Chap. I.] OF A NERVOUS SYSTEM. 19 



to what obtains amongst the members of the vegetable 

 world. 



As to the mode by which, in Medusae or other low types 

 of animal life, the first rudiments of a nervous system are 

 evolved, only a few brief statements can be made. On 

 this subject inferences have only too often to take the 

 place of positive knowledge. Fortunately, however, the 

 data on which such inferences may be based are now fairly 

 well established, thanks more especially to the writings 

 of Herbert Spencer* — whose speculations on this subject 

 have been to some extent confirmed by the recent investi- 

 gations of Komanes and Eimer. 



In the lower forms of animal life, we have to do with 

 a body substance composed, as already stated, almost 

 wholly of undifferentiated protoplasm. This substance, 

 if not * sensitive ' in the strict sense of the term, is 

 highly impressible — or capable of receiving a stimulus — 

 and is also highly contractile. But neither the impressi- 

 bility nor the contractility of the protoplasm in lower 

 forms of animal life is localized — both properties are, so 

 far as they exist, uniformly possessed by all parts of the 

 organism. In some of the larger Ciliated Infusoria, in 

 Gregarinse, and in the hydroid Polyps, distinct rudimen- 

 tary * muscles ' become differentiated, and such tissues are, 

 moreover, now known to exist in many other organisms in 

 which no traces of a nervous system are to be found. 

 Muscular tissue, therefore, makes its appearance before 

 nervous tissue, and it becomes developed in those situa- 

 tions where the protoplasm is stimulated to undergo 

 frequent contractions. 



It is, in fact, one of the most fundamental truths in 

 biology that the performance of functions, or, in other 



* " Principles of Psychology," vol. ii. p. 69. 



