September 6, 1907] 



SCIENCE 



321 



pods which are positive, change the sense of 

 their response when thrown into water. Many- 

 animals change the sense of their response to 

 light with age and sexual condition. The 

 larvae of the king crab react positively in their 

 earlier stages, negatively later. Maggots of 

 the honse-fly respond negatively at the end of 

 their larval period, but are quite indifferent 

 to light both before and after this stage in 

 their existence. At the time of sexual ma- 

 turity, both ants and bees have exhibited posi- 

 tive responses to light. 



Further, it is a highly interesting fact that 

 certain caterpillars, notably Porthesia chrys- 

 orrhoea, are positively heliotropic when starved, 

 unresponsive when well fed. The suggestion 

 at once arises that the diffusion of chemical 

 substances into the body from the digestive 

 canal may cause the modification of the reac- 

 tion. Acting on the hint here given. Professor 

 Loeb initiated a series of experiments to see 

 whether the immediate effect of the light in 

 causing the heliotropic reaction is of a chem- 

 ical nature. Such a supposition could be put 

 to the test by placing heliotropic organisms in 

 an artificial chemical environment. The re- 

 sults of experimentation in this direction have 

 been productive of the most important results. 

 GammaniSj Daphnia and Cyclops^ all fresh- 

 water Crustacea, were used. Gamrnarus pulex 

 is, if anything, negatively heliotropic. By 

 the addition to the water containing a number 

 of individuals of this species of slight amounts 

 of various chemical substances — esters, hydro- 

 chloric, acetic, oxalic and carbonic acids (the 

 last itself a product of the metabolism of 

 the animal), alcohol, paraldehyde and am- 

 monium salts — in each case the animals be- 

 come positively heliotropic. Similarly, Cy- 

 clops, either negative or indifferent, may be 

 made positive by the addition of hydrochloric 

 acid or carbon dioxide. 



It is clear from the facts just recited that 

 the heliotropic reaction of an animal is not 

 necessarily constant, but that it may vary 

 widely and suddenly in sense, or disappear al- 

 together, in accordance with internal changes 

 which are immediately chemical in character. 

 There is little in such phenomena to suggest 



that tropic reactions are products of carefully 

 selected trial movements. On the contrary, 

 they suggest most strongly the possibility of 

 identifying such reactions with de Vriesian 

 mutations. 



With the demonstration that chemical 

 changes are connected in an important way 

 with the reactions of organisms to light, the 

 analysis of the tropic reaction has only begun. 

 Eecent experiments have achieved further re- 

 sults. I may quote, in translation, from a 

 recent paper by Professor Loeb: 



It might be assumed that acids call forth posi- 

 tive heliotropism among fresh water organisms 

 because they accelerate the formation of a certain 

 substance upon which the positive heliotropism 

 depends. This conjecture, however, can be dis- 

 proved. We know, namely, that reaction velocity 

 increases with the temperature, and that the tem- 

 perature coefficient is in these cases very high, 

 namely, in general for each 10° rise in temperature 

 ^ 2. Now I determined for freshwater copepods 

 how large the minimal amount of carbonic or 

 acetic acid is that is necessary to make indifferent 

 animals positively heliotropic. It became apparent 

 that for temperatures of about 10°-15°, not more 

 but actually less acid is required to call forth 

 positive heliotropism than at 20°-25°. That shows 

 that the acid in this case can not act through the 

 formation of a substance that conditions positive 

 heliotropism. A similar experiment resulted even 

 more strikingly for Daphnia. Here a fall in tem- 

 perature below that of the room lessened in the 

 clearest way the amount of acid necessary for the 

 production of positive heliotropism. Now it ap- 

 pears to be generally the case, that when the tem- 

 perature influences especially the sense of helio- 

 tropism in animals, this, so far as at present 

 known, always happens in the sense of making it 

 more positively heliotropic. We can accordingly 

 draw the conclusion with absolute safety that the 

 production of positive heliotropism is not due to 

 nn acceleration in the formation of a positive 

 heliotropic substance — to use an expression which 

 may be permitted for the sake of brevity. Rather 

 are we forced by all these facts to the conclusion 

 that the production of positive heliotropism in 

 animals by means of acids rests upon the inhibi- 

 tion of the formation or action of an antipositive 

 substance. It is conceivable that the conditions 

 of positive heliotropism (therefore the positive 

 heliotropic substance) are present in the organ- 

 isms that here interest us, that, however, their 



