24 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



asserted by the theory of " taxis " to occur if the intensity 

 of the original stimulus was increased, or if other stimuli 

 came into play. The facts were quite true, but their real 

 explanation now proves to be of a much more general kind. 

 In fact, there may also be " acclimatisation," say to 

 chemical stimuli ; then the avoiding reaction shown at first 

 will not be shown any longer after a certain time : " nega- 

 tive chemotaxis ' : will cease to exist. And other kinds of 

 stimuli, coming into competition with the original one, 

 may result in the same effect. 1 



But now we come to two classes of modifications of 

 single motor acts, which possess a great importance for all 

 that is to follow. 



There may be a typical series of consecutive different 

 single motor reactions, whenever the first or any following 

 one of these reactions has not avoided the external stimulus 

 or has not reached the condition " desired," and this typical 

 series may go on until the " desired " state is actually reached. 

 Such typical lines of different single reactions have been 

 well studied by Jennings and his followers in many cases, 

 the most typical ones occurring in the infusorium Stentor 

 and in Actinians. If a Stentor is disturbed, say by some sort 

 of light powder falling upon it, it first bends to one side 



1 A very remarkable fact of this class lias recently been discovered by 

 Minkiewicz (Arch. Zool. exp. et gen. 4 ser. 7, notes, 1907) : the crab Maia 

 may change the quality not the "sense" of its " chromotropism," which 

 is independent of its reaction to light in general, according to the colour 

 of the ground it lives upon, and another crab, Hippolyte, changes its colour 

 and its chromotropism correspondingly. In this case the whole phenomenon 

 falls most markedly under the concept of what we have called "physiological 

 adaptation " in the first volume of this book. Indeed, the question may arise, 

 whether all modifications of primitive motor irritability may not be considered 

 under this heading in further analytical studies. Of course, what Minkiewicz 

 calls chromo-" tropism " ought rather to be styled chromo-" taxis," and, most 

 probably, is no real "taxis." 



