48 The Mechanistic Conception of Life 



instance, Leo Loeb has found that the substances which are 

 set free by the bursting of an egg foUicle cause a special sensi- 

 tiveness in the non-pregnant uterus, so that every mechanical 

 stimulus causes the latter to form a decidua. In this way he 

 could cause the formation of any number of deciduae in non- 

 pregnant uteri, while without the circulation of follicle sub- 

 stance in the blood the uterus did not react in this manner. 



It is a common phenomenon that animals in certain larval 

 stages are positively heliotropic, while in others they are not 

 sensitive to light or are even negatively heliotropic. I will 

 not discuss these facts further in this place, but refer my readers 

 to my earlier papers. 



This change in the heliotropic sensitiveness, produced by 

 certain metabolic products in the animal body, is of great biologi- 

 cal significance. I pointed out in former papers that it serves 

 to save the lives of the above-mentioned yoimg larvae of Chrysor- 

 rhoea. When the young larvae are awakened from their winter 

 sleep by the sunshine of the spring they are positively helio- 

 tropic. Their positive heliotropism leaves them no freedom 

 of movement, but forces them to creep straight upward to the 

 top of a tree or branch. Here they find the first buds. In this 

 way their heliotropism guides them to their food. Should 

 they now remain positively heliotropic they would be held fast 

 on the ends of the twigs and would starve to death. But we 

 have already mentioned that after having eaten they once more 

 lose their positive heliotropism. They can now creep down- 

 ward, and the restlessness which is characteristic of so many 

 animals^ forces them to creep downward until they reach a 

 new leaf, the odor or tactile stimulus of which stops the pro- 

 gressive movement of the machine and sets their eating 

 activity again in motion. 



The fact that ants and bees become positively heliotropic 

 at the time of sexual maturity plays an important role in the 



1 The physico-chemical cause of this "restlessness" which is noticeable in 

 many insects and crustaceans is at present unknown. 



