1858.] THE LETTER TO DR. GRAY. 125 



of land will yield a greater weight, if cropped with several 

 species of grasses, than with two or three species. Now every 

 single organic being, by propagating rapidly, may be said to 

 be striving its utmost to increase in numbers. So it will be 

 with the offspring of any species after it has broken into 

 varieties, or sub-species, or true species. And it follows, I 

 think, from the foregoing facts, that the varying offspring of 

 each species will try (only few will succeed) to seize on as 

 many and as diverse places in the economy of nature as 

 possible. Each new variety or species when formed will 

 generally take the place of, and so exterminate its less well- 

 fitted parent. This, I believe, to be the origin of the classifi- 

 cation or arrangement of all organic beings at all times. 

 These always seem to branch and sub-branch like a tree 

 from a common trunk ; the flourishing twigs destroying the 

 less vigorous the dead and lost branches rudely representing 

 extinct genera and families. 



This sketch is most imperfect ; but in so short a space I 

 cannot make it better. Your imagination must fill up many 

 wide blanks. Without some reflection, it will appear all 

 rubbish ; perhaps it will appear so after reflection. 



C. D. 



P.S. This little abstract touches only the accumulative 

 power of natural selection, which I look at as by far the most 

 important element in the production of new forms. The laws 

 governing the incipient or primordial variation (unimportant 

 except as the groundwork for selection to act on, in which 

 respect it is all important), I shall discuss under several 

 heads, but I can come, as you may well believe, only to very 

 partial and imperfect conclusions. 



[The joint paper of Mr. Wallace and my father was read at 

 the Linnean Society on the evening of July 1st. Sir Charles 

 Lyell and Sir J. D. Hooker were present, and both, I believe, 

 made a few remarks, chiefly with a view of impressing on those 



