2IO Animal Life and Intelligence. 



generations more and more adapted to its functional work. 

 To give but one example. It is well known that certain 

 hoofed creatures are divisible into two groups — first, those 

 which, like the horse, have in each limb one large and 

 strong! digit armed with a solid hoof ; and, secondly, those 

 which, like the ox, have in each limb two large digits, so 

 that the hoof is cloven or split. It is also well known that 

 the ancestral forms from which both horse-group and ox- 

 group are derived were possessed of five digits to each 

 limb. Professor Cope regards the differentiation of these 

 two groups as the result of the different modes of use 

 necessitated by different modes of life. " The mechanical 

 effect," he says, " of walking in the mud is to spread the 

 toes equally on opposite sides of the middle line. This 

 would encourage the equal development of the digits on 

 each side of the middle line, as in the cloven-footed types. 

 In progression on hard ground the longest toe (the third) 

 will receive the greatest amount of shock from contact with 

 the earth." * Hence the solid-hoofed types. Here, then, 

 the middle digit in the horse-group, or two digits in the ox- 

 group, having the main burden to bear, increase through 

 persistent use, while the other digits dwindle through dis- 

 use. f 



On the other hand, one who holds the opposite view wiU 



* Cope, « Origin of the Fittest," p. 374. 



t It would appear, from certain passages of his " Darwinism," that Mr. 

 A. K. Wallace {e.g. p. 139, note) holds or held similar views. " The 

 genera Ateles and Colobus" he says, " are two of the most purely arboreal 

 types of monkeys, and it is not diificult to conceive that the constant use of 

 the elongated fingers for climbing from tree to tree, and catching on to 

 branches while making great leaps, might require all the nervous energy and 

 muscular growth to be directed to the fingers, the small thumb remaining 

 useless." I should also have quoted Mr. Wallace's account of the twisting 

 round of the eyes of flat-fishes — where he says that the constant repetition 

 of the efibrt of twisting the eye towards the upper side of the head, when 

 the bony structure is still soft and flexible, causes the eye gradually to move 

 round the head till it comes to the upper side — had he not subsequently dis- 

 claimed this explanation (see Nature, vol. xl. p. 619). It is possible that Mr. 

 Wallace, notwithstanding the words " constant use" in the passage I liave 

 quoted, merely intends to imply that the elongated fingers are of advantage 

 in climbing, and are thus subject to natural selection, the thumb diminishing 

 through economy of growth. 



