On his part the English scientist does full justice to the French scientist’s incomparable mastery in the study of insects. We have often mentioned the title of “inimitable observer” which he gives him in his work on the Origin of Species. In a letter dated the 16th of April 1881, he wrote to Mr. Romanes, who was preparing a book on Animal Intelligence: “I do not know whether you would care to discuss in your book some of the more complicated and marvellous instincts. It is an ungrateful task.... But if you discuss some of these instincts, it seems to me that you could not take a more interesting point than that of the animals that paralyse their prey, as Fabre has described in his astonishing memoir in the Annales des sciences naturelles, a memoir which he has since amplified in his admirable Souvenirs.” When he wrote this Darwin was acquainted only with the first volume of the Souvenirs. [142] What would he have said if he could have enjoyed the whole of the learned entomologist’s masterly work? In reading this first volume, the attention of the English naturalist had been especially struck by the operations of the Hunting Wasps, which were peculiarly upsetting to his theories. Darwin was visibly preoccupied by the problem of instinct as propounded by the irrefutable observations of the French entomologist, but he did not despair of finding a solution in conformity with his system. Fabre, on his side, believed that his position was inexpugnable, and was not without hope of converting Darwin by what appeared to him to be the evidence of the facts. Nowhere does the theory of evolution come full tilt against so immovable an obstacle. Darwin, a true judge, did not fail to realise this. He greatly dreaded the problem of the instincts. My first results in particular had left him anxious. If he had known the tactics of the Hairy Ammophila, the Mantis-hunting Tachytus, the Philanthus apivorus, the Calicurgus, and other predatory insects which have since been investigated, his anxiety, I believe, would have become a frank avowal of his inability to get instinct to enter the world of his formula. Alas! the philosopher of Down left us when the discussion was only just beginning, with experiment to fall back upon, a method superior to all arguments. The little that I had published at that period left him still some hope of explanation. In his eyes instinct is always an acquired habit.