INTRODUCTORY REMARKS 5 



impossible that further experiments on the egg may aid us in solving 

 this problem. In certain forms, e.g. the starfish, the mature egg, if 

 not fertilized, dies rapidly, while the fertilized egg continues to hve. 

 For this egg the act of impregnation is a life-saving act. 



We shall now attempt to give a short sketch of the phenomena and 

 problems of self-preservation similar to that just given of the phe- 

 nomena of development. Among the phenomena of self-preservation 

 are such facts as the existence of automatic mechanisms for the union 

 of the sexual cells wherever there exist separate sexes, or the existence 

 of automatic mechanisms for the deposition of eggs in places which 

 furnish food for the young larvae, and so on. What do we know con- 

 cerning the nature of these automatic mechanisms? Metaphysics 

 has supplied us in these cases with the terms "instinct" and "will." We 

 speak of instinct when an animal, apparently unconsciously, executes 

 motions or actions which are necessary for the preservation of the in- 

 dividual or the species; while we speak of will when motions are exe- 

 cuted consciously. We call it instinct when the female fly deposits 

 her eggs on meat on which the young larvae can feed. An analysis of 

 the instinctive actions has yielded the result that the purposeful motions 

 of animals frequently depend upon mechanisms which are a function 

 of the symmetrical structure and the symmetrical distribution of irrita- 

 bility on the surface of the body of the organisms. Symmetrical points 

 on the surface of an animal have usually an equal irritability, i.e. if 

 such spots are stimulated equally the same amount of motion, but in 

 opposite directions, is produced. Points on the surface which are 

 nearer the oral pole of the animal usually have a higher irritabihty 

 than, or a different irritabihty from, points which are nearer the aboral 

 pole. If lines of force, e.g. rays of light, current curves, lines of gravi-. 

 tation, Hnes of diffusion, strike one side of an animal in greater density 

 than the other side, the tension of the muscles, or contractile elements, on 

 both sides of the organism does not remain the same, and if the animal 

 moves, a tendency to turn to one side must result. This will continue 

 until symmetrical points on both sides of the animal are again struck 

 at the same angle by the lines of force. As soon as this occurs the ten- 

 sion of the muscles, or contractile elements, on both sides of the animal 

 becomes equal, and there is no more reason why the animal should 

 deviate toward the right or the left; it will therefore continue to move 

 in the original direction. This automatic orientation in a field of force 

 toward or away from the center of force is called a tropism. It has 

 been possible to dissolve a number of animal instincts into tropisms. 

 The analysis of various tropisms has shown that there exists a great 

 variety, and often a great complexity due to the combination of several 



