PROGRESS, BIOLOGICAL AND OTHER 23 



of an adjuster 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 or- 

 gans. Striated muscles, the modelling of the skele- 

 ton 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 loco- 

 motor 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, improve- 

 ments which involve and imply improvements of 

 mind. 



This stage was reached by mammals and birds 

 quite early in the Tertijry 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 extinguished and replaced by another type 



