462 POPULAR SCIENCE MONTHLY. 



have taken place by a process which involved simultaneous changes in 

 both fore and hind limbs. The support of the body, hitherto laid upon 

 all four limbs, could not have been taken over at once in its final 

 adequacy and security by the legs alone, unless we conceive of a spon- 

 taneous variation of improbably large extent. The animal at first 

 raised itself hesitatingly upon its hind limbs, supporting its weight in 

 part by the grasp of the hands upon higher portions of the trunk and 

 branches, thus distributing the function as heretofore among the whole 

 set of limbs, but in such a way that the fore limbs were adapted to 

 their new specific use while performing their old generic function. 

 The body, in this stage of development, was sustained in part by sup- 

 port from beneath and in part by suspension from above. Either of 

 these factors may be conceived as appropriating a chief place in the 

 locomotive function; and in different animal species these divergent 

 directions of development are both presented, progression by swinging 

 from limb to limb in the long-armed apes, and by the sole use of the 

 legs in man. 



It is probable that the progenitor of man, together with the whole 

 group of anthropoid apes to which he belongs, maintained the quadru- 

 pedal position longer than those types which, like the Cebidse, e. g., 

 the tee-tees and Capuchin monkeys, present no opposition in the mem- 

 bers of the fore limbs. If we conceive the semi-upright position to 

 have been assumed at a time anterior to the development of opposition 

 in the hind limb, say at the beginnings of arboreal existence, so that 

 from the outset each pair of limbs was modified under different condi- 

 tions of function, it will be found difficult to imagine the causes which 

 under these unlike circumstances brought about a similar modifica- 

 tion in each set of limbs. If, on the other hand, both fore and hind 

 limbs were used to support the animal in a quadrupedal position upon 

 the branch beneath it during the early period of arboreal life, it will 

 be as difficult to imagine a reason why both sets of limbs should not 

 present the same type of adaptation. The condition which predisposes 

 to conservation of the phenomenon of opposition is support, not 

 suspension; it is peculiarly a modification of the foot. All that is 

 involved in successful adaptation to the function of suspension is the 

 existence of sufficient elongation in the digits, flexibility in the joints 

 and strength in the muscles — the development of a strong and supple 

 member, but not necessarily one possessing an opposable thumb. Even 

 a single series of joints may form an efficient instrument of suspension, 

 as in the case of the prehensile tail of the monkey tribe. For support 

 upon the rounded branch beneath, on the other hand, some sort of 

 forking is almost the only modification which could give security, 

 and in the man-like ape this has taken the form of an opposition be- 

 tween a single member and the rest of the group. 



We may therefore conceive that the progenitors of the Capuchins 



