618 APPENDIX B. 



conspicuous proofs. Many of the muscles of the face and some 

 of the bones, are distinctly co-operative ; and these respectively 

 vary in such ways as to produce in each person a different com 

 bination. What we see in the face we have reason to believe 

 holds in the limbs and in all other parts. Indeed, it needs but to 

 compare people whose arms are of the same lengths, and observe 

 how stumpy are the fingers of one and how slender those of 

 another ; or it needs but to note the unlikcnesses of gait of 

 passers-by, implying small unlikenesses of structure ; to be con 

 vinced that the relations among the variations of co-operative 

 parts are anything but fixed. And now, confining our attention 

 to limbs, let us consider what must happen if, by variations 

 taking place miscellaneously, limbs have to be partially changed 

 from fitness for one function to fitness for another function have 

 to be re-adapted. That the reader may fully comprehend the 

 argument, he must here have patience while a good many ana 

 tomical details are set down. 



Let us suppose a species of quadruped of which the members 

 have, for immense past periods, been accustomed to locomotion 

 over a relatively even surface, as, for instance, the &quot; prairie-dogs &quot; 

 of North America ; and let us suppose that increase of numbers 

 has driven part of them into a region full of obstacles to easy 

 locomotion covered, say, by the decaying stems of fallen trees, 

 such as one sees in portions of primeval forest. Ability to leap 

 must then become a useful trait ; and, according to the hypo 

 thesis we are considering, this ability will be produced by the 

 selection of favourable variations. What are the variations 

 required ? A leap is effected chiefly by the bending of the hind 

 limbs so as to make sharp angles at the joints, and then suddenly 

 straightening them ; as any one may see on watching a cat leap 

 on to the table. The first required change, then, is increase of the 

 large extensor muscles, by which the hind limbs are straightened. 

 Their increases must be duly proportioned ; for if those which 

 straightened one joint become much stronger than those which 

 straightened the other joint, the result must be collapse of the 

 other joint when the muscles are contracted together. But let 

 us make a large admission, and suppose these muscles to vary 

 together ; what further muscular change is next required ? In a 

 plantigrade mammal the metatarsal bones chiefly bear the reac 

 tion of the leap, though the toes may have a share. In a digiti- 

 grade mammal, however, the toes form almost exclusively the 

 fulcrum, and if they are to bear the reaction of a higher leap, 

 the flexor muscles which depress and bend them must be pro 

 portionately enlarged : if not, the leap will fail from want of a 

 firm point d appui. Tendons as well as muscles must be modified ; 

 and, among others, the many tendons which go to the digits and 



