274 THE PHILOSOPHY OF BIOLOGY 



the liver, etc., transform these ingested proteids, fats, 

 and carbohydrates of the food into the proteids, fats, 

 and carbohydrates of the animal itself ; the heart, 

 blood, and lymph vessels carry this food material to 

 the muscles and nervous organs ; the respiratory 

 organs absorb oxygen which is distributed throughout 

 the body in the blood stream ; the execretory organs, 

 that is, the lungs, skin, and kidneys, remove noxious 

 materials like carbonic acid and urea, or its precursors ; 

 and purposeful changes of functioning of all these 

 organs are brought about by changes in motor activity. 

 Round the sensori-motor system all the rest of the 

 structure of the animal body is built up. 



What we see clearly in the evolution of the animal 

 body is the progressive increase of activity of the 

 sensori-motor system. The animal becomes more and 

 more mobile. It is in this way that dominance has 

 been attained and all the directions of structural 

 evolution in the past that have not tended in this 

 direction have been unsuccessful, irreversible, evolu- 

 tionary processes. Great size has not succeeded in the 

 animal kingdom, and so the gigantic reptiles and 

 mammals of the secondary and tertiary periods have 

 become extinct. Defence against enemies by the de- 

 velopment of dermal armour has not succeeded, and so 

 the Dinosaurs, and other armed animals of the Tertiary 

 Age have also become extinct. The transformation of 

 the fore limbs of the reptile into wings, or the legs of the 

 mammal into flappers, did not succeed, because all 

 the rest of the structure of these animals had become 

 adapted to locomotion on dry land, and the change of 

 structure had become too profound to be modified : 

 so the Pterodactyls passed away, as the whales of 

 our own period are also passing. Only in the lightly 

 boned, feathered bird, with the possibility of the 



