There is, I confess, a particular class of observation that has always arrested my attention more forcibly than any other: the alteration of an animal's manners, not by the slow accumulation of generations, but seemingly within the compass of a single lifetime, when the pressure of danger is newly applied or newly removed. I recall vividly my own surprise upon the islands of the Galapagos archipelago, where the birds, having known no enemy among men, would allow me to approach within a yard, or even to push them from a branch with the muzzle of my gun, so wholly unacquainted were they with the notion of fear. And I recall, too, the converse case among the foxes and birds of the Falkland Islands, which, after but a few years of persecution by sealers and settlers, grew markedly more wary of man than their fellows on parts of the coast less frequented. If, then, I am told that boar and deer which had grown accustomed to safety, perhaps through the long absence of the wolf from their country, begin again to show caution, to break from cover sooner, to post sentinels as it were, upon the wolf's return—I own I should not be in the least astonished, but should instead see in it a beautiful and immediate instance of a principle I have long laboured to establish.

The question that naturally presents itself is whether such rapid change is to be attributed to instinct newly selected, or to something more properly called sagacity or memory, acquired by the individual animal in the course of its own experience and perhaps communicated, by example, to its young. I must own some hesitation here, for the two causes are not easily disentangled in a short span of years. An animal may learn caution as a child learns to dread the fire it has once been burned by, without any alteration whatsoever in the inherited constitution of the species; and such learning, however striking to the observer, would be no proper evidence of natural selection at work, but only of that more common faculty of association which we see so plainly in the dog and the horse. Yet I think it exceedingly probable that beneath this acquired wariness there lies also a groundwork of true instinct, never wholly extinguished even in generations unmolested by the wolf, but merely lying dormant, as the instinct of the pointer dog to point, or of the retriever to retrieve, may lie dormant in the puppy until called forth by occasion, or as I have shown the tumbling habit of the pigeon to appear in fanciers' breeds without ever having been taught.

Here, I think, is the true interest of the matter for one who, like myself, has spent so many years pondering the origin of species by means of natural selection. For if the wariness be in part instinctive, then its sudden apparent reappearance is no contradiction of my theory, but rather its fullest illustration: the variation in timidity among individual boar or deer, always present though unobserved while the wolf was absent, becomes at once a matter of life and death the moment the wolf returns, and those individuals least disposed to caution will, generation upon generation, be weeded out, while the more wary leave the greater number of offspring. What appears to the casual observer as an instantaneous adaptation is, I would gently suggest, more probably the swift unmasking of a selective process that had been proceeding, unseen, since the question of safety was first laid open to doubt.

I offer this only as a probable explanation, resting as it must upon analogy with cases I have myself examined, rather than upon direct observation of the wolf and its prey, which has not fallen within my own study. But I confess I know of no fact in natural history better fitted to bring home to the plain understanding what is meant by the struggle for existence, nor any spectacle better calculated to show how closely, and how constantly, the fates of the timid and the bold are weighed against each other in the silent economy of nature.