ZZ THE HISTORY OF BIOLOGY 



the liver as that of sensuality. His estimation of the brain places Democritus 

 once more in front of Aristotle, who believed the brain to serve only the 

 purpose of cooling the blood. Democritus considered life and the soul to be 

 one and the same thing, and the latter he believed, as already mentioned, to 

 consist of fire atoms which, owing to their lightness and mobility, are con- 

 stantly being given off by the body. Through inhalation the body receives a 

 fresh supply of them; if respiration ceases, then life departs from the body. 

 Sleep and asphyxia he declared to be also due to a loss of soul-atoms, but on 

 a smaller scale. Hydrophobia in dogs and human beings he considered to be 

 caused by inflammation of the nerves. Epidemics he believed to be the result 

 of atoms falling upon the earth out of other celestial bodies. Sensation is due 

 to the movement of atoms which emanate from the objects perceived. In 

 connexion with Democritus' materialistic idea of the soul he believed in 

 spiritual beings and revelations, a belief which other thinkers who regarded 

 the soul as matter — ■ as, for instance, Swedenborg — have shared with him. 

 On the other hand, he denied the divine beings of popular belief, without, 

 however, like Xenophanes, substituting any unified and eternal divine power 

 in their stead. Necessity, which, according to his views, governed the uni- 

 verse, was purely impersonal. 



Democritus on the whole represents the climax of the endeavour of 

 Greek philosophy to arrive at an explanation of existence based on a natural 

 connexion of causes, an endeavour which, with Anaximander as its instigator, 

 gave rise to a long series of heterogeneous explanations of the cosmos, of 

 which only the most important can be considered here. In several funda- 

 mental respects — for instance, in the strict theory of causation, the atomic 

 theory, and, in connexion therewith, the principle of motion, the emphasiz- 

 ing of the importance of the brain for the function of thinking, the insistence 

 upon the complicated organism of the lower animals — Democritus achieves 

 results similar to those that have been attained by natural research in our 

 own day, although his speculations were in detail often very primitive, even 

 when compared with the achievements of philosophers of later antiquity. 

 The promising idea was not pursued, however, by succeeding generations; 

 for certain reasons which will be more clearly explained when accounting 

 for Aristotle's theoretical views, shortly after the age of Democritus the 

 Greek natural philosophy started to work out a method of explaining the 

 cosmic process entirely different from that indicated by the atomic theory. 

 Democritus thus represents the close of the first and the purely natural 

 scientific period of Greek philosophy. The results achieved by research dur- 

 ing that period have been mentioned above; what it failed to achieve may 

 also be briefly described here. The most serious defect from which it suffered 

 was undoubtedly the lack of material for investigation, which rendered it 

 difficult to follow up the principle of causation which had already been 



