240 The Unity of the Organism 



branches. This rather complex and, to the insects, highly 

 useful performance Loeb and others have proved to consist 

 of a series of reflexes so interconnected as to come under 

 the tropistic type of activity. And Loeb, e.g., in the chap- 

 ter, On the Theory of Animal Instincts (Physiology of the 

 Brain) uses the case to good effect in support of his 

 contention that the traditional instinct-and-nerve-center ex- 

 planation of such phenomena is utterly inadequate. 



So far, good. As to the straightforward presentation of 

 facts, Loeb's position seems unassailable. But what about 

 the causal explanation of the facts? What, exactly, is it 

 that sends the larvae up the branches? What causes the 

 eating activities? What makes the creatures then turn 

 about and finally sends them down the branches? That 

 several environmental factors, the warm weather, the 

 sunlight, the character of the plant buds and leaves, and so 

 on, are involved is brought out clearly enough. But what 

 about the factors pertaining to the larvae themselves ? The 

 body-shape, the skin, the sense drgans, the muscles are, as 

 was emphasized in the previous discussion of tropisms, freely 

 recognized after a fashion by the tropism theory. But 

 deeper still than these what? Chemical substances "ac- 

 cording to requirement," in the language of the cook books. 

 Until the caterpillars have taken food they are positively 

 "heliotropic," that is, literally, are induced by sunlight to 

 move toward the sun, after the higher spring temperature 

 has caused chemical changes in their bodies essential to 

 such movement. But by eating to satiety the chemical 

 changes essential to the positive heliotropism are inhibited 

 and a negatively heliotropic state comes on. "We can im- 

 agine," writes Loeb, "that the taking up of food leads to 

 the destruction of the substances in the skin of the animal 

 which are sensitive to light, upon which substances the helio- 

 tropism depends, or that through the consumption of food 

 the action of these substances is indirectly prevented." 1 In 



