GREAT STEPS IN ORGANIC EVOLUTION 393 



ing life, was replaced among the worms by bilateral sym- 

 metry, suited for more strenuous life, such as that involved 

 in chasing victims, avoiding enemies, pursuing mates, and 

 shepherding the offspring. This was another literally epoch- 

 making step ; it was the beginning of our knowing our right 

 hand from our left. It meant a distinction between head 

 and tail, right and left 3 it was the beginning of head-brains 

 and cephalisation. 



This opened up great possibilities of integration, which 

 means more perfect unity and control, especially through the 

 nervous system. Differentiation may be compared to the 

 extension of an empire and to the complex division of labour 

 that is established in different parts of it; integration is 

 the binding of the whole into harmonious federation and 

 unified control. 



This is not the place to follow the long succession of 

 achievements in differentiation and integration which mark 

 organic evolution, but we shall simply mention a few: — an 

 open food-canal, a body-cavity or coelom between it and the 

 body-wall, striated or swiftly-contracting muscle, a fluid tis- 

 sue such as blood or lymph, circulatory organs for keeping 

 this in movement and thus distributing throughout the body 

 digested food and oxygen and collecting waste, oxygen-cap- 

 turing pigments such as haemoglobin, a segmented body as 

 in earthworms, a renewable external armour as in crusta- 

 ceans, muscular appendages unjointed to start with and by 

 and by jointed, specialised sense-organs such as eyes and 

 balancers and chemo-ceptors, improved respiratory arrange- 

 ments reaching extraordinary perfection among insects where 

 the blood hardly becomes impure, delicate adjustments for 

 filtering out the poisonous nitrogenous waste of the body, 

 for checkmating intrusive parasites, irritants, and poisons, 



