320 



SCIENCE 



[N. S. Vol. XXYL No. 662 



To these conclusions I believe we may justly 

 add another that has not yet been formulated 

 in the discussion. 



If all definite directive responses to stimuli 

 have been produced by the selection of re- 

 sponses " that favor the rtormal life processes " 

 as Jennings appears to believe, then such 

 directive responses must be adaptive, must be 

 of distinct advantage to the organism possess- 

 ing them in the struggle for existence. There 

 are, however, among certain organisms and in 

 connection with certain classes of stimuli defi- 

 nite reactions which do not appear to serve 

 the organism in any way. 



In the first place, the phenomena of galvano- 

 tropism are obviously not in any way related 

 to a possible adaptive value. In the second 

 place. Professor Loeb mentioned long ago the 

 caterpillar of the willow borer, and Diastylis 

 (Cuma) rathhii, as two animals that live away 

 from the light, the one buried in the wood of 

 trees, the other in the mud of bays and lagoons, 

 that yet react positively when exposed to light. 

 Such instances are brushed aside as insignifi- 

 cant by Jennings, because few have been re- 

 corded. I feel confident, however, that in- 

 stances of this sort will multiply. And far 

 from being insignificant, it is most fortunate 

 for the analysis of behavior that, in a world 

 where the struggle for existence is so intense, 

 even a few organisms have been found whose 

 behavior has remained unaffected by it. 



I do not think it is necessary to go farther 

 into the facts to make it clear that the hy- 

 pothesis advanced by Jennings is not suffi- 

 ciently broad to encompass all the phenomena 

 it is devised to explain. As a method of an- 

 alysis, it is essentially historical. It seeks to 

 derive all forms of organic behavior from a 

 simple -type or unit assumed to be funda- 

 mental. We have seen, however, that the 

 assumed fundamental unit is really not funda- 

 mental physiologically, since it is based 

 squarely upon a psychological conception. 

 The method, therefore, prescribes the inter- 

 pretation of purely physiological phenomena, 

 such as reflexes and tropisms, in terms of psy- 

 chology. From the standpoint of effective an- 

 alysis, this is surely putting the cart before 



the horse. The trial and error program looks 

 very much like a modern recrudescence of the 

 attitude toward the problems of behavior that 

 could tolerate the interpretation of the be- 

 havior of a motli toward a flame as an ex- 

 hibition of curiosity. 



We may now examine somewhat more closely 

 than has been possible so far, another inter- 

 pretation, known by the name of " tropism 

 hypothesis," which has been applied to certain 

 aspects of the behavior of organisms and has 

 been sharply attacked by Jennings. Far from 

 pretending to be a universal formula, it has 

 been suggested in various forms by various 

 investigators for the purpose of testing the 

 applicability to the problems of organic be- 

 havior of the data of physics and chemistry. 

 It is a guide for analysis along experimental 

 rather than historical lines, and in accord 

 with its reason for being, is dependent upon 

 no psychological data of any sort. 



It has appeared from the preceding discus- 

 sion that there is ground for believing in the 

 existence of two classes of stimuli in nature, 

 and that according to the tropism hypothesis 

 both may elicit primary responses. 



The view that the definitely directive re- 

 sponses known as tropisms are primary does 

 not rest, however, merely on whatever pre- 

 sumptive evidence the curtailment of the trial 

 and error program may admit. There are 

 numerous examples in nature of the depend- 

 ence of the tropic reaction upon the physiolog- 

 ical condition of the organism. The larvae of 

 Polygordius, a marine annelid worm, when 

 taken, are negatively heliotropic. Two hours 

 later, they may be positively heliotropic. This 

 change may be obtained immediately by cool- 

 ing them down to a temperature of 7° C. 

 The response may again be reversed by sud- 

 denly diluting the salt water containing the 

 larvae with one third to two thirds its volume 

 of fresh water. The sense of the resulting 

 response may in turn be reversed by increasing 

 the concentration of the water. I have al- 

 ready referred to the barnacle larvse that are 

 positively heliotropic immediately after hatch- 

 ing, but, after a certain limited exposure to 

 light, become negative. Terrestrial amphi- 



