CHAP. I.] OF A NERVOUS SYSTEM. 11 



isin's increasing powers of movement more definitely 

 ministering to this capacity. Its motions, instead of 

 being wholly at random, show more and more signs of 

 purposiveness they become, to an increasing degree, 

 subservient to the capture of food. 



Look, then, at the differences already indicated both in 

 grade of organization and mode of life, by virtue of which 

 even the simpler kinds of animals become strikingly un- 

 like vegetal organisms. 



The unit of vegetal life before it has attained any great 

 size exhibits, by reason of its lower degree of inherent 

 activity, a tendency to undergo the first stage of organiza- 

 tion, that is, to develop a cell- wall which imprisons the 

 more active living matter within and causes it to under- 

 go certain secondary modifications. Before this occurs, 

 however, the vegetal unit, if it does not divide, may seg- 

 ment or bud ; the bud grows into a unit similar to its 

 parent, and this in its turn may also segment or bud. By 

 repetition of such a process motionless cellular organisms 

 are produced, which, though presenting almost endless 

 differences in form and in the ultimate arrangement of 

 their units, are in the main composed of mere aggrega- 

 tions of similar parts these being not solid units of pro- 

 toplasm, but mostly vesicular elements, in which a cavity 

 filled with fluid contents is bounded by a layer of pro- 

 toplasm and outside this by an inert cell- wall. We may 

 have, in the more simple combinations, long strings of 

 such elements forming cellular filaments, as in the Con- 

 fervae and other thread-like algse; or we may have flat 

 cellular expansions, such as exist and brighten many a 

 rock pool, in the rich green fronds of Ulva. Organisms like 

 this present us with life changes of extreme simplicity. 

 If they move it is because they are swayed to and fro by 



