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necessity for such a mutation taking place is this (as I have 

 observed in the Introduction to my Translation of Aristotle's 

 History of Animals), that all the parts of the universe are 

 unable to participate the providence of divinity in a similar 

 manner, but some of its parts enjoy this perpetually, and 

 others only for a time ; some in a primary, and others in a 

 secondary degree. For the universe, being a perfect whole, 

 must have a first, a middle, and a last part. But its first 

 part, as having the most excellent subsistence, must always 

 exist according to nature; and its last part must sometimes 

 subsist according to, and sometimes contrary to, nature. 

 Hence the celestial bodies, which are the first parts of the 

 universe, perpetually subsist according to nature, both the 

 whole spheres and the multitude co-ordinate to these wholes*; 

 and the only alteration which they experience is a mutation 

 of figure, and variation of light at different periods ; but in 

 the sublunary region, while the spheres of the elements 

 remain, on account of their subsistence as wholes, always 

 according to nature, the parts of these wholes have some- 

 times a natural, and sometimes an unnatural subsistence ; 

 for thus alone can the circle of generation unfold all the 

 variety which it contains. 



The different periods in which these mutations happen 

 are called by Plato, with great propriety, periods of fertility 

 and sterility ; for in these periods a fertility or sterility of 

 men, irrational animals, and plants takes place; so that 

 in fertile periods mankind will be both more numerous, and 

 upon the whole superior in mental and bodily endowments, 

 to the men of a barren period. And a similar reasoning 

 must be extended to animals and plants. The so much 

 celebrated heroic age was the result of one of these fertile 



* See the Introduction to my Translation of the Timaeus of 

 Plato. 



