PROGRESS as 



planning and imagination are entirely functions of 

 an adjustor mechanism, and without such a mechanism, 

 great enlargement of sensory power would only result 

 in an organism reacting too often and unnecessarily to 

 events in its environment. 



There is, in fact, an obvious limit to the perfection 

 which can be attained by receptor and effector organs. 

 Striated muscles, the modelling of the skeleton and 

 joints for speed in a horse or greyhound, the eye and 

 ear of higher vertebrates, the mammalian sense of 

 smell — no doubt it would be possible for life to have 

 produced more perfect and more efficient mechanisms 

 — ^but not, apparently, mechanisms much more perfect 

 or much more efficient. They stand near the limit 

 of biological efficiency. 



There thus comes a time when it is impossible or 

 extremely difficult to give an organism advantage in 

 the struggle by improving its sense-organs or its 

 locomotor system, or indeed any of its general physical 

 construction, whereas it is still possible to confer the 

 most important advantages upon it by means of im- 

 provements in the adjustor mechanism, improvements 

 which involve and imply improvements of mind. 



This stage was reached by mammals and birds quite 

 early in the Tertiary period ; and one of the most 

 striking spectacles of biology, revealed in the fossils 

 of successive strata, is to see Mind coming into its 

 own after this epoch. Over and over again a group 

 of animals is seen to appear and spread, only to be 



