4 INTRODUCTION 



to a change in the environment, an appropriate physiological 

 reaction. 



One difference, then, between the chemical and the 

 nervous co-ordination is a difference of speed. Another 

 difference lies in the greater variety of response which the 

 intricate nature of the nervous system makes possible. 



It is sometimes stated that life is simpler in a single- 

 celled than in a multi-cellular animal. It may be ques- 

 tioned which is the more complex, an organism in which 

 different functions are pigeon-holed in different tissues, 

 admirably co-ordinated though these be, or an organism 

 in which all the animal functions are performed in an 

 orderly manner in one cell. Specialisation of function does 

 not necessarily mean greater complexity of the biological 

 process. In what sense, then, is a multi-cellular animal 

 such as a mammal " higher " than a unicellular organism 

 such as Amoeba? Simply in this, that with division of 

 labour goes an increase of stability in face of changes in 

 the environment, an increase in the power of response to 

 external disturbing factors, an indifference to adverse cir- 

 cumstances. The Amoeba is completely at the mercy of 

 the shghtest changes in the physical and chemical con- 

 dition of the water in which it hves. Its hold upon life 

 is of the slenderest. Contrast with this the comparative 

 security of hfe possessed by the mammal. In the follow- 

 ing pages we shall have reason to see the extraordinary 

 stability possessed by different bodily systems. The 

 reaction of the blood, the volume of the blood, the 

 arterial blood-pressure are, within wide limits, maintained 

 constant in spite of external forces tending to disturb them. 

 Another example is seen in the regulation of body 

 temperature. 



But the most potent factor in the stabilising of the body 

 is the evolution of the central nervous system — the develop- 

 ment of instincts, memory, association of ideas, and other 

 intellectual processes. It is to the greater security of hfe 



