ORGANIC MOVEMENTS 15 



Nothing of course would be explained by calling any 

 process of movement of this sort " taxis " : but " taxis ' : 

 certainly would be a good name for embracing a rather 

 simple class of co-ordinated movements, which have a 

 very apparent common feature in the fixed relation of the 

 directions between the stimulus or cause and the final 

 effect, reached without any interruption in an unbroken 

 line. 



It is true, the phenomena of this so-called taxis were 

 known not to be so simple as described here ; there were 

 all the kinds of complications known from the phenomena 

 of tropisms. Taxis was called " positive " in the case when 

 the anterior end of the organism was finally placed towards 

 the stimulating source, and it was called " negative " in the 

 opposite case. Now it was found that the same organism, 

 which had proved to be positively phototactic or chemo- 

 tactic, could react negatively when the intensity of the 

 stimulus increased, and conversely. But the point of this 

 change was by no means fixed for a given individual ; the 

 organism could become adapted or acclimatised to a stimulus 

 which at first had caused the avoiding or negative reaction, 

 and could thus become positive without any change of the 

 medium. But other conditions of the medium, such as its 

 salinity or temperature, were also found to have an influence 

 upon the " sense " of taxis, say with regard to the rays of 

 the sun (J. Loeb). 



That was the general state of the apparently well 

 established theory of taxis about six years ago. Was it 

 possible to explain all these facts as being simple and 

 machine -like in the same way as simple reflexes? The 

 difficulties, as with tropisms, lay in the variability of the 



