THE VITAL IMPETUS 147 



however, the orientation would be affected through the 

 chain of sense-organ, afferent nerve, nerve centre, 

 efferent nerve, and effector organ. This is the chain 

 of events which on this hypothesis causes a moth to 

 fly into a flame, or a sea-bird to dash itself against the 

 lantern of a lighthouse. 



A taxis is, then, an inevitable response by move- 

 ment in a definite direction, to a directed stimulus. 

 Including also tropisms it may be admitted that the 

 movement is a purposeful, or at least, a useful one in 

 some cases, as for instance the heliotropism and 

 geotropism of the green plant. If we admit that Loeb's 

 description of the feeding of the caterpillar, as a tactic 

 act, is true, we may also call this a useful act. But in 

 the majority of cases tropisms and tactes are acts which 

 appear to be of no use to the organism. The invasion 

 of a part of the body which is irritated by a poison (as 

 in inflammation) by leucocytes, is useful to the body 

 itself, but we must regard the leucocytes as organisms, 

 and their tactic motion leads to their destruction, and 

 so also with other analogous acts. Just because of 

 this we find difficulty in accounting for their origin in 

 terms of natural selection. 



This does not matter so much, since it can hardly 

 be maintained now that the tropistic or tactic act has 

 any reality except in a very few cases — the motions 

 of plants, galvano-taxis, the chemico-taxic movements 

 of bacteria and leucocytes, and some other analogous 

 cases, perhaps, are these exceptions. It can hardly 

 be doubted that the extension of the concept to cover 

 the motions of many invertebrates, and even some 

 vertebrate actions, by Loeb and his school is a straining 

 after generality which has not been justified. The 

 hypothesis, as Loeb has stated it, is evidently almost 

 certainly a logical one and was obviously elaborated 



