If, then, as we have previously observed, we follow Kerr in regarding the fish kidney, not as a simple mesonephros, but as an opisthonephros (_i.e._ a combination of mesonephros and metanephros), there is no warrant for interpreting the embryonic mesonephros of man and mammals generally as the fish-kidney stage. But waiving this consideration, and assuming, for the sake of argument, that the fish kidney is a perfect homologue of the human mesonephros, the mere fact of the adoption by the human embryo of a temporary solution of its excretory problem similar to the permanent solution of that problem adopted by the fish, would not, of itself, imply the common ancestry of men and fishes. Such a coincidence would be fully explicable as a case of convergent adaptation occurring in the interest of embryonic economy. It is, indeed, a well-known fact that larval and embryonic organisms are often obliged to defer temporarily the construction of the more complex structures of adult life, and to improvise simpler substitutes for use until such a time as they have accumulated a sufficient reserve of energy and materials to complete the work of their more elaborate adult organization. The young starfish, for example, arising as it does from an egg but scantily supplied with yolk, is forced, from the very outset, to shift for itself, in coping with the food-getting problem. Under stress of this necessity, it economizes its slender resources by constructing the extremely simple digestive and motor apparatus characteristic of the larva in its bilaterally-symmetrical _Bipinnaria_ stage, and postponing the development of the radially-symmetrical structure characteristic of the adult stage, until it has stored up the wherewithal to complete its metamorphosis.