254 ESSAYS OF A BIOLOGIST 



crease of the control exercised by living matter over 

 the environment, and of its independence of the en- 

 vironment — two reciprocal aspects of a single proc- 

 ess. When we look more closely into the means by 

 which this has been achieved, we shall see an increase 

 of the maximum not only in size, but in complexity, 

 in length of life, in efficiency of particular organs, 

 in co-ordination of parts and general harmony, in 

 improvement of sense-organs, and, continuing even 

 after other tendencies have reached their limits, in 

 brain-size and consequently in complexity of mode 

 of reaction and behaviour. 



If we turn to the psychological side, we fmd that 

 there has been an increase in the intensity of mental 

 process. This is apparent in all aspects of mind, 

 on that of emotion equally with that of knowledge, 

 of volition equally with that of emotion. To be an 

 amoeba or a worm is to live a life almost without 

 windows. Perfection of sense-organs makes it pos- 

 sible for life to be aware of the different types of 

 outer events, whilst memory and, later, associative 

 memory give the possibility of understanding their 

 history. In higher forms volition can be maintained 

 for longer and longer intervals, can attain greater in- 

 tensity, and can fix itself upon ever more and more 

 distant objects. With depth of feeling comes also 

 differentiation, so that finally we find in ourselves the 

 possibility of organizing various blends of the sim- 

 ple emotions into the compound emotional forms 



