EVOLUTION OF THE HUMAN HAND. 457 



THE EVOLUTION OF THE HUMAN HAND. 



By Professor ROBERT MacDOUGALL, 



NEW YORK UNIVERSITY. 



n ^HE succession of organic modifications which resulted in the 

 -*- formation of the human hand is part of the general process of 

 evolution by which in the animal series the means of progression and 

 of the taking of food were shaped by the environmental conditions 

 under which life was carried on. Antecedent to the appearance of 

 vertebrate limbs a series of manifold devices had originated by which 

 the body could be transported from place to place and appropriate 

 foodstuffs seized and carried to the mouth. These consisted of more 

 or less permanent extensions of the body substances, naked or clothed 

 in protective shields of denser material. In some types the limbs were 

 created in the act of extension itself and were retracted by absorption 

 and disappearance into the general body mass; in some they were 

 formed of erectile tissues which could be protracted or withdrawn 

 as occasion demanded; in some the whole body was thus contractile, 

 and alternately elongated and shortened as the animal progressed; in 

 some the organs of locomotion consisted of definitely formed limbs, 

 which, while subject to loss by violence or even sudden shock, might 

 be repeatedly and perfectly regenerated in the course of the individual 

 life. In the forms to which they are molded and the mechanical prin- 

 ciples upon which they depend, these organs of movement present the 

 utmost variety, including ameboid extensions, flagellate cilia, pulsating 

 bells, contractile stalks and bodies, suckered tentacles, swimming fins 

 and tails, wings and articulated legs. They appear as a great series 

 of adaptive levels through which the evolution of this particular 

 mechanism passed toward more highly integrated and developed types. 

 The functions of life which call into service the bodily limbs are 

 chiefly two — locomotion, an activity which has arisen in connection 

 with the search for food and flight from enemies; and prehension, 

 which is concerned primarily with the grasping and tearing of food, 

 but secondarily also with processes assistive of locomotion and other 

 biological functions, such as sexual congress, the care of the body, 

 burrowing and climbing. Of these two functions, if we regard the 

 vertebrate class only, the former is the more primitive. Upon the 

 office of locomotion the prehensive and manipulative activities of the 

 limb have been superposed as subsequent and more specialized adapta- 

 tions. In vertebrates of less modified types the food is seized and 

 manipulated by the mouth parts directly. Fish, reptiles and birds 



