DIVISION OF LABOUR. 15 



duction. When income exceeds expenditure in a young 

 animal, growth goes on, and the inherited qualities of the 

 organism are more and more perfectly developed. At the 

 limit of growth, when the animal has reached " maturity," 

 it normally reproduces, that is to say, liberates parts of 

 itself which give rise to new individuals. It is this power 

 of growing and reproducing which most distinguishes an 

 organism from an inanimate thing. 



Division of Labour. 



All the ordinary functions of life are exhibited by the 

 simple unicellular animals or Protozoa. Take the Amoeba 

 for example. It moves by contracting its living substance, 

 it draws back sensitively from hurtful influences, it engulfs 

 and digests food, it gets rid of waste, and it absorbs oxygen, 

 without which its living matter cannot continue active or 

 indeed alive. For activity implies, in part, an oxidation, a 

 combustion of material, and respiration in plants and animals I 

 alike consists in absorbing oxygen, and in liberating the car- 

 bonic acid gas which is one of the waste products both of 

 life and burning. ' 



But all these activities occur in the Amceba within the 

 compass of a unit mass of living matter, — a single cell, 

 physiologically complete in itself. There is no division of 

 labour, there are as yet no parts. 



In all other animals, from Sponges onwards, there is a 

 " body " consisting of hundreds of unit masses or cells. It 

 is impossible for these to remain the same, for some are 

 internal and others external, nor would it be well for the 

 organism that all its units should retain the primitive and 

 many sided qualities of Amcebfe. Division of labour, con- , 

 sequent on diversity of conditions, is thus established in the 

 organism. In some cells one kind of activity predominates, 1 

 in others a second, in others a third. And this division of' 

 labour is followed by that complication of structure which 

 we call differentiation. 



Thus, in the fresh water Hydra, which is one of the 1 

 simplest many celled animals, the units are arranged in 1 

 two layers, and form a tubular body. Those of the outer 

 layer are protective, nervous, and muscular ; those of 



