BIOLOGICAL PHILOSOPHY 405 



a difference in the velocity of chemical reaction between (the two sides)." 

 The organism has no control over its behavior. 



But what proof have we that such chemical changes as Loeb assumes do 

 occur in the organism? If we suspend a stem of a plant in a horizontal posi- 

 tion, it soon bends downward, taking the form of a U. This bending is not 

 due to sagging of the stem as a rope sags, but rather to unequal growth of 

 the two sides, which can be proven by marking equal distances on upper 

 and lower sides by Hnes of India ink and later measuring the amount of 

 growth occurring between the marks. If the amount of bending in such a 

 stem with leaves attached be compared with that in a stem lacking leaves, 

 it will be found to be much jjreater in the former due to the greater amount 

 of growth material available, and similarly there is greater bending in a stem 

 furnished with a complete leaf than in one with a leaf which has been 

 partly cut away. "What has been demonstrated in this case explains prob- 

 ably also why the apex of many plants when put into a horizontal position 

 grow upward, and why certain roots under similar conditions grow down- 

 ward. It disposes also in all probability of the suggestion that the apex 

 of a positively geotropic root has 'brain functions.' It is chemical mass ac- 

 tion and not 'brain functions' which are needed to produce the changes 

 in growth underlying geotropic curvature." 



The purely mechanical response of an animal to stimuli is beautifully il- 

 lustrated by the behavior of the caterpillar of the butterfly (Porthesia 

 chrysorrhoea). This butterfly lays its eggs upon a shrub, on which the 

 larvae hatch in the fall and on which they hibernate, as a rule, not far from 

 the ground. They leave the nest in the spring when the first leaves have 

 begun to form on the shrub. After leaving the nest they crawl directly up- 

 ward on the shrub where they find the leaves on which they feed. If the 

 caterpillars should move down the shrub they would starve. What gives the 

 caterpillar this never-failing certainty which saves its life? It is merely posi- 

 tive heliotropism and the light reflected from the sky guides the animals up- 

 ward. If we put these caterpillars into closed test tubes which lie with their 

 horizontal axes at right angles to the window they will all migrate to the 

 window end where they will stay and starve, even if we put their favorite 

 leaves into the test tubes close behind them. These larvae are in this condi- 

 tion slaves of the light. 



The light which saved its life by making it creep upward where it finds 

 its food would cause it to starve could the animal not free itself from the 

 bondage of positive heliotropism. It can be shown that a caterpillar after 

 having been fed loses its positive heliotropism almost completely and per- 

 manently. If we submit fed and unfed caterpillars of the same nest to the 

 same source of light in two diiferent test tubes the unfed will creep to the 

 light and stay there until they die, while those that have eaten will pay 

 little or no attention to the light. It can be shown with a reasonable degree 

 of probability however that even here what we call "instinct" may be 



