1 88 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



central nervous system plays no other role in this than 

 that it forms the protoplasmic bridge for the conduc- 

 tion from the skin to the muscles. In organisms in 

 which this conduction is possible without a central 

 nervous system, in plants, for instance, we also find 

 the same reactions. 



5. We find another instance of a preservative in- 

 stinct in the young caterpillars of many butterflies. 

 The larvae of Portkesia chrysorrhcea creep out of the 

 eggs in the autumn and winter in colonies in a nest on 

 trees or shrubs. The warm spring sun drives them 

 out of the nest and they crawl up on the branches of 

 the tree or shrub to the tip, where they find their first 

 food. After having eaten the tips, they crawl about 

 until they find new buds or leaves, which in the mean- 

 time have come out in great numbers. It is evident 

 that the instinct of the caterpillars to crawl upwards, 

 as soon as they awake from the winter sleep, saves 

 their lives. Were they not guided by such an in- 

 stinct, those that crawled downwards would die of 

 starvation. What role does the central nervous sys- 

 tem play in these instincts ? 



I have found that the young caterpillars of Por- 

 thesia are oriented by the light. Until they have 

 taken food they are positively heliotropic. This 

 positive heliotropism leads them to the tips of the 

 branches where they find their food. During the win- 

 ter they are stiff and do not move. The higher tem- 

 perature of the spring brings about chemical changes in 

 their bodies, and these chemical processes cause them 



