COMPAKATIVE LOCOMOTION 263 



sections of a museum of comparative anatomy, in 

 the hope that such a series might be able to throw 

 some light upon the subject. The principal difficulty 

 in this undertaking lies not so much in the actual 

 collection of many different species, as in the sub- 

 jection of them to the best conditions for observation. 



Domestic animals or tame species can easily be dealt 

 with, but the natural paces or movements of others 

 can only be observed under special conditions, the 

 nature of which can only be discovered after patient 

 research. 



A frightened animal never moves about in a normal 

 fashion, and if it is compelled to advance in a pre- 

 determined direction, it instinctively goes the other way. 

 Sometimes the animal becomes scared by the bright 

 light with which it is necessarily illuminated ; at other 

 times it resents an abnormal support under its feet. 

 In some cases the animal has to be brightly illu- 

 minated against a dark background, at others it has 

 to be silhouetted against a dark background. In all 

 these cases a straight pathway from which it cannot 

 diverge is essential. 



Being compelled to curtail the discussion of these 

 researches, we can only give a few examples of 

 comparative locomotion, and a brief account of the 

 arrangements for taking the photographs. 



Comparative Locomotion of Tortoises and Lizards. —A 

 water tortoise was placed in a glass aquarium and 

 exposed to translucent illumination, just as in the case 

 of the sea-horse described (p. 212). The animal dived 

 and crawled about at the bottom of the aquarium, but 

 after a time it had to rise to the surface for breath, 

 and it was this movement of which advantage was 

 taken. 



Fig. 185 shows the tortoise moving about like a 

 quadruped in water, the successive movements of the 



