190 The Unity of the Organism 



connecting more or less directly the sensory areas with the 

 muscles. In a word, tropistic movements are determined by 

 a definite number of definite kinds of parts or organs, defi- 

 nitely arranged with reference to one another and to the 

 whole; that is, upon an organized body called a living being, 

 or briefly an organism. A morphological entity is funda- 

 mental to the tropism conception. Tropisms are explained, 

 partly, by these organizations. Loeb does not avoid, in fact 

 at times appears not to try to avoid, recognizing this : "The 

 irritable structures at the surface of the body and the ar- 

 rangement of the muscles determine the character of the 

 reflex act." "The explanation of them [tropisms] depends 

 first upon the specific irritability of certain elements of the 

 body-surface, and, second, upon the relations of symmetry 

 of the body." 5 



The Organismal Character of the Segmental Theory of the 

 Nervous System 



Another province in which Loeb has done distinguished 

 work and into which the organism as such constantly ob- 

 trudes itself and will not accept relegation to a secondary 

 place, is that of the coordinated action of the central nerv- 

 ous system in those animals in which that system consists 

 of an axial series of ganglia. Here again, as in the tropism 

 theory, the basal perception underlying the segmental theory 

 of nerve physiology is the truth that nerve "centers" as the 

 "seats" of particular activities and functions in the old sense 

 (in the sense, that is, of being the real source of these 

 activities) do not exist. According to the segmental theory 

 the real unit of activity of an animal made up of metameres 

 or segments is the segment itself, the nerve ganglion of the 

 segment being only one element in the complex. The idea 

 is set forth with fullness and perspicacity in the chapters 

 of the Physiology of the Brain dealing with worms and 



