of the substance which elsewhere becomes muscular, and that Dr. Davy was justified in denying the organ to be muscular.[71] That it is substituted for muscle cannot be doubted. Now, although we are entirely ignorant of the conditions which cause this differentiation of substance which elsewhere becomes muscular, but here becomes electric organs, we can understand that, when once such a development had taken place, if it in any way profited the fish in its struggle for existence, Natural Selection would _tend_ to its further increase and propagation. So far Mr. Darwin carries us with him; but we decline proceeding further. The development of these organs in fishes so widely removed, does not imply an ancestral community. It is interpretable as mere growth on a basis once laid; and therefore would occur with or without any advantage in the struggle with rivals. The similarity in concurrent conditions is quite enough to account for the resemblance in structure. This, with his accustomed candor, Mr. Darwin admits. “If the electric organs,” he says, “had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would be specially related to each other. Nor does Geology at all lead to the belief that formerly most fishes had electric organs which most of their modified descendants have lost.” 121. It may seem strange that he should urge a difficulty against his hypothesis when it could be avoided by the simple admission that even among nearly allied animals great differences in development are observable, and the electric organs might be ranged under such diversities. But Mr. Darwin has so thoroughly wrought out his scheme, that he foresees most objections, and rightly suspects that if this principle of divergent development be admitted, it will cut the ground from under a vast array of facts which his hypothesis of Descent requires.