WHAT was it, exactly, that Charles Darwin achieved with his theory of evolution? It’s well beyond our abilities to describe the fullness of Darwin’s accomplishment, and we’ll probably regret attempting this essay, but we’ll give it our best. Please, dear reader, don’t think less of Darwin because of your Curmudgeon’s shortcomings.
Darwin began with information that was common knowledge. Some had been around for a few generations (like geology), and in some cases for centuries (the apparent relationships among various organisms), and in other cases from a time before history — improving our breeds of agricultural products and domesticated animals.
Darwin added more recent observations of how living things are distributed around the world. And then he did something that was extraordinarily original — he not only tied all of that information together into a comprehensive description of life on earth, but then he explained — using natural means only — how it all occurred.
Let’s consider some of the pre-existing ideas Darwin used.
1. Geology, pioneered by James Hutton, one of the great thinkers of the Scottish Enlightenment. He first published his ideas in 1785, a quarter-century before Charles Darwin was born. Darwin probably couldn’t have made any sense of what he saw without an understanding of the geological processes that had altered the earth and the millions of years they required.
2. Visible patterns of physical similarities among living things — an idea that had been obvious for centuries. Carl Linnaeus published the system of Linnaean taxonomy more than a century before Darwin published Origin of Species. The Linnaeus system established a hierarchical structure of classification based upon observed physical characteristics. That system has been revised considerably since it was first proposed, but when today’s students are introduced to the biological classifications of kingdom, phylum, class, order, family, genus, and species, they are learning the basics of the Linnaeus system. He gave gave linguistic rigor to what thoughtful observers had noticed since the time of Aristotle.
3. Geographic patterns of relationships among living things. Regional wildlife descriptions had long been available, but what Darwin observed on a global scale was inconsistent with his expectations as a student of theology. The patterns that emerged from the evidence strongly suggested a history of natural dispersion rather than special creation, and in Origin of Species he dealt with this at length — it’s the only subject to which two specific chapters are devoted. For a sample, in Chapter 11, he wrote:
In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. … There is hardly a climate or condition in the Old World which cannot be paralleled in the New at least as closely as the same species generally require; … yet these are not inhabited by a peculiar fauna or flora. Notwithstanding this parallelism in the conditions of the Old and New Worlds, how widely different are their living productions! [Emphasis supplied.]
A second great fact which strikes us in our general review is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. …
A third great fact, partly included in the foregoing statements, is the affinity of the productions of the same continent or sea, though the species themselves are distinct at different points and stations. … We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties nearly like each other.
And in Chapter 12:
He who admits the doctrine of the creation of each separate species, will have to admit, that a sufficient number of the best adapted plants and animals have not been created on oceanic islands; for man has unintentionally stocked them from various sources far more fully and perfectly than has nature.
I have carefully searched the oldest voyages, but have not finished my search; as yet I have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above 300 miles from a continent or great continental island; and many islands situated at a much less distance are equally barren.
The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species.
The law which causes the inhabitants of an archipelago, though specifically distinct, to be closely allied to those of the nearest continent, we sometimes see displayed on a small scale, yet in a most interesting manner, within the limits of the same archipelago. Thus the several islands of the Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world.
The principle which determines the general character of the fauna and flora of oceanic islands, namely, that the inhabitants, when not identically the same, yet are plainly related to the inhabitants of that region whence colonists could most readily have been derived, the colonists having been subsequently modified and better fitted to their new homes, is of the widest application throughout nature.
4. Individual differences (Darwin’s term for what we call mutations) that are inherited. Darwin knew nothing of genetics, but he knew what all farmers and cattlemen knew. In Chapter 1 of Origin he wrote — note the ambiguous use of the word “race” which was common at the time:
Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. Some little effect may, perhaps, be attributed to the direct action of the external conditions of life, and some little to habit; but he would be a bold man who would account by such agencies for the differences of a dray and race horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy. … We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in several cases, we know that this has not been their history. The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to make for himself useful breeds.
And in Chapter 2 he wrote, with a hint of theory thrown in:
Again, we have many slight differences which may be called individual differences, such as are known frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen, from being frequently observed in the individuals of the same species inhabiting the same confined locality. No one supposes that all the individuals of the same species are cast in the very same mould. These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions.
In Darwin’s final chapter of Origins, Chapter 14, he said:
Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability. But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed. It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by an uneducated eye. This process of selection has been the great agency in the production of the most distinct and useful domestic breeds.
So Darwin had observed, collected, and described. He could see — as had others — the natural pattern of life’s diversity. Because of his travels and his widespread correspondence with other naturalists, perhaps he saw these things more clearly and comprehensively than any of his predecessors or contemporaries. He was obviously a splendid naturalist.
But up to that point he hadn’t addressed the great unknown — How had life’s diversity occurred? He knew that in a small way and over a brief span of time it could happen, as with human efforts in breeding varieties (or “races”) of dogs, horses, and pigeons. And he could see a worldwide geographical pattern to the diversity of life. He suspected that inheritance was at the root of it all. But what drove the system in nature? What was the cause of speciation?
In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement ‘Malthus on Population,’ and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here then I had at last got a theory by which to work; but I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it.
Contrary to the assumption of creationists, Darwin didn’t begin with the theory of evolution in mind, hoping for evidence to support that idea. Rather, in proper scientific sequence, he first assembled all the evidence that was then available, observed that the global distribution of species didn’t support the concept of special creation, and only afterward did he conceive of natural selection as the explanation.
After a delay of 20 years, Darwin published Origin of Species. He summarized his evidence in Chapter 14, the book’s final chapter, which starts out saying:
As this whole volume is one long argument, it may be convenient to the reader to have the leading facts and inferences briefly recapitulated.
After reviewing much evidence, and rebutting many objections, he takes the well-known phenomenon of human-influenced domestic plant and animal breeding as a model, applies Malthusian dynamics, and says:
There is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under nature. In the preservation of favoured individuals and races, during the constantly-recurrent Struggle for Existence, we see the most powerful and ever-acting means of selection. The struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings. This high rate of increase is proved by calculation, by the effects of a succession of peculiar seasons, and by the results of naturalisation, as explained in the third chapter. More individuals are born than can possibly survive. A grain in the balance will determine which individual shall live and which shall die, — which variety or species shall increase in number, and which shall decrease, or finally become extinct. … The slightest advantage in one being, at any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance.
Man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and every one admits that there are at least individual differences in species under nature. …
If then we have under nature variability and a powerful agent always ready to act and select, why should we doubt that variations in any way useful to beings, under their excessively complex relations of life, would be preserved, accumulated, and inherited? Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products? What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature, — favouring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life.
That’s all we can do in a brief essay. Darwin’s ideas seem familiar to us now, and the concept of natural selection seems almost obvious. But throughout human history, although much of the evidence was available, it wasn’t obvious at all. It required Charles Darwin to reason it out.
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