1 6 PIONEERS OF EVOLUTION. 



action of an external cause, called Nous, which also 

 is material, although the " thinnest of all things and 

 the purest," and " has power over all things," there 

 arose plants and animals. It is probable, as Pro- 

 fessor Burnet remarks, " that Anaxagoras substituted 

 Nous, still conceived as a body, for the LOVE and 

 STRIFE of Empedocles simply because he wished 

 to retain the old Ionic doctrine of a substance that 

 ' knows ' all things, and to identify this with the 

 new theory of a substance that ' moves ' all things." 

 Thus far speculation has run largely on the ori- 

 gin of life forms, but now we find revival of specula- 

 tion about the nature of things generally, and the 

 formulation of a theory which links Greek cosmology 

 with early nineteenth-century science with Dalton's 

 ATOMIC THEORY. Democritus of Abdera, who was 

 born about 460 B. c., has the credit of having elab- 

 orated an atomic theory, but probably he only further 

 developed what Leucippus had taught before him. 

 Of this last-named philosopher nothing whatever is 

 known; indeed, his existence has been doubted, but 

 it counts for something that Aristotle gives him the 

 credit of the discovery, and that Theophrastus, in 

 the first book of his Opinions, wrote of Leucippus as 

 follows : " He assumed innumerable and ever-mov- 

 ing elements, namely, the atoms. And he made their 

 forms infinite in number, since there was no reason 

 why they should be of one kind rather than another, 

 and because he saw that there was unceasing becom- 

 ing and change in things. He held, further, that 



