BIOLOGICAL INTRODUCTION 23 



which they first take root, whereas the power of locomotion 

 carries with it, at least potentially, the ability to choose between 

 many more environmental factors. It is only the free-moving 

 animals that have anything to gain by looking ahead in the 

 world, and here only do we find well-developed distance recep- 

 tors, i. e., sense organs adapted to respond to impressions from 

 objects remote from the body. And the distance receptors, as 

 we shall see, have dominated the evolution of the nervous sys- 

 tem in vertebrates and determined the lines it should follow. 



The net result of this discussion can be briefly stated. The 

 differences between various kinds of organisms are, in the main, 

 incidental to the extent and character of their relations with the 

 forces of surrounding nature. A species which can adjust itself 

 to few elements of its environment we call low; one that can 

 adapt itself to a wide range of environmental conditions in a 

 great variety of ways we call higher. The supremacy of the 

 human race is directly due to our capacity for diversified living. 

 If man finds himself in an unfavorable climate, he may either 

 move to a more congenial locality or adapt his mode of life by 

 artificial aids, such as clothing, houses, and fire. And in these 

 adaptations he is not limited to a narrow range of inherited 

 instincts, like the hive of bees, but his greater powers of obser- 

 vation and reflection enable him to discover the general uni- 

 formities of natural process (he calls these laws of nature) and 

 thus to forecast future events and prepare himself for them in- 

 telligently. In other words, to return to our original point of 

 view, our advantage in the struggle for existence lies in our 

 ability to correlate our bodily activities with a wide range of 

 natural forces so as to make use of these forces for our good rather 

 than our hurt. (Of course, it should be borne in mind that this 

 formula makes no pretense of being an exhaustive account of 

 human faculty; but only that, in so far as biological evolutionary 

 factors have operated in the human realm, they act in accord- 

 ance with this principle.) The apparatus by which these exter- 

 nal adjustments are effected and by which the inner parts of the 

 body are kept in working order is the nervous system. 



