4o6 The Living Plant 



present biological opinion, biologists differ much in their opinions 

 as to the method by which it has been effected, — for of course the 

 fact or non-fact of evolution is one thing, and the method whereby 

 it has been brought about is another. Evolution may be true 

 and yet every one of the explanations of the method thereof, 

 given heretofore by scientific men, may be false. These explana- 

 tions, however, are so important in many ways that we must now 

 proceed to consider them. 



Of all the explanations of the method of evolution, the greatest 

 and best known is Darwin's, embodied in his principle of Natural 

 Selection. It was, indeed, the first logically-satisfactory explana- 

 tion ever given of the way in which evolution may have been 

 brought about; and, because it was logical, it enabled thoughtful 

 men for the first time to believe in evolution as a fact, — for they 

 could not believe in its reality so long as they could not under- 

 stand how it might have been effected. Herein consists Darwin's 

 greatest service to science; and this, moreover, is the reason why 

 his name is associated with evolution so closely that most people 

 regard the two words as practically synonymous. And the case 

 is not at all affected by the fact that Natural Selection may yet 

 prove not to be the real explanation of evolution. It is a possible 

 and a logically-adequate explanation, but not necessarily on that 

 account the correct one. 



So important is this principle of Natural Selection, historically 

 as well as scientifically, that we must now consider it sufficiently 

 to make its significance clear; and this is the more needful be- 

 cause it is commonly misunderstood even by many of those who 

 talk much about it. I shall try first to present the subject as 

 I think that Darwin conceived it, giving later the modifications 

 introduced by subsequent investigation. 



In essence Natural Selection is a deduction from the inter- 

 operation of five factors, all of which are familiar to observation, — 

 viz., variation, overproduction, struggle for existence, survival of 

 the fittest, heredity. 



