MOVEMENT 



pliragm, which completely divides the abdominal and 

 thoracic cavities ; the various muscles, from the blending 

 of which it has been formed, still remain separate in the 

 other vertebrates. 



The many organs by means of which our human or- 

 ganism accomplishes its manifold movements are just 

 the same as in the apes, and the mechanism of their 

 action is in no way different. The same two hundred 

 bones, in the same order and composition, form our in- 

 ternal bony skeleton; the same three hundred muscles 

 effect our movements. The differences we find in the 

 form and size of the various muscles and bones (and 

 which are, as is well known, also found between lower 

 and higher races of men) are due to differences in 

 growth in consequence of divergent adaptation. On 

 the other hand, the complete agreement in the con- 

 struction of the whole motor apparatus is explained 

 by heredity from the common stem-form of the apes 

 and men. The most striking difference between the 

 movements of the two is due to man's adaptation to 

 the erect posture, while the climbing of trees is the 

 normal habit of the ape. However, it is unquestion- 

 able that the former is an evolution from the latter. 

 A double parallel to this modification is seen in the 

 jerboa among the ungulates, and in the kangaroo 

 among the marsupials. Both these, in springing, use 

 only the strong hinder extremities, and not the weaker 

 fore-limbs; as a result of this their posture has become 

 more or less erect. Among the birds we have an analo- 

 gous case in the penguins (aptenodytes); as they no 

 longer use their atrophied wings for flight, but only in 

 swimming, they have developed an erect posture when 

 on land. 



The human will is also not specifically different from 

 that of the ape or any other mammal; and its micro- 

 scopic organs, the neurona in the brain and the muscular 



285 



