DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 199 



The figures, which have been executed with extreme care, are, with very few excep- 

 tions, from the specimens themselves, either living or dead. They arc 78 in number; 

 and of these 72 are original. 



I have also to thank Mr. A. D. Bartlett, the intelligent and zealous superintendent of 



the Zoological Society's splendid collection,— that gentleman having, with characteristic 

 urbanity, on various occasions enabled me to study the movements of the animals placed 

 more immediately under his care, under unusually favourable circumstances. 



Lastly, I have to acknowledge my unqualified obligations to my friend and former col- 

 league, Dr. James Murie, the prosector to the Zoological Society, whose advice, t ime, and 

 valuable services have on all occasions been unreservedly placed at my disposal. 



LOCOMOTION ON LAND. 



Walking of the Quadruped, Biped, Reptile, 8fc. — As the earth, because of its solidity, 

 will bear any amount of pressure to which it may be subjected, the size, shape, 

 and weight of animals destined to traverse its surface are matters of little or no 

 consequence. As, moreover, the surface trod upon is rigid or unyielding, tbe ex- 

 tremities of quadrupeds are, as a rule, terminated by small feet (Plate XII. fig. 1). 

 In this there is a double purpose — the limited area presented to the ground affording 

 the animal sufficient support and leverage, and enabling it to disentangle its feet 

 with the utmost facility, it being a condition in rapid terrestrial progression that tin 

 points presented to the earth should 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 diminshed, as shown 

 in the tardy movements of the mollusk, caterpillar, and slowworm, 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 surfaces are fur- 

 nished with suctorial disks, which enable the animals to walk, if need be, in an inverted 

 position; and "the Tree-frogs {Hyla) have a concave disk 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 Cuttlefish. 



The movements of the extremities in land-animals vary according to class and the 



degree of speed aimed at. 



In the Kangaroo the posterior extremities only are used, and the animal advances per 

 saltum, or by I series of leaps. In the Giraffe the legs of opposite sides move together 

 and alternate, whereas in the pace known as the trot 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. In the hexapod 

 insects, according to Muller, the fore and hind foot of the one side and the middle one 



Comp. Anat. and Phys. of Vertebrates, by Professor Owen, vol. i. pp. 2G2-263. Lon.l. 1866. 

 * 2B 2 



