Darwin and Wallace papers

24 Abstract of a letter from Darwin to Asa Gray

II. Abstract of a Letter from C. DARWIN, Esq., to Prof. ASA GRAY, Boston, U.S., dated Down, September 5th, 1857.

In broad outline this letter resembles some of the subject matter in The Origin of Species, which we covered in Part 2 of this course. In the letter to Asa Gray, Darwin discusses artificial selection in domesticated animals, and then moves on to natural selection. He also discusses his principle of divergence, albeit in simplified form. Note also that Darwin touches on some other important ideas; very long periods of time, so that there might be ‘millions on millions of generations‘, and gradual change – ‘natura non facit saltum‘.

“It is wonderful what the principle of selection by man, that is the picking out of individuals with any desired quality, and breeding from them, and again picking out, can do. Even breeders have been astounded at their own results. They can act on differences inappreciable to an uneducated eye. Selection has been methodically followed in Europe for only the last half century; but it was occasionally, and even in some degree methodically, followed in the most ancient times.”

“I am convinced that intentional and occasional selection has been the main agent in the production of our domestic races; but however this may be, its great power of modification has been indisputably shown in later times.”

“Now suppose there were a being who did not judge by mere external appearances, but who could study the whole internal organization, who was never capricious, and should go on selecting for one object during millions of generations; who will say what he might not effect?

“We have almost unlimited time; no one but a practical geologist can fully appreciate this. Think of the Glacial period, during the whole of which the same species at least of shells have existed; there must have been during this period millions on millions of generations.”

“I think it can be shown that there is such an unerring power at work in Natural Selection (the title of my book), which selects exclusively for the good of each organic being.

“Reflect that every being (even the elephant) breeds at such a rate, that in a few years, or at most a few centuries, the surface of the earth would not hold the progeny of one pair. I have found it hard constantly to bear in mind that the increase of every single species is checked during some part of its life, or during some shortly recurrent generation. Only a few of those annually born can live to propagate their kind. What a trifling difference must often determine which shall survive, and which perish!”

Asa Gray, an American botanist with who Darwin corresponded regularly. Author: John A. Whipple, via Wikimedia Commons (CC-PD-Mark)
Asa Gray, an American botanist with who Darwin corresponded regularly. Author: John A. Whipple, via Wikimedia Commons (CC-PD-Mark)      

“Now take the case of a country undergoing some change. This will tend to cause some of its inhabitants to vary slightly—not but that I believe most beings vary at all times enough for selection to act on them. Some of its inhabitants will be exterminated; and the remainder will be exposed to the mutual action of a different set of inhabitants, which I believe to be far more important to the life of each being than mere climate. Considering the infinitely various methods which living beings follow to obtain food by struggling with other organisms, to escape danger at various times of life, to have their eggs or seeds disseminated, &c. &c., I cannot doubt that during millions of generations individuals of a species will be occasionally born with some slight variation, profitable to some part of their economy. Such individuals will have a better chance of surviving, and of propagating their new and slightly different structure; and the modification may be slowly increased by the accumulative action of natural selection to any profitable extent.”

“Multiform difficulties will occur to every one, with respect to this theory. Many 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 individuals undergoing change at any one time, answers others. The extreme imperfection of our geological records answers others.”

“Another principle, which may be called the principle of divergence, plays, I believe, an important part in the origin of species.”

“Now, every 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 become diversified into varieties, or subspecies, 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 thus exterminate its less well-fitted parent. This I believe to be the origin of the classification and affinities of organic beings at all times; for organic beings always seem to branch and sub-branch like the limbs of a tree from a common trunk, the flourishing and diverging 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 very wide blanks.”

C. DARWIN.

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