166 Dynamic Theory. 



the scapula, the ischium to the coracoid, and the pubis to the anterior 

 coracoid, or clavicle ; the parts being thus arranged from front to rear, 

 in the same order at each end of the animal, as if precisely the same 

 muscular movements were required and the same sort of strains im- 

 posed at each end. But that divergencies should take place between 

 the two ends was as inevitable as that they could not possibly be al- 

 ways subject to the same sort of stimulation. Like the fore wheels of 

 a wagon, the fore limbs of the vertebrate must steer the animal, exert- 

 ing their muscular force in many various ways to give the head new 

 directions, while the duties of the hind limbs are much more monoto- 

 nous. The skeleton of the fossil ancient reptile Ichthyosaurus shows 

 how far such divergence was carried in a class of animals whose chief 

 movements were connected with swimming. But in the land verte- 

 brates, whose habits require such different services from the limbs at 

 either end of the body, we find the greatest divergence of structure, a 

 divergence caused, however, in much the greatest degree by alterations 

 in the pectoral arch. For throughout the class there is a striking same- 

 ness in the number and character of the pelvic bones. They vary in 

 size and strength in relation to the strain put upon them. 



In the enormous fossil Megatherium, which is supposed to have had 

 the habit of throwing its whole weight upon the hind legs while secur- 

 ing its food from the branches of trees, the pelvic arch is proportionally 

 very strong and wide. It is also strong and large in the Kangaroo 

 which stands upon and leaps with its hind limbs. The same is true of 

 most of the rodents ( Squirrels, Rabbits, &c. ), animals that run by 

 springing from their hind feet, rather than by creeping. In the erect 

 Apes, and still better in Man, the pelvic bones form a basin, as the 

 name implies, very well adapted to receive the falling viscera of a four- 

 legged animal assuming an erect position. 



FIG. 89. Ribs of Ichthyosaurus. 

 C. Centrum of backbone. 

 rr Ribs. 

 vo. Ventral Ossifications. 



The common Sloth, (an Edentate), lives up- 

 on trees, suspended on the underside of branches 

 around which he clasps his hind legs and feet, 

 and one of his fore legs or arms. With the 

 other arm he reaches the leaves on which he 

 feeds. In consequence of this habit the pel- 

 vis is large, and the thighs are directed out- 



\^ / ward, so that the knees cannot be brought to- 



gether. The arms are very much longer than 

 the legs. He is extremely slow and awkward 

 anywhere but in the trees in which he lives, but is extremely well fit- 

 ted for his habit of life. 



