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. 

 Isow 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 unpublished by him.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 



