tqc 



THE INADEQUACY OF "NATURAL SELECTION." 25 



the fingers of one and how slender those of another ; or it needs 

 but to note the unlikeness of gait of passers-by, implying small 

 unlikenesses of structure; to be convinced 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 anatomical details are. set down. 



Let us suppose a species of quadruped of which the members 

 have for long past periods been accustomed to locomotion over a 

 relatively even surface, as, for instance, the "prairie dogs" of 

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

 driven part of them into a region full of obstacles to easy locomo- 

 tion covered, say, by the decaying stems of fallen trees, such as 

 one sees in portions of primeval forest. Ability to leap must be- 

 come a useful trait ; and, according to the hypothesis we are con- 

 sidering, this ability will be produced by the selection of favor- 

 able 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 mus- 

 cles, by which the hind limbs are straightened. Their increases 

 must be duly proportioned, for if those which straighten one 

 joint become much stronger than those which straighten 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 reaction of the 

 leap, though the toes may have a share. In a digitigrade mam- 

 mal, however, the toes form almost exclusively the fulcrum, and 

 if they are to bear the reaction of a higher leap, the flexor mus- 

 cles which depress and bend them must be proportionately en- 

 larged ; 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 their phalanges. 

 Stronger muscles and tendons imply greater strains on the joints ; 

 and unless these are strengthened, one or other dislocation will be 

 caused by a more powerful spring. Not only the articulations 

 themselves must be so modified as to bear greater stress, but also 

 the numerous ligaments which hold the parts of each in place. 

 Nor can the bodies of the bones remain unstrengthened ; for if 

 they have no more than the strengths needed for previous move- 



VOL. XLIII. 3 



