HOURS WITH NATURE. 



though imperfectly, all the various functions by which 

 life is maintained: it receives impressions from without, 

 e. g. of the proximity of food, it is locomotive, it takes 

 hold of and digests prey, it respires, it circulates its 

 assimilated nutriment, and it excretes its useless waste, 

 and in most "Protozoa" as you must have noticed in 

 AmxbaS) "it does all this with any and every part of- its 

 simple unicellular body. 



\ * But when a multitude of cells is incorporated to- 

 gether to form a large many-celled animal, it is not 

 economical for the incorporation that each of its cell- 

 units should, so to speak, go its own way and continue 

 to perform all its functions independently for itself. 

 It is more economical, both for the incorporation as a 

 whole, and each of these cell-units, that each of the 

 functions requisite for the common weal should be 

 assigned to particular groups of cells to some group 

 the receiving of impressions, to other groups the functions 

 of digestion and assimilation, and so on : much as hap- 

 pens in communities of men, where the various indus- 

 tries and activities of the community are not performed 

 by all the citizens alike, but each citizen is devoted to 

 one particular occupation. In this way, since 'practice 

 makes perfect' each function comes to be dexterously 

 and economically performed. 



'This apportioning of functions is known as the 

 "physiological division of labour," and we must now 

 consider the changes to which it gives rise. 



'Let us imagine a colony of cells, all of which are 



