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300 clusion that they have not been limited in their scope to recent times, but must have existed in past epochs, and even, to some extent at least, in very remote epochs." When gadflies are about, the Ox "seems to be seized with an unreasoning paroxysm of fear." "In modern times the gadfly merely causes some fear and a little discomfort to an animal, and some loss of money and temper to its owner when he finds that the hide has been perforated, and is therefore held cheap by the tanner. But there must have been occasions when the war between gadflies and cattle was a much more serious affair. So strongly marked a protective instinct can only have been produced at a time when the very existence of the species was threatened by parasites of this order." Sir Charles Lyell, as early as 1836, and before much had been thought or expressed on the subject—for Darwin had not then returned from his epoch-making voyage—appears to have had clear conception of the phenomena, though based on very different philosophical views to those he embraced and enunciated later on. In a letter to Sir John Herschel, he advances probable causes that may aid a species' duration in time. "Now, if it be an insect, it may be made in one of its transformations to resemble a dead stick, or a lichen, or a stone, so as to be less easily found by its enemies; or, if this would make it too strong, an occasional variety of the species may have this advantage conferred upon it; or, if this would be still too much, one sex of a certain variety. Probably there is scarcely a dash of colour on the wing or body of which the choice would be quite arbitrary, or what might not affect its duration for thousands of years. I have been told that the leaf-like expansions of the abdomen and thighs of a certain Brazilian Mantis turn from green to yellow as autumn advances, together with the leaves of the plants among which it seeks for its prey. Now, if species come in succession, such contrivances must sometimes be made, and such relations predetermined between species, as the Mantis, for example, and plants not then existing, but which it was foreseen would exist together with some particular climate at a given time."