198 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



urged against us that instincts should be explained 

 historically and not physiologically or causally. It 

 seems to me that living organisms are machines and 

 that their reactions can only be explained according 

 to the same principles which are used by the physicist. 

 Our ultimate aim in the analysis of instincts is to find 

 out by which physical and chemical properties of pro- 

 toplasm they are determined. Of course the physicist 

 finds it useful to illustrate the mechanism of compli- 

 cated machines by the comparison with simpler or 

 older machines of the same kind. We have made use 

 of this same method and heuristic principle by utilis- 

 ing in this book the reactions of simpler forms for the 

 analysis of more complicated forms. Even if we 

 were in possession of a scientific phylogeny instead of 

 the fairy tales that go by that name at present, it 

 would not relieve us of the task of explaining the 

 instincts on the basis of the physical and chemical 

 qualities of protoplasm. 



ii. At first sight it may seem a hopeless task to 

 find a connection between the instinctive actions of 

 animals and the properties of their protoplasm. And 

 yet the task is not so great if we choose the right 

 method. This method, in my opinion, consists in 

 varying the instincts of an animal at desire. If we 

 succeed in this we are able to find out how the physi- 

 cal qualities of protoplasm may affect the instincts. I 

 have tried this in one case. A number of marine ani- 

 mals (Copepods, larvae of Polygordius) which go away 

 from the light can be forced to go to the light in two 



