322 



SCIENCE 



[N. S. Vol. XXVI. No. 602 



(photoehemie?) activity is inhibited by the contin- 

 uous formation of certain stuffs in the body, e. g., 

 in the eyes. If, now, we assume that acid inhibits 

 the formation of these latter antibodies, then the 

 positive effect of the acid is intelligible. Just so 

 is the positive effect of the fall in temperature 

 intelligible, since thereby the rapidity of the for- 

 mation of the inhibitive antibodies is diminished. 



Though the nature of the substances con- 

 nected with the heliotropic reaction is not 

 definitely known at present, experiments have 

 suggested strongly that they may be of the 

 nature of oxidases. Researches soon to be 

 published confirm this view in a striking 

 manner. 



These facts still further emphasize the im- 

 probability of the production of tropic reac- 

 tions as the result of the selection of a series 

 of trial movements; while they further em- 

 phasize the probability that such reactions, 

 dependent upon the presence of definite chem- 

 ical substances, have sprung suddenly into 

 existence in the manner of the mutations of 

 de Vries. As such, they provide material for 

 natural selection, along with every other varia- 

 tion, whenever they tend to preserve the life 

 of any organism in its struggle for existence. 



It has already been said that both Unter- 

 schiedsempfindlichkeit and heliotropism (or 

 some other tropism) may be associated in the 

 same organism. Such cases are common, and 

 in every one of them the possibility of con- 

 fusing the two reactions (as shown for Eu- 

 glena) exists. To cite but a single instance, 

 certain positively heliotropic butterflies will 

 not move toward the brightest light when their 

 bodies chance to be in contact with a pane of 

 glass under which they have crept. Further- 

 more, a weak light may produce no reaction 

 upon organisms where a light of greater in- 

 tensity would. It often happens that under 

 a light stimulus too weak to produce its ap- 

 propriate definite directive effect, an organism 

 may waver about, swinging now toward, now 

 away from the source of the stimulus. These 

 have been called trial movements. I do not 

 believe they belong in that category, for two 

 reasons. In the first place, when the organ- 

 ism comes into a proper orientation for an 



organism whose line of least resistance runs 

 toward the light, it does not stay so oriented. 

 In the second place, when the light is suffi- 

 ciently streng-thened, the organism may make 

 for it with the utmost directness. 



The earthworm, a much used and abused 

 animal in this connection, has recently af- 

 forded a case in point. Perichwta is an un- 

 usually active worm, and reacts, as all earth- 

 worms do, negatively to light. To quote from 

 Harper : 



2. The body is less sensitive to light when con- 

 tracted than when extended, owing to the fact that 

 when extended the sensitive elements are spread 

 out over a greater surface and become more sus- 

 ceptible. 



3. In locomotion, as there are alternate exten- 

 sions and contractions, there is an alternation of 

 the condition of lower and higher sensibility. 

 This is important particularly in the sensitive 

 anterior end. 



4. As the worm begins each extension in a con- 

 dition of lower sensibility, it may project its an- 

 terior end toward the source of light. This move- 

 ment is checked as soon as the increased sensi- 

 bility of the extended anterior end appreciates the 

 stimulus. Movements away from the light do not 

 meet such a check and so are prolonged farther. 

 Orientation is the result of a trial and error 

 method. 



Up to this point the reaction comes under 

 the head of Unterschiedsempfindlichkeit. 



5. In strong enough light, random movements 

 toward the light are suppressed altogether, and 

 the worm appears to move directly away from the 

 light without noticeable trial movements. This 

 applies to worms which have been kept in the dark 

 and are in a perfectly fresh condition, as after a 

 time they lose their discrimination and begin to 

 make random movements. 



This section should be noted, especially the 

 last sentence. Just as it has been seen that 

 larvEe of barnacles may change their response 

 when exposed for a time to light, so Perichceta 

 becomes, after a certain exposure, indifferent 

 to stimulation that produces a typical helio- 

 tropic reaction in the fresh worm. The ani- 

 mal remains, however, unterschiedsempfind- 

 lich. It contracts whenever, by extension of 

 the anterior end, a sufficiently large sensitive 

 area is exposed to the light. In Perichceta, 



