conditions of life, then the difficulty of be- lieving that a perfect and complex eye could be formed by natural selection, though. insuperable by our imagination, can hardly be considered real."
Admitting the full force of this reason- ing, and though anatomy shows that the elementary eye arises by a simple involution of the integument, the difficulty is, that the same, or nearly the same, occurs for the ear; and we have no knowledge which would enable us to predict from such beginning the formation of organs so elaborate as these ultimately become, for that can hardly be called knowledge which but deals with facts by contemplation alone. We cannot be said to have acquired a clue to the idea of organic operations until we are able to try our knowledge by prediction, and to foretell what must arise under given conditions. Although, however, physiology is obliged to assume the unit of each organ, we cannot survey the relations of organic beings throughout their whole extent without the conviction that we have before us the varied
