io8 CORRELATION OF PHYSICAL FORCES. 



more than probable that planets, not differing in their con- 

 dition so much from this as salt water, in which lives a whale, 

 does from the air in which an eagle disports himself, are 

 not barren while this is densely thronged with suitable 

 organisms ; he would think it improbable that, by a singular 

 accident, his lot should have been cast on the only planet 

 which is crowded at the present time with living organisms, 

 and whose history shows that it has been so crowded in the 

 past, that some of its rocks consist of a congeries of organic 

 remains. Would it not appear reasonable to deduce the 

 probability that there are other worlds, not peopled by crea- 

 tures identical with or very similar to those which struggle 

 for each spot of this earth, but as varied from those and 

 from each other as the nature of their surroundings may 

 require ; and thence, finding by more perfect acquaintance 

 the inevitable connection of surrounding conditions with the 

 character of the organisms capable of existing among them, 

 might he not more or less approximately deduce, from know- 

 ledge of the conditions, the character of the organisms ? 



As by the relation of inorganic matter to light, heat, &c. 

 we have now tolerably clear proofs of the substances of which 

 the sun and fixed stars are composed, it seems only a more 

 perfected knowledge that may give the more complex deduc- 

 tions which I venture, at the risk of being thought fanciful, to 

 indicate as possible in the distant future. 



Light was regarded, by what was termed the corpuscular 

 theory, as being in itself a specific matter emanating from 

 luminous bodies, and producing the effects of sensation by 

 impinging on the retina. This theory gave way to the undu- 

 latory one, which is generally adopted in the present day, 

 and which regards light as resulting from the undulation of a 

 specific fluid to which the name of ether has been given, 

 which hypothetic fluid is supposed to pervade the universe, 

 and to permeate the pores of all bodies. 



In a lecture delivered in January 1842, when I first pub- 

 licly advanced the views advocated in this essay, I stated that 

 it appeared to me more consistent with known facts to regard 

 light as resulting from a vibration or motion of the molecules 

 of matter itself, rather than from a specific ether pervading it ; 



