Animal Instincts and Tropisms 255 



may be considered as a manifestation of one of those 

 wonderful instincts which form the delight of readers 

 of Maeterlinck's romances from insect life. Imagine 

 the social foresight of the sterile workers who when the 

 occasion demands it ''raise" eggs to preserve the stock 

 from extinction ! And yet what really happens is that 

 these workers, when there are no larva), can consume 

 the food which would otherwise have been devoured 

 by the larvas; and some substance contained in this 

 food induces the development of eggs in the otherwise 

 dormant ovaries. What appeared at first sight as a 

 mysterious social instinct is revealed as an effect 

 comparable to that of thyroid substance upon the 

 growth of the legs of tadpoles in Gudcrnatsch's 

 experiment (Chapter VII). 



2. If we wish to show in an unmistakable way the 

 mechanistic character of instincts we must be able to 

 reduce them to laws which are also valid in physics. 

 That instinct, or rather that group of instincts, for 

 which this has been accomplished are the reactions 

 of organisms to light. The reader is familiar with the 

 tendenc}^ of many insects to fly into the flame. It 

 can be shown that many species of animals, from the 

 lowest forms up to the fishes, are at certain stages — 

 very often the larval stage — of their existence, slaves of 

 the light. When such animals, e. g., the larva; of the 

 barnacle or certain winged plant lice or the cater- 

 pillars of certain butterflies, are put into a trough or 



