TROPrSMS AND RELATED PHENOMENA 159 



The more fertile a principle is, the more we can afford to be conserva- 

 tive in applying it. It is obvious that certain reactions have been called 

 tropisms which have nothing to do with them; possibly Roux's cyto- 

 tropism belongs to this group. Roux has observed motion of the cleav- 

 age cells of the germ of the frog's egg to and from each other; he has 

 called these cases cytotropism. Driesch has pointed out that these are 

 phenomena which are caused purely by capillary forces between the eggs. 

 If this be correct, as it seems to be, and if we are not dealing in this case 

 with a reaction of hving matter to an outside stimulus, we are not deal- 

 ing with a tropism ; for by the latter we mean distinctly a class of com- 

 pulsory reactions of the organism to outside stimuli ; but not the passive 

 motions of bodies caused by capillary forces. That these bodies consist 

 of living protoplasm does not influence this discrimination. 



Another warning to be careful in applying this principle was shown 

 by recent investigations of E. P. Lyon.* It is a well-known fact that 

 many fishes put their bodies into the direction of a current of water, and 

 try to swim against the current. It was commonly supposed that this 

 orientation was caused by the streaming of the water, possibly its fric- 

 tion against the sides of the body. Lyon has shown that this behavior 

 is an optical reflex caused by the apparent motion of the object while 

 the animal is moved passively by the stream. When he inclosed the 

 fish in glass jars and dragged these jars through the water, the fish inside 

 the jars oriented themselves in the direction opposite to that in which 

 the jar was moved. There is no objection to calhng this a tropic re- 

 action, but it is certain that it should no longer be called rheotropism. 



Attention should also be called to the fact that while the tropisms 

 form in many cases the mechanism by which the preservation of the 

 individual and the species is brought about, there are many cases of 

 tropism which are of no use to the species ; the whole field of galvano- 

 tropism is an example of this. Galvanotropism is purely a laboratory 

 phenomenon; outside of the laboratory no animal ever comes into a 

 situation which might call forth a galvanotropic reaction; yet galvano- 

 tropism is not uncommon among animals. Among the positively heho- 

 tropic animals, we find forms which are never exposed to the light, e.g. 

 the caterpillar of the willowborer, or Cuma Rathkii, a Crustacean which 

 lives in the mud at Kiel. I pointed out sixteen years ago that these 

 cases speak against the assumption that the tropisms could have been 

 acquired by the way of natural selection, f and Morgan has recently 

 taken the same ground; J but I do not wish to enter upon a criticism of 



* E. p. Lyon, Am. Jour. Physiology, Vol. 12, 149, 1904. 



t Loeb, Der HeliotropUmm der Thiere, 1889. 



\ T. H. Morgan, Adaptation and Evolution, New York, 1904. 



