I THALES TO LUCRETIUS 17 



force of heredity, that still unsolved problem of 

 biology. Speaking broadly of his teaching, the 

 details of which would fill pages, its main features 

 are (i) His insistence on observation. In his 

 History of Animals he says : ' We must not accept a 

 general principle from logic only, but must prove its 

 application to each fact. For it is in facts that we 

 must seek general principles, and these must always 

 accord with facts. Experience furnishes the particular 

 facts from which induction is the pathway to general 

 laws.' (2) His rejection of chance and assertion of 

 law, not, following a common error, of law personified 

 as cause, but as the term by which we express the 

 fact that certain phenomena always occur in a certain 

 order. In his Physics Aristotle says that ' Zeus 

 rains not that corn may be increased, but from 

 necessity. Similarly, if some one's corn is destroyed 

 by rain, it does not rain for this purpose, but as an 

 accidental circumstance. It does not appear to be 

 from fortune or chance that it frequently rains in 

 winter, but from necessity.' (3) On the question of 

 the origin of life-forms he was nearest of all to its 

 modern solution, setting forth the necessity 'that 

 germs should have been first produced, and not 

 immediately animals ; and that soft mass which first 

 subsisted was the germ. In plants, also, there is 

 purpose, but it is less distinct ; and this shows that 

 plants were produced in the same manner as animals, 

 not by chance, as by the union of olives with grape 

 vines. Similarly, it may be argued, that there should 

 be an accidental generation of the germs of things, 

 but he who asserts this subverts Nature herself, for 

 Nature produces those things which, being continu- 



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