of the Vertebrate Skeleton, 277 



heavier in proportion ; but neither the leg-bones nor scapulge 

 have increased in length proportionally with the increased 

 dimensions of the remainder of the skeleton. All of which is 

 in accord with the law of pressure and tension, the increase of 

 bulk of the tame animal depending merely on luxuriant diet. 

 The leg-bones, less exercised, experience less central com- 

 pression, and are consequently relatively heavier ; and simi- 

 larly, from less exercise of the parts usually most exercised, 

 they become relatively shorter. And with respect to cattle, 

 Prof. Tanner finds that in improved breeds the lungs and 

 liver are considerably reduced in size when compared with 

 those organs in animals having perfect liberty — thus changing 

 the form of their bones by respiration and nutriment. 



But the kind of evidence which more particularly concerns 

 the subject now is the converse of this. Thus ungulate ani- 

 mals which are light of body (deer, horses, &c.) have the limbs 

 longer than have most unguiculate animals ; and as a rule, 

 those hoofed animals are more active, and strike the ground 

 with greater force, so that the bones can act on each other 

 more powerfully. And in man, where the position of the body 

 is erect, and the habit not active, so that the weights of the 

 upper parts of the body act on each other with no violent 

 pressure, and that alternating with rest in sleep, the vertebra? 

 will be seen to steadily enlarge, from the neck down to the 

 sacrum. But in quadruped animals with a large head carried 

 erect in running and pendent in seeking food, like the deer, 

 the cervical vertebree will be seen to be longer and larger than 

 the dorsal vertebrae ; and here it is to be remarked that the 

 neck-bones have to support the weight of the head, and that 

 their processes experience the pressure and tension caused by 

 its movements, while the back-bones only have to share be- 

 tween them the general weight and tension of the carcass. 

 In animals which walk erect, and chiefly use the hind limbs, 

 the hind limbs are longer than the fore limbs, as in man and 

 the ostrich, and in jumping animals, such as kangaroos, jer- 

 boas, frogs, &c. On the other hand, animals which use their 

 fore limbs more than the hind limbs, have them longer than 

 the hind limbs : familiar examples of this condition are seen 

 in the tribe of bats and in most birds, such as the albatross or 

 the swan. Here the pressure and tension experienced by the 

 bones in flight is very great in comparison with the influences 

 which could stimulate growth in the hind limbs ; and the 

 growth is greater. 



Special modification of structures in relation to modified 

 function may be seen in the humerus of the burrowing mole : 

 this bone experiences enormous lateral tension, and accordingly 



