INDIRECT EQUILIBRATION. 445 



sought to express in mechanical terms, is that which Mr Dar- 

 win has called " natural selection, or the preservation of 

 favoured races in the struggle for life." That there is going 

 on a process of this kind throughout the organic world, 

 Mr Darwin's great work on the Origin of Species has shown 

 to the satisfaction of nearly all naturalists. Indeed, when 

 once enunciated, the truth of his hypothesis is so obvious as 

 scarcely to need proof. Though evidence may be required 

 to show that natural selection accounts for everything ascribed 

 to it, yet no evidence is required to show that natural selec- 

 tion has always been going on, is going on now, and must 

 ever continue to go on. Recognizing this as an a priori cer- 

 tainty, let us contemplate it under its two distinct aspects. 



That organisms which live, thereby prove themselves fit to 

 live, in so far as they have been tried ; while organisms which 

 die, thereby prove themselves in some respects unfitted for 

 living ; are facts no less manifest, than is the fact that this 

 self-acting purification of a species, must tend ever to insure 

 adaptation between it and its environment. This adaptation 

 may be either so maintained or so produced. Doubt- 



less many who have looked at Nature with philosophic eyes, 

 have observed that death of the worst and multiplication of 

 the best, must result in the maintenance of a constitution 

 in harmony with surrounding circumstances. That the aver- 

 age vigour of any race would be diminished, did the diseased 

 and feeble habitually survive and propagate ; and that the 

 destruction of such, through failure to fulfil some of the con- 

 ditions to life, leaves behind those which are able to fulfil the 

 conditions to life, and thus keeps up the average fitness to 

 the conditions of life ; are almost self-evident truths. But 

 to recognize " natural selection " as a means of preserving 

 an already-established balance between the powers of a spe- 

 cies and the forces to which it is subject, is to recognize it 

 only in its simplest and most general mode of action. It is 

 the more special mode of action with which we are here con- 

 rerned. This more special mode of action, Mr Dar- 



