38 



ANIMAL LOCOMOTION. 



utmost facility, it being a condition in rapid terrestrial pro- 

 gression that the points presented to the earth be few in 

 number and limited in extent, as this approximates the feet 

 of animals most closely to the wheel in mechanics, where the 

 surface in contact with the plane of progression is reduced to 

 a minimum. When the surface presented to a dense resisting 

 medium is increased, speed is diminished, as shown in the 

 tardy movements of the mollusc, caterpillar, and slow worm, 

 and • also, though not to the same extent, in the serpents, 

 some of which move with considerable celerity. In the gecko 

 and common house-fly, as is well known, the travelling sur- 

 faces are furnished with suctorial discs, which enable those 

 creatures to walk, if need be, in an inverted position ; and 

 " the tree-frogs [Ryld) have a concave disc at the end of each 

 toe, for climbing and adhering to the bark and leaves of trees. 

 Some toads, on the other hand, are enabled, by peculiar 

 tubercles or projections from the palm or sole, to clamber up 

 old walls." ^ A similar, but more complicated arrangement, 

 is met with in the arms of the cuttle-fish. 



The movements of the extremities in land animals vary 

 considerably. 



In the kangaroo and jerboa,^ the posterior extremities 

 only are used, the animals advancing "per salinm^ i.e. by a 

 series of leaps.^ 



The deer also bounds into the air in its slower movements; 

 in its fastest paces it gallops like the horse, as explained at 

 pp. 40-44. The posterior extremities of the kangaroo are 

 enormously developed as compared with the anterior ones ; 

 they are also greatly elongated. The posterior extremities 

 are in excess, likewise, in the horse, rabbit,^ agouti, and guinea 



1 Comp. Anat. and Phys. of Vertebrates, by Professor Owen, vol. i. pp. 

 262, 263. Lond. 1866. 



2 The jerboa when pursued can leap a distance of nine feet, and repeat the 

 leaps so rapidly tiiat it cannot be overtaken even by the aid of a swift horse. 

 The bullfrog, a much smaller animal, can, when pressed, clear from six to 

 eight feet at each bound, and project itself over a fence five feet high. 



3 The long, powerful tail of the kangaroo assists in maintaining the equi- 

 lil)rium of the animal prior to the leaps ; the posterior extremities and 

 tall forming a tripod of support. 



4 The- rabbit occasionally takes several short steps with the fore legs and 



