484: SEGREGATION. 



are brought into the most obvious relation, are supplied by 

 the limbs. The metacarpal bones (those which in man sup- 

 port the palm of the hand) are separate from each other in 

 the majority of mammalia: the separate actions of the 

 toes entailing on them slight amounts of separate move- 

 ments. This is not so however in the ox-tribe and the 

 horse-tribe. In the ox-tribe, only the middle metacarpals 

 (third and fourth) are developed; and these, attaining mas- 

 sive proportions, coalesce to form the cannon bone. In the 

 horse-tribe, the segregation is what we may distinguish 

 as indirect: the second and fourth metacarpals are present 

 only as rudiments united to the sides of the third, while 

 the third is immensely developed; thus forming a cannon 

 bone which differs from that of the ox in being a single 

 cylinder, instead of two cylinders fused together. The 

 metatarsus in these quadrupeds exhibits parallel changes. 

 Now each of these metamorphoses occurs where the differ- 

 ent bones grouped together have no longer any differ- 

 ent functions, but retain only a common function. The 

 feet of oxen and horses are used solely for locomotion are 

 not put like those of unguiculate mammals to purposes 

 which involve some relative movements of the metacarpals. 

 Thus there directly or indirectly results a single mass of bone 

 where the incident force is single. And for the inference 

 that these facts have a causal connexion, we find confirma- 

 tion throughout the entire class of birds; in the wings 

 and legs of which, like segregations are found under like 

 conditions. While this sheet is passing through the 



press, 'a fact illustrating this general truth in a yet more 

 remarkable manner, has been mentioned to me by Prof. 

 Huxley ; who kindly allows me to make use of it while still 

 unpublished by him. The Glyptodon, an extinct mammal 

 found fossilized in South America, has long been known as a 

 large uncouth creature allied to the Armadillo, but having a 

 massive dermal armour consisting of polygonal plates close- 

 ly fitted together so as to make a vast box, inclosing the body 



