propagating, this variation, which again will be slowly increased by the accumulative action of Natural 
selection; and the variety thus formed will either coexist with, or more commonly will exterminate its 
parent form. An organic being like the woodpecker or misletoe may thus come to be adapted to a score 
of contingencies: 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 everyone on this theory. Most can I think be satisfactorily 
answered.— "Natura non facit saltum" answers some of the most obvious.— The slowness of the 
change, and only a very few undergoing 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 20 species belonging to 18 
genera),— or in the plants and insects, on any little uniform islet, belonging almost to 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 2 or 3 species. Now every single organic 
being, by propagating so 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 places of and so exterminate its less well-fitted 
parent. This, 1 believe, to be the origin of the classification 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 1 cannot make it better. Your imagination must fill 
up many wide blanks.— Without some reflexion it will appear all rubbish; perhaps it will appear so after 
reflexion.— | C. D. 
This little abstract touches only on 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 to 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 & imperfect conclusions.— 
