insects did not venture out into the open to seek their mates and to charm them with their painted pinions, some bolder rivals would carry off the prize, and so leave the cowards unrepresented in future ages. Thus, in the course of generations, a great many butterflies have come to have two sets of colours-the one set attractive for their own kind, and the other set protective against their enemies. The lower sides of the wings are coloured like the leaves or