502 COMPARATIVE LOCOMOTION OF DIFFERENT ANIMALS. 



different types of locomotion is enconntered not so raucli in obtaining 

 great numbers of species of animals alive, but in finding snitable 

 methods for pbotograpliing each of tbem in its normal gait. 



The greater part of domestic animals lend tbemselves very well to 

 these studies; they are readily led to a track i)repared and will travel 

 over it regularly. With wild birds the difficulty is greater; we have 

 however succeeded in obtaining a number of types. 



Fishes, reptiles, mollusks, and insects are more difficult to manage; 

 it is necessary to devise for each species some method which will com- 

 pel it to travel regularly before the camera. IMoreover, we must, accord- 

 ing to circumstances, so vary the conditions of illumination that the 

 animal will sometimes show dark on a light ground, and sometimes 

 appear light upon a dark background. I have succeeded, nevertheless, 

 in obtaining good pictures of a considerable number of different species, 

 as may be judged from the illustrations (PI. xxiii-xxv). This series 

 of figures shows certain analogies in the mode of progress of species 

 which approach each other in their anatomical characters. Thus 

 the adder and the eel both progress by means of horizontal undulations 

 which move over the entire length of the body from the head to the 

 tail (PI. xxiii). The analogy would be still greater if the eel and 

 the serpent both swam in the water, or crawled upon the earth, for 

 it is the resistance of the medium, or in other words the nature of 

 the point of support, which governs the motions of crawling. In water 

 the unduhitions of the body are more regular and more efficacious than 

 on the ground, while at the same time they are less extended, and the 

 retrograde speed of what we may call the wave of motion is but little 

 less than the animal's rate of progress. That is, by the time an undu- 

 lation has run from head to tail the animal has advanced by nearly the 

 length of its body. On level ground, and still more on a slippery sur- 

 face, the undulations of the serj)ent and eel are very much extended 

 and progress is slow. 



Among coleopterous and orthopterous insects progress is much as it 

 has been described by naturalists. Carlet and M. de Moore have shown 

 that insects rest on three legs while the other three move. The sup- 

 porting legs constitute a triangular base, formed by the first and third 

 leg of one side, and the middle leg of the opposite side. (PI. xxiii, 

 xxiv.) 



Among arachnids there are on either side two supporting legs, and 

 two legs raised at the same time. But in the spider and scorpion which 

 we have taken as types, the walk is so rapid that it is not easy to follow 

 the successive motions in their jiroper order although they were photo- 

 graphed at the rate of 60 a second. In such cases it is necessary to 

 increase the number of figures, and above all, to resort to such methods 

 of illumination as we have adopted in studying the spider. This con- 

 sists in so illuminating the animal above and below, that while it is 

 clearly shown in outline, its shadow is proiected upon the track over 



