September 18, 1908] 



SCIENCE 



357 



as distinguished from permanent anatom- 

 ical conditions.' " Though he does not 

 claim novelty for his view, I am not aware 

 that it has ever been so well stated. Ex- 

 ternal stimuli are supposed to act by alter- 

 ing this physiological state; that is, the 

 organism is temporarily transformed into 

 what, judged by its reactions, is practi- 

 cally a different creature. 



This may be illustrated by the behavior 

 of Stentor, one of the fixed infusoria.' If 

 a fine jet of water is directed against the 

 disc of the creature, it contracts "like a 

 flash" into its tube. In about half a 

 minute it expands again and the cilia re- 

 sume their activity. Now we cause the 

 current to act again upon the disc. This 

 time the Stentor does not contract, which 

 proves that the animal has been in some 

 way changed by the first stimulus. This 

 is a simple example of "physiological 

 state." When the Stentor was at rest, 

 before it received the first current of 

 water, it was in state 1, the stimulus 

 changed state 1 into sta,te 2, to which con- 

 traction is the reaction. When again 

 stimulated it passed into state 3, which 

 does not produce contraction. 



We can not prove that the contraction 

 which occurred when the Stentor was 

 first stimulated was due to a change of 

 state. But it is a fair deduction from the 

 result of the whole experiment, for after 

 the original reaction the creature is un- 

 doubtedly in a changed state, since it no 

 longer reacts in the same way to a repeti- 

 tion of the original stimulus. 



Jennings points out that, as in the case 

 of plants, spontaneous acts are brought 

 about when the physiological state is 

 changed by unknown causes, whereas in 

 other eases we can point to an exter- 

 nal agency by which the same result is 

 effected. 



* Jennings, " Behavior of the Lower Organisms," 

 1906, p. 170. 



MORPHOLOGICAL CHANGES 



Let US pass on to the consideration of 

 the permanent or morphological changes 

 and the stimuli by which they are pro- 

 duced, a subject to which, in recent years, 

 many workers have devoted themselves. 

 I need only mention the names of Vocht- 

 ing, Goebel and Klebs, among botanists, 

 and those of Loeb, Herbst and Driesch 

 among zoologists, to remind you of the 

 type of research to which I refer. 



These morphological alterations pro- 

 duced by changes in environment have 

 been brought under the rubric of reaction 

 to stimulation, and must be considered as 

 essentially similar to the class of tempor- 

 ary movements of which I have spoken. 



The very first stage in development may 

 be determined by a purely external stim- 

 ulus. Thus the position of the first cell- 

 wall in the developing spore of Equise- 

 tum is determined by the direction of 

 incident light.' In the same way the 

 direction of light settles the plane of 

 symmetry of Marchantia as it develops 

 from the gemma.^" But the more inter- 

 esting cases are those where the presence 

 or absence of a stimulus makes an elabor- 

 ate structural difference in the organism. 

 Thus, as StahP^ has shown, beech leaves 

 developed in the deep shade of the middle 

 of the tree are so different in structure 

 from leaves grown in full sunlight that 

 they would unhesitatingly be described as 

 belonging to different species. Another 

 well-known case is the development of the 

 scale-leaves on the rhizome of Circcea into 

 the foliage leaves under the action of light.^^ 



The power which the experimenter has 

 over the lower plants is shown by Klebs, 

 who kept Saprolegnia mixta, a fungus 

 found on dead flies, in uninterrupted veg- 



' StaU, Ber. d. Bot. Ges., 1885, p. 334. 

 '° Pfeffer, in Sachs's Arbeiten, I., p. 92. 

 '^Jenaische Zeitschr., 1883, p. 162. 

 "Goebel, in Bot. Zeitung, 1880. 



