254 ESSAYS OF A BIOLOGIST 



disease-germs and vast organisms like whales and 

 elephants, there has been a gradual steady increase 

 (tending to a limit) in the size of the largest organisms 

 existing at any one period. 



If we confine ourselves for the moment to the 

 material side, we find that the directional change in 

 organic evolution can be reduced to this — to an 

 increase of the control exercised by living matter 

 over the environment, and of its independence of 

 the environment — two reciprocal aspects of a single 

 process. 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 com- 

 plexity, 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 find 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. 



