In regard to the operation of the two kinds of selection in the evolution of man from a lower form of animal, we find the theory to be this: That organic beings with peculiar habits and structure have passed through transitions which have converted the primordial animal into one of totally different habits and structure; that, in these transitions, organs adapted to one condition and mode of life have become adapted to another; that such organs are homologous, and that in their widely varied uses they have been formed by transitional gradations, so that, for example, a floating apparatus, or swim-bladder, existing in a water-animal for one purpose--flotation--has become converted in the vertebrate animals into true lungs for the very different purpose of respiration. Thus, by ordinary generation, from an ancient and unknown prototype, not only have organs, by minute and successive transitions, become adapted to changed conditions of life, but the whole organism has become changed, and this has resulted in the production of an animal vastly superior to his ancient and unknown prototype; and yet to that prototype, of which we have no specimen and no record, are to be traced the germs of all the peculiarities of structure which we find in the perfect animals of different kinds that we thoroughly know, until we come to man, these successive results being brought about by the two kinds of selection--natural and sexual. There can be no better illustration of the character of Mr. Darwin's theory than that to which he resorts when he means to carry it to its most startling length, while he candidly admits that he has felt the difficulty of this application of it far too keenly to be surprised at the hesitation of others. This illustration is the eye. Here he very justly says it is indispensable that reason should conquer imagination; but on which side of the question reason or imagination is most employed might, perhaps, be doubtful. Mr. Darwin's hypothesis concerning the eye begins with the fact that in the highest division of the animal kingdom, the vertebrata, we can start from an eye so simple that it consists, as in the lancelet,[25] of a little sack of transparent skin, furnished with a nerve, and lined with pigment, but destitute of any other apparatus. From this prototype of a visual organ, up to the marvelous construction of the eye of man or of the eagle, he supposes that extremely slight and gradual modifications have led, by the operation of natural and sexual selection; and by way of illustrating this development, he compares the formation of the eye to the formation of the telescope. "It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects, and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference