Darwin, a true judge, made no mistake about it. (Charles Robert Darwin, born the 12th of February, 1809, at Shrewsbury, died at Down, in Kent, on the 19th of April, 1882. For an account of certain experiments which the author conducted on his behalf, cf. "The Mason-bees": chapter 4.--Translator's Note.) He greatly dreaded the problem of the instincts. My first results in particular left him very anxious. If he had known the tactics of the Hairy Ammophila, the Mantis-hunting Tachytes, the Bee-eating Philanthus, the Calicurgi and other marauders, his anxiety, I believe, would have ended in a frank admission that he was unable to squeeze instinct into the mould of his formula. Alas, the philosopher of Down quitted this world when the discussion, with experiments to support it, had barely begun: a method superior to any argument! The little that I had published at that time left him with still some hope of an explanation. In his eyes, instinct was always an acquired habit. The predatory Wasps killed their prey at first by stabbing it at random, here and there, in the softest parts. By degrees they found the spot where the sting was most effectual; and the habit once formed became a true instinct. Transitions from one method of operation to the other, intermediary changes, sufficed to bolster up these sweeping assertions. In a letter of the 16th of April, 1881, he asks G.J. Romanes to consider the problem: "I do not know," he says "whether you will discuss in your book on the mind of animals any of the more complex and wonderful instincts. It is unsatisfactory work, as there can be no fossilised instincts, and the sole guide is their state in other members of the same order, and mere PROBABILITY. "But if you do discuss any (and it will perhaps be expected of you), I should think that you could not select a better case than that of the sand-wasps which paralyse their prey as described by Fabre in his wonderful paper in the 'Anales des sciences naturelles,' and since amplified in his admirable 'Souvenirs...'" I thank you, O illustrious master, for your eulogistic expressions, proving the keen interest which you took in my studies of instinct, no ungrateful task--far from it--when we tackle it as it should be tackled: from the front, with the aid of facts, and not from the flank, with the aid of arguments. Arguments are here out of place, if we wish to maintain our position in the light. Besides, where would they lead us? To evoking the instincts of bygone ages, which have not been preserved by fossilization? Any such appeal to the dim and distant past is quite unnecessary, if we wish for variations of instinct, leading by degrees, according to you, from one instinct to another; the present world offers us plenty.