ORGANIC EVOLUTION 29 
as ten or twenty,’ on the supposition that a consider- 
able proportion of the sun’s substance was made up of 
tadio-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 
