8 THE USES AND ORIGIN 



ization, above referred to in connection with the Conferva 

 unit, does not take place. So long as the Amoeba remains 

 in full vigour and constantly changes its shape, a cell- wall 

 cannot be formed. 



Secondly, during the movements of the organism from 

 place to place, portions of its projected body-substance 

 come into contact with other more minute organisms, 

 such as unicellular algae and diatoms, or with small 

 portions of organic refuse, and these are oftentimes 

 drawn into its interior when the projections with which 

 they are in contact are retracted. The activity , of the 

 Amoeba and its allies is excited by contact with matter of 

 this and of other kinds, though inorganic fragments are 

 subsequently rejected. 



The surplus inherent activity of the Amoeba being, 

 therefore, one of the immediately determining causes of its 

 absorbing solid food, may also be regarded as one of the 

 causes of its departure from the more elementary mode 

 of nutrition met with amongst the simpler or less vitalized 

 organisms from which it has been derived. 



A word, however, is required as to the ' selective ' 

 power which the Amoeba seems to manifest. 



A magnet ' selects ' minute fragments of iron or steel 

 from any heap of heterogeneous particles containing 

 such matter with which it may be brought into contact. 

 Certain plants, also, such as the Sun-dew and the Venus 

 fly- trap, ' select,' and seem capable of discriminating, 

 nitrogenous from other substances with which they come 

 into contact. The leaves of these plants, however, possess 

 no nervous tissues of any kind ; so that the fact that they 

 seem to ' select ' nitrogenous substances merely implies 

 the existence of some relation between the molecular com- 

 position and activities of the leaves and those of such 

 substances by virtue of which mutual contact keeps 



