Chap. L] OF A NERVOUS SYSTEM. 7 



and animals in tlieir mode of nutrition is so fundamental, 

 BO much depends upon it, that we shall find it worth our 

 while to inquire a little more particularly how the depar- 

 ture from the more primordial mode of nutrition, met 

 •with amongst animals, can be accounted for. 



If we examine some simple vegetal unit through a 

 microscope — the germ from which a Conferva grows, for 

 instance — we find it exhibiting no distinct changes of 

 form ; and, if unprovided with one or more vibratile 

 filaments, it also shows no movements from place to 

 place. It manifests no tendency to seize, nor has it any 

 means of taking, solid food. As soon, therefore, as the 

 changes incident upon the active growth of such a unit 

 have ceased, the outer portion of its substance remains 

 constantly in contact with the medium in which it lives, 

 and shortly becomes modified. It condenses and is 

 otherwise changed into an investing envelope, which 

 commonly goes by the name of a ' cell- wall.' In the 

 Amoeba, on the other hand, we have an organism which, 

 like the fabled Proteus, is for ever changing its form. 

 It is composed of a clear jelly-like material, endowed 

 with a superabundance of that intrinsic activity character- 

 istic of animal life generally. Those internal molecular 

 movements, indeed, which are inferred to occur to a 

 marked extent in all living matter, seem to take place in 

 it in a pre-eminent degree. Its whole substance shows a 

 mobility of the most striking kind. It continually moves 

 through the water or over surfaces, by alternate projec- 

 tions and retractions of its active body-substance. 



Two consequences flow from this high inherent 

 activity of the Amoeba. In the first place, owing to the 

 creature's rapid alterations in shape, no one portion of its 

 substance is continuously exposed to contact with its 

 medium, and, as a consequence, that first step in orgau- 



