RECENT DEVELOPMENT OF BIOLOGY 19 



performs, without foresight of the ends, those acts by which the 

 preservation of the individual or the species is secured. The term 

 " will "is reserved for those cases where these processes form constitu- 

 ents of consciousness. The words " instinct " and " will " do, however, 

 not give us the variables by which we can analyze or control the 

 mechanism of these actions. Scientific analysis has shown that the 

 motions of animals which are directed towards a definite aim depend 

 upon a mechanism which is essentially a function of the symmetrical 

 structure and the symmetrical distribution of irritability. Symmetri- 

 cal points of the surface of an animal, as a rule, have the same irrita- 

 bility, which means that, when stimulated equally, they produce the 

 same quantity of motion. The points at the oral pole as a rule pos- 

 sess a qualitatively different or greater irritability than those at the 

 aboral pole. If rays of light or current curves, or lines of diffusion or 

 gravitation, start from one point and strike an organism, which is 

 sensitive for the form of energy involved, on one side only, the tension 

 of the symmetrical muscles or contractile elements does not remain the 

 same on both sides of the body, and a tendency for rotation will result. 

 This will continue until the symmetrical points of the animal are 

 struck equally. As soon as this occurs there is no more reason why the 

 animal should deviate to the right or left from the direction of its 

 plane or axis of symmetry. These phenomena of automatic orienta- 

 tion of animals in a field of energy have been designated as tropisms. 

 It has been possible to dissolve a series of mysterious instincts into 

 cases of simple tropisms. The investigation of the various cases of 

 tropism has shown their great variety, and there can be no doubt that 

 further researches will increase the variety of tropisms and tropism- 

 like phenomena. I am inclined to believe that we possess in the 

 tropisms and tropism-like mechanisms the independent variable of 

 such functions as the instinctive selection of food and similar regu- 

 latory phenomena. 



As far as the mechanism of consciousness is concerned, no scientific 

 fact has thus far been found that promises an unraveling of this me- 

 chanism in the near future. It may be said, however, that at least the 

 nature of the biological problem here involved can be stated. From a 

 scientific point of view we may say that what we call consciousness 

 is the function of a definite machine which we will call the machine 

 of associative memory. Whatever the nature of this machine in living 

 beings may be, it has an essential feature in common with the phono- 

 graph, namely, that it is capable of reproducing impressions in the 

 same chronological order in*which they come to us. Even simultane- 

 ous impressions of a different physical character, such as, for instance, 

 optical and acoustical, easily fuse in memory and form an inseparable 

 complex. The mechanism upon which associative memory depends 

 seems to be located, in higher vertebrates at least, in the cerebral 



