MAN AND THE VERTEBRATE SERIES. y^j 



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 

 advantao-e 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 impor- 

 tant of the perceptive powers being specially active at the highest ele- 

 vation of the body. It must rest upon the earth in such a way as to re- 

 duce 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 fea- 

 tures 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 interme- 

 diate 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 pos- 

 ture 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 bal- 

 anced, 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 repro- 

 duction 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. There- 

 fore adaptation tends to produce in it four fins, and four only an an- 

 terior and a posterior one on each side. 



