iv PREFACE. 



the phenomena of life are scrutinized, the more carefully they are 

 studied in their various aspects, the more certain does the conclu- 

 sion become that the processes attributed to physico-chemical forces 

 in reality obey much more complicated laws. To illustrate, it was 

 at one time conceded that the phenomena of resorption and nutri- 

 tion were explainable by diffusion and endosmosis; Dutrochet. 

 upon his discovery of endosmosis, imagined even that he had dis- 

 covered the principle of life. At the present time we know that the 

 walls of the intestines do not in any wise act like the inanimate 

 membrane used in experiments in endosmosis. They are covered 

 with epithelial cells, each of which is an organism endowed with a 

 complex of properties. The protoplasm of these cells lays hold of 

 food by an act of prehension, exactly as the ciliate Infusoria and 

 other unicellular organisms do, that lead an independent life. In 

 the intestines of cold-blooded animals the cells emit prolongations 

 wnich seize the minute drops of fatty matter and, carrying them 

 into the protoplasm of the cell, convey them thence into the chyli- 

 factive ducts. There is still another mode of absorption of fatty 

 matters, met with among cold-blooded as well as warm-blooded 

 animals: the lymphatic cells pass out from the adenoid tissue which 

 contains them, so that upon arriving at the surface of the intestines 

 they seize the particles of fatty matter there present and, laden 

 with their prey, make their way back to the lymphatics. 



Accordingly, the faculty of seizing food and of exercising a 

 choice among foods of different kinds a property essentially psy- 

 chological appertains to the anatomical elements of the tissues, 

 just as it does to all unicellular beings, in the manner shown in our 

 treatise. It is plainly impossible to explain these facts by the in- 

 troduction of physico-chemical forces. They are the essential phe- 

 nomena of life and are the exclusive appurtenance of living pro- 

 toplasm. 



If the existence of psychological phenomena in lower organ- 

 isms is denied, it will be necessary to assume that these phenom- 

 ena can be superadded io the course of evolution, in proportion as 



