458 POPULAR SCIENCE MONTHLY. 



feed in this way. In these as well as in mammalian forms which 

 present relatively slight limb specialization, the mouth parts have in 

 many cases undergone modifications which render them effective in- 

 struments for grasping, rending, digging, picking and the like. Such 

 adaptations are shown in the snouts of the mullet and pig; thebeaks 

 of the paddle-fish, the duck-billed otter, the humming-bird and the 

 secretary; the tusks of the boar, the horn of the rhinoceros, the 

 proboscis of the tapir, the tongue of the chameleon and the trunk of 

 the elephant. In all these cases the specialization of the limbs which 

 accompanied such modifications of the mouth parts consists in an 

 adaptation of the function of locomotion in connection with the partic- 

 ular conditions under which the life of the species is carried on; in 

 consequence of which their general features have diverged very widely 

 from those of manipulative organs. 



The earliest form of locomotion which vertebrate limbs fulfilled 

 was propulsion through the water. The problem to be solved did 

 not include the support of the body, which was buoyed up by the dense 

 medium in which the animal moved. The same dense medium afforded 

 a sufficient resistance to allow of a relatively slow and weak movement 

 on the part of the locomotive organs. The earliest vertebrate limbs, 

 or the body extensions which foreshadowed them in times still earlier, 

 needed neither the strength and rigidity of the terrestrial leg nor the 

 expanse and velocity of stroke of the aerial wing. 



If we conceive the progenitor of the limbed vertebrate to have 

 progressed by means of an undulatory motion of the whole body, 

 brought about by a peristaltic wave of contraction passing from front 

 to rear of the animal, it is not difficult to infer the advantages which 

 would accrue to those individuals in which a modification appeared 

 in the form of flexible extension parallel to the longitudinal axis of 

 the body, by the independent undulations of which progression became 

 possible. The economy resulting from reduction of movement in the 

 whole body mass would be accompanied by a decrease in the likelihood 

 of attracting notice, a greater control of movements in taking food, 

 and a more exact process of perception in adjusting the body to sur- 

 rounding changes. 



Though the series of limb forms is obscure in its earlier parts, the 

 whole group is generally supposed to have its prototype in the lateral 

 fold of the primitive fishes, in which locomotion took place through 

 a wave-like movement passing backward along the length of the web. 

 Out of this primitive lateral fold the various fin-formed limbs which 

 characterize the aquatic progenitors of the land vertebrates arose by 

 a series of modifications in which the following stages may be noted : 

 In the undifferentiated swimming folds first developed a system of 

 parallel rods extending from the body surface to the margin of the 

 web, which probably both served the purpose of increasing the resistance 



