28 EVOLUTION 



as ten or twenty,' on the supposition that a consider- 

 able proportion of the sun's substance was made up of 

 radio-active material. 



The above remarks may serve to illustrate the im- 

 portance of the theory of evolution as applied to the 

 two sciences of astronomy and geology. We pass next 

 to a brief historical consideration of the development 

 of the evolution theory as a method of describing the 

 origin of the species of animals and plants. 



The views of the ancient Greeks cannot be said to 

 have much more than a purely speculative interest. 

 Some rudiments of the idea of evolution have been 

 attributed to Empedocles as well as to several other 

 early writers, and in the writings of Aristotle, for 

 whom the too great faith of his successors for many 

 ages has been followed by a somewhat unmerited 

 degree of contempt in modern times, we find that the 

 evolution idea had reached quite a respectable degree 

 of development. 



In the Middle Ages the adoption of the Jewish cos- 

 mogony by the Christian Churches effectually annihi- 

 lated all useful thought upon the subject of species, 

 since the hypothesis of separate creation affords no 

 scope for further speculation or experiment, and it is 

 not until the end of the seventeenth century that we 

 find thoughtful men beginning to struggle against the 

 ecclesiastical bondage. Thus Erasmus Darwin de- 

 rived the idea of generation rather than creation of 

 the world from David Hume, and himself waxes 

 enthusiastic over the thought : 



' That is, it (the world) might have been gradually 



