, PKOGRESSION ON THE LAND. 



39 



pig As a consequence these animals descend declivities with 

 difficulty. They are best adapted for slightly ascending ground. 

 In the giraffe the anterior extremities are longer and more 

 powerful, comparatively, than the posterior ones, which is 

 just the opposite condition to that found in the kangaroo. 



In the giraffe the legs of opposite sides move together and 

 alternate, whereas in most quadrupeds the extremities move 

 diagonally — a remark which holds true also of ourselves in 

 walking and skating, the right leg and left arm advancing 

 together and alternating with the left leg and right arm (fig. 1 9). 



Fig. 19.— Diagram showing the figure-of-8 or double-waved track produced by 

 the alternating of the extremities in man in walking and running; tlie 

 right leg (r) and left arm (s) advancing simultaneously to form one step ; 

 and alternating with the left leg {t) and right arm {u), which likewise ad- 

 vance together to form a second step. The continuous line (r, t) gives the 

 waved track made by the legs ; the interrupted line {s, u) that made by the 

 arms. The curves made by the right leg and left arm, and by the left leg 

 and right arm, form ellipses. Compare with fig. 18, j). 37. — Original. 



In the hexapod insects, according to Muller, the fore and 

 hind foot of the one side and the middle one of the opposite 

 side move together to make one step, the three corresponding 

 and opposite feet moving together to form the second step. 

 Other and similar combinations are met with in the decapods. 



The alternating movements of the extremities are interest- 

 ing as betokening a certain degree of flexuosity or twisting, 

 either in the trunk or limbs, or partly in the one and partly 

 in the other. 



This twisting begets the figure-of-8 movements observed in 

 walking, swimming, and flying. (Compare figs. 6, 7, and 26 x, 

 pp. 28 and 55 ; figs. 18 and 19, pp. 37 and 39 ; figs. 32 and 50, 

 pp. 68 and 97 ; figs. 71 and 73, p. 144 : and fig. 81, p. 157.) 



Locomotion of the Horse. — As the limits of the present 

 volume forbid my entering upon a consideration of the move- 

 ments of all the animals with terrestrial habits, I will describe 

 briefly, and by way of illustration, those of the horse, ostrich, 



one long one with the hind legs ; so that it walks with the fore legs, and leaps 

 with the hind ones. 



