COMPARATIVE LOCOMOTION OP DIFFERENT ANIMALS. 503 



which it runs. Tliis shadow gives much iufoi-matioii in regard to the 

 position of the legs for when the feet are resting on the ground the leg 

 and its shadow touch at their extremities. (PI. xxiv.) 



One of the most interesting points in these physiological comparisons 

 is to see how the anatomical resemblances of different animals corre- 

 spond with their functional resemblances. 



Among fishes, for example, we meet in varying degree, with the reptil- 

 ian undulation which forms the eel's sole mode of progress, but find 

 that it has lost much of its importance. Still very apparent in tlie dog- 

 fish (PL XXV) it is found only in the caudal region of those fishes whose 

 thick set bodies have lost the greater part of their flexibility, but in 

 these cases the widened tail acts more efficiently for it meets with 

 great resistance in the water. 



Batrachians in different ijhases of their development have modes of 

 locomotion corresponding to the state of their organs. The tadpole of 

 a to;\d, in which the feet are still imperfectly developed (Pl.xxv, fig. 2, 

 ui>]' " line) swims with its tail after the fashion of a fish. Later on 

 (low or Une) the legs begin to be used for locomotion, but the tail still 

 ireeps no its energetic action and vibrates continually, while the legs 

 move in alternate jerks. Still later (middle line) the tail has disappeared 

 and the hind legs are alone nsed in progression. This role of the hind 

 limbs which presents so striking an analogy to the swimming of man 

 is effected in the following" manner.* 



The animal flexes its legs, bringing them well under the body, then 

 spreads them wide apart in such manner that the two legs, directed 

 laterally, form a right angle with the axis of the body. Propulsion is 

 effected by bringing the outstretched feet quickly together, after which 

 they are gradually flexed and brought towards the body, and the series 

 "<" movements recommences. 



The lizards, which anatomically approach the serpents, also pre- 

 serve in their j)rogressiou something of the undulatory movement 

 wiiich we have represented above, but this undulation is complicated 

 by the action of the limbs, which play the leading part in the crawling 

 of these reptiles (PI. xxv). In the gecko (PI. xxv) the undulation of the 

 body is plainly to l)e seen; it is scarcely apparent in the gray lizard. 

 In both species it is impossible for the eye to follow the iiicessive move- 

 ments of the feet, and to compare them with those of other quadru- 

 peds, but from their chrono-photographic images it is easy to see that, 

 taking the order of the movements of the limbs as a standard, the 

 lizards are trotting animals. The limbs, in short, move diagonally— 

 that is, the right fore leg and left hind leg, move simultaneously. 



The undulations of the body are so combined with the movements 

 of the legs that the feet are brought close together on the concave side 



* Several of the views sbown in the illnstratious have been reversed. 



