i853.] THE LETTER TO DR. GRAY. 43 1 



gencies ; natural selection, accumulating those slight variations 

 in all parts of its structure which are in any way useful to it, 

 during any part of its life. 



V. Multiform difficulties will occur to every one on this 

 theory. Most can, I think, be satisfactorily answered, — 

 '" Natura non facit saltum " answer some of the most obvi- 

 ous. The slowness of the change, and only a very few under- 

 going change at any one time answers others. The extreme 

 imperfections of our geological records answers others. 



VI. One other principle, which may be called the principle 

 of divergence, plays, I believe, an important part in the origin 

 of species. The same spot will support more life if occupied 

 by very diverse forms : we see this in the many generic forms 

 in a square yard of turf (I have counted twenty species 

 belonging to eighteen genera), or in the plants and insects, 

 on any little uniform islet, belonging to almost as many 

 genera and families as to species. We can understand this 

 with the higher animals, whose habits we best understand. 

 We know that it has been experimentally shown that a plot 

 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 a 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 

 gsnerally 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 7/iosf imperfect ; but in so short a space I 

 22 



