DIVISION OF LAB O UR. 1 1 



— growth and reproduction. When income exceeds ex- 

 penditure in the life of a young animal, growth goes on, 

 and the inherited qualities of the organism are more and 

 more perfectly unfolded. 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. 



Division of Labour.^-\n the first chapter of the fourth 

 edition of Prof Foster's Text Book of Physiology, you will 

 find a graphic account of the activities of the Amceba. It 

 moves by contracting its living substance, it draws back sensi- 

 tively from hurtful influences, it engulfs and digests food, 

 it gets rid of waste, and it absorbs the oxygen, without which 

 Its living matter cannot continue active or indeed alive. 

 For activity means, in part, an oxidation, a combustion 

 of material, and respiration in plants and animals alike 

 essentially consists in absorbing oxygen, and in liberating 

 the carbonic acid gas which is one of the waste products 

 both of life and burning. 



But the physiological ir(terest of the Amceba, and minute 

 animals like it, is that all the activities occur within the 

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

 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 persist with the 

 primitive and many-sided qualities of Amoebae. Division of 

 labour, consequent on diversity of conditions, is thus estab- 

 lished in the organism. In some cells one kind of activity 

 predominates, 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 

 simplest many-celled animals, the units are arranged in two 

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

 layer are protective, sensitive, and muscularj those of 

 the inner layer absorb and digest the food, and are also 

 muscular. 



In worms and higher organisms, there is a middle layer 



