4 PRINCIPLES OF ANIMAL BIOLOGY 



Aristotle. — A reaction set in against the materialistic conceptions 

 of Democritus and others. Philosophy came to be dominated by 

 Socrates, who was interested in ethics, and by Plato, who found true 

 reality in the world of abstract thought. The latter says expressly 

 that no true knowledge is to be attained through observations of the 

 senses. One leading philosopher who came under Plato's influence was 

 Aristotle (384-322 b.c.) (Fig. 2), the greatest of the early biologists, to 

 whom the essence of living things was their form. Everything that 

 happens, he taught, is due to a supreme intelligence, everything is done 



Fig. 2.— Aristotle, 384-322 b.c. {From Heklcr, "Greek and Roman Portraits," G. P. Pul- 



nam,'s Sons.) 



for a purpose, and the primary purpose in nature is the development 

 of a higher form. As a result of this continuing purpose, there has been 

 an evolution from lower types to higher ones. 



Despite his leaning to supernatural causes, Aristotle made some excel- 

 lent observations in biology and sought to organize them wherever possi- 

 ble. He classified animals according to their mode of life and their 

 structure and knew over five hundred kinds, all Greek; those from other 

 countries he knew only from descriptions. He insisted that the study of 

 anatomy should be comparative, which is a fruitful procedure at the 

 present time. The heart was regarded as the organ of the soul and 

 intelligence; here Aristotle drops behind his predecessor Democritus. 

 Digestion was to him a process of ''cooking." Nerves were confused with 

 tendons; the brain was thought to be cold and the spinal cord hot. Fleas 



