EXTERNAL INFLUENCES AS CAUSES OF VARIATION 271 



Tumors. Those overgrowths of various parts of the body, 

 called tumors, arising from causes not well understood, have 

 their specific characters as truly as if derived from inherit- 

 ance. Whether the character of the tumor is derived primarily 

 from the tissues affected or from some outside specific cause is 

 not known, but it is a significant fact that the protoplasm, which 

 derived its characters originally by inheritance, has undergone 

 permanent and definite alteration through the operation of causes 

 absolutely distinct from inheritance, whether internal or external 

 to the organism, thus showing the possibility of diverting the ener- 

 gies of inherited material into absolutely new channels through 

 apparently slight causes. 



At this point it is well to call our attention to the profound 

 changes (permanent variations) wrought upon the constitution 

 of the individual by such internal-external circumstances as vacci- 

 nation or the injection of antitoxin, as well as to the immunity 

 acquired through a single attack of an infectious disease. 



Germination of seeds. It is a well-known fact that certain 

 chemicals accelerate and others retard the process of germina- 

 tion. Just why this is true is not clear on any other ground 

 than that of the ordinary susceptibility of growing protoplasm 

 to stimulants and sedatives. The process of germ development 

 requires, in addition to what is contained within the seed, only 

 oxygen, water, and a favorable temperature ; the influence of 

 other chemicals must be indirect. 



Chemotaxis and chemotropism. 1 The influence of chemical 

 substances upon the locomotion of free-moving organisms is tech- 

 nically known as chemotaxis, and their influence upon the direc- 

 tion of growth (in plants) is known as chemotropism. 2 The 

 distinction is hardly worth observing for our purposes, because 

 both phenomena arise from a direct influence of chemical sub- 

 stances upon living protoplasm, either attractive or repellent. If 

 attractive, it (the protoplasm) will move toward the material in 



1 C. B. Davenport, Experimental Morphology, Part I, 32-45 ; Part II, pp. 

 335-342; Loeb, Physiology of the Brain, pp. 50, 88-90, 118, 186-188. 



2 As has already been noted, the same distinction is often observed between 

 geotaxis, the influence of gravity upon locomotion, and geotropism, its influence 

 upon the direction of growth ; also between heliotaxis and heliotropism as cover- 

 ing corresponding influences of the sun (light). 



