788 THE POPULAR SCIENCE MONTHLY. 



The fish-form governs that of the amjihibian and of the reptile. 

 The fins become limbs, which are variously modified. In some am- 

 phibians, as in the frog, the limbs become different in functions, the 

 hind-limbs being adapted to a leaping motion, the fore-limbs to sup- 

 j^ort, A differentiation of another kind takes place, eventually, in 

 the reptile stock, the fore-limbs becoming organs of aerial support. 

 They become wings, and we obtain the bird type, with fore-limbs 

 adapted to aerial, hind-limbs to solid support. 



These, however, are but aberrations from the general path of de- 

 velopment, which is toward a continuance of the horizontal position of 

 the fish-body : the fins become four similar supporting limbs. 



Adaptation from this point takes one general direction, that of the 

 reduction of weight to the lowest point consistent with the proper ex- 

 ercise of the nutritive functions, and the requisite strength of bone and 

 vigor of muscle. The general shape attained by the body is governed 

 by several mutually assisting or opjDosing functions. The require- 

 ment of speed needs that it shall be narrow and long, so as best to 

 avoid the resistance of the aii\ But too great elongation would be a 

 disadvantage, since the mid regions of the body would remain with- 

 out support by the limbs, and could only sustain themselves by muscles 

 acting to ojipose gravity. These muscles would add to the weight, 

 and would form additional consumers of force. The best form of the 

 body, then, is one sufficiently narrowed to partly avoid aerial resistance, 

 but not so elongated as to diminish its proper support upon the limbs. 

 The chief aberrations from these requirements are those of the prone 

 serpents, and, among mammals, of the weasel family, whose mode of 

 life requires a very narrow body, so that digestive space can only be 

 gained by its elongation. We may return here to the serpents to say 

 that their elongation is probably due in great part to the same cause. 

 Their mode of concealment and of motion requires great narrowness 

 of body, so that space for the nutritive functions can only be gained 

 by elongation. The requirement of narrowness is, in fact, so great 

 that there is not sufficient room for the larger organs to be bilater- 

 ally reproduced, therefore respiration is confined to a single lung, the 

 other being atrophied. 



But, with a comparatively few exceptions, caused by highly special- 

 ized life-habits, the land vertebrates possess the general form requisite 

 for the fullest adaptation to these three conditions that of reduced 

 resistance to motion, proper support, and the necessary room for the 

 exercise of the nutritive functions. 



The weight of the body is reduced to the lowest possible limit con- 

 sistent with its general size and the exercise of its functions. The 

 limbs, for instance, take no part in the nutritive function, and are 

 reduced in size to the weight of bone requisite for support and of 

 muscle necessary to move the body. The head is just large enough 

 to hold the brain, the organs of special sense, and the mouth with its 



