attained by any animal, the best suited it is to cope with nature. And if this variety of experiences extend through the life of a species, it becomes adapted to escape from more varied descriptions of peril and to obtain food in greater variety and quantity.
Thus, in seeking the form of animal most fairly planted in the path of true development, we must look for one capable of attaining to wide experience of nature, and adapted to evade perilous or to take advantage of beneficial conditions of the most diverse character.
These considerations lead us to a conception of the description of animal most likely to appear as a development from the low amphibian and reptilian forms. It must have sensory organs acutely adapted to all of Nature's most active forces, as light, heat, sound, and contact in its several kinds. It must be elevated above the surface sufficiently to give it the widest range of vision and hearing, these most important of the perceptive powers being specially active at the highest elevation of the body. It must rest upon the earth in such a way as to reduce friction to a minimum, so far as is consistent with proper support; and its powers of flight, of concealment, and of physical strength, must be sufficiently reduced to force it to seek other sources of safety, to adapt its organs to more varied functions, and to develop new features of mental activity.
What will be the form and the conditions of exposure to and defense from danger, of this most highly developed animal, adapted to gain the widest experiences, and to the greatest organic division of labor? It must, of necessity, display in its evolution every intermediate gradation of form upward, from that of the lowest vertebrate. Its form must be founded upon that natural to the fish.
Now, the fish naturally and inevitably assumes the horizontal posture from the requirements of its mode of life. It has developed fins as organs of movements. The fin is, in fact, as closely adapted to the fish-form as the oar is to the boat-form. The vertebral fins are reduced in number to four. There is a decided advantage in this reduction, in the saving of muscular exertion needed to move the fin, and also of the weight of extra muscles and fins. But a smaller number than four would be a disadvantage in the varied movements requisite to safety.
The fish-body is necessarily narrow and long, so as best to avoid friction. For its most effective movement it must be properly balanced, its two sides being alike in form and equal in weight. This produces bilateral symmetry of form, and perhaps also of organs, equal weight being most easily and completely attained by a reproduction of organs on the opposite sides of the body, but flexibility and full control of such a long, narrow body could not be gained except by a separate power of motion at each extremity and at each side. Therefore adaptation tends to produce in it four fins, and four only—an anterior and a posterior one on each side.