STIMULI AND THEIR ACTIONS 429 



peculiar effects. But the sensations of our childhood are again 

 awakened when we meet with effects analogous to those that the 

 magnet exerts upon the needle, namely, attraction and repulsion , 

 transferred to living nature as results of a great variety of stimuli ; 

 and when we see that these stimuli are capable of exercising upon 

 organisms an effect that forces them under certain circumstances 

 to turn toward or away from the source of the stimulus with the 

 same irresistible power and the same unfailing certainty as the 

 magnetism forces the iron. The moth returns with deadly certainty 

 to the light ; although it has singed its wings innumerable times, 

 it cannot resist the fascinating power and finally falls into the flame. 



Since, in the higher animals, as a result of the co-operation of the 

 nervous system, these phenomena possess a complexity that renders 

 their examination more difficult and not rarely diminishes the 

 certainty of the reaction, it is advantageous to consider them more 

 especially from the standpoint of cell-physiology. 



It is necessary to the occurrence of the phenomena in question 

 that differences in stimulation exist in different parts of the body. 

 If stimuli act equally upon all sides, all the effects of stimula- 

 tion described in the preceding section occur, but a directive 

 effect is necessarily absent. Only unsymmetrical stimulation can 

 control the direction of motion. 



1. Chemotaxis. 1 



The word chemotaxis is applied to that property of organisms 

 that are endowed with the capacity of active movement by which 

 when under the influence of chemical stimuli acting unilaterally 

 they move toward or away from the source of the stimulus. Where 

 there is an approach to the source of the stimulus, there is positive 

 chemotaxis, where there is a removal from the source negative 

 chemotaxis. Unilateral stimulation with chemical stimuli is only 

 realised when the concentration of the substance in question 

 gradually increases from the living object in one direction. 



Discovered first by Engelmann in Bacteria, observed by Stahl in 

 Myxomycetes, studied systematically and more fully by Pfeffer, and 

 recently investigated in leucocytes by Massart, Leber, Gabrits- 



1 Although the words ' ' chemotropism " " heliotropism," etc., have been long in 

 use, I have decided, after considerable delay, to exchange them in this edition of 

 the book for the words " chemotaxis," " phototaxis," etc. ; my reason is that the 

 former not only sound heavy, but suggest objections from the philological stand- 

 point. I come to this conclusion with some reluctance because for some time I 

 have been endeavouring to extend the former terminology from the few, earlier known 

 "tropisms" to the corresponding phenomena associated with other stimuli, and 

 admit new tropisms. thus indicating at once by the term used the fact that all 

 the phenomena belong in the same class. Now, however, when the analogy of 

 the phenomena that result from the various kinds of stimuli is fully recognised, I 

 believe it advantageous to replace the less fortunate terms by the newer expres- 

 sions, already much employed. 



