As revolutionary scientific works go, Charles Darwin’s On the Origin of Species is remarkably accessible to the ordinary reader. True, some of the language is occasionally heavy going—Darwin wrote in haste, had a thing for double negatives and rogue commas, and occasionally embarked on convoluted, heavily nested sentences requiring several deep breaths to read out loud—but, minor stylistic concerns aside, over a century and a half after its publication, Origin is still a rewarding read for anyone wanting to get inside the mind of one of the most important figures in the history of science. Darwin’s great theory of evolution by means of natural selection forms the foundation of modern biology. To read Origin is to have the theory explained to you by the man himself. That should be a good enough reason for anyone to read it.
Darwin describes his masterpiece as ‘one long argument’. He knows his views will be seen as controversial by some of his contemporaries, and he’s out to convince. But he realises some of his audience—especially some of the old guard—will not be able to accept his evidence and line of reasoning. Indeed, at one point, he humorously states anyone who has read his friend Charles Lyell’s book Principles of Geology and is still unconvinced about the great age of the earth might as well close Origin right away.
Darwin’s great theory is brilliant in its simplicity: individual organisms within a species vary; those with beneficial variations will have a better chance of competing for limited resources, and of passing on those beneficial variations to future generations; over time, these beneficial variations will become more widespread in local populations. The composition of the population will have changed. The species will have evolved. Given enough time, more changes will accrue. New species will emerge from old—often, but not always, supplanting them.
Darwin begins his long argument in what should be comfortably familiar territory for his readers, describing how, consciously or otherwise, humans have selected preferred variants of domesticated animal and plant species over decades and centuries, leading to all manner of highly adapted breeds and strains. He goes on to describe how wild species also vary, and how far more individuals are produced than can possibly be supported by the environment. This, he says, leads to a struggle for existence in which many individuals will perish. Those individuals that vary from the rest in some advantageous way will stand a better chance of surviving, and of passing on those variations to future generations. While humans select between variants of domesticated species based on some personal preference, Nature selects between variants of wild species based on whether or not they are well enough adapted to meet the challenges of life:
I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. […] As man can produce and certainly has produced a great result by his methodical and unconscious means of selection, what may not nature effect? Man can act only on external and visible characters: nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends.
In the fourth chapter of Origin, Darwin brilliantly illustrates his principle of natural selection with a number of hypothetical examples. These are not intended to show how certain adaptations did indeed evolve, for that could never be proved retrospectively; Darwin’s aim is simply to show how such adaptations could have evolved through natural selection.
Famously, Darwin never got to the bottom of what causes the observed variations that are central to his theory, nor how they are inherited by future generations. He did, however, identify a number of important phenomena associated with such variations, including the atrophying and occasional total disappearance of organs which no longer have any use, occasional throwbacks to former organs, the skipping of variations in some generations, and the correlation of certain variations in different parts of individual organisms. He made a few stabs at explaining these phenomena, but they would not be properly understood until after his death, when the new field of genetics was merged with his theory. Darwin would no doubt have been delighted that his theory continued to evolve, but remains the single most important idea in the field of biology.
Having worked on his theory for two decades prior to publication, Darwin was painfully aware of the objections that would most likely be raised against it. He meets these objections head on in Origin, dedicating an entire chapter to addressing some of them, while still being prepared to admit when he doesn’t yet have entirely satisfactory answers. He addresses other potential objections to his theory in later chapters, which we shall return to later.
Darwin then goes on to discuss how animal instincts are also subject to natural selection, before exploring hybridism. I have to admit, even my eyes began to glaze over during the hybridism chapter: Darwin does go on a bit. The key point he is trying to get across is at last stated boldly in the final phrase of the chapter: ‘…there is no fundamental distinction between species and varieties’. Under Darwinian evolution, new species are simply old varieties writ large.
Darwin next dedicates an entire chapter to the imperfection of the geological record. If species evolved from other species, where are all the intermediate fossils? He is careful to explain it is tempting but wrong to envisage ‘intermediates’ directly between two closely related modern species—what we would nowadays describe as the ‘missing link’ fallacy. What really links modern species is their common ancestor, and it is far from necessarily true that this common ancestor would resemble some weird 50:50 chimera of its living descendants. Even if, as Darwin convincingly argues, the fossil record weren’t hopelessly incomplete, it is quite possible we might sometimes simply not recognise fossils of common ancestors for what they really are. Of course, since Darwin’s day, the fossils of many common ancestors, or their close relatives, have been identified.
One important challenge Darwin needed to overcome was to explain the current geographical distribution of species. If all species descend from other species, each must have originated in some single geographical location. But many individual species, or closely related species, have now spread far and wide across the planet. How could salt-water-intolerant plants and animal, for example, have crossed entire oceans? Darwin brilliantly addresses this problem in two of Origin’s more entertaining chapters, citing his own weird experiments on plant dispersal, and invoking climate change, among other phenomena, to explain how currently inhospitable areas might once have acted as corridors for species dispersal. Darwin would dearly have loved the twentieth-century theory of plate tectonics, which explains how continents themselves can move, transporting species with them. To his great credit, although he mentions the contemporary theory of numerous ‘land bridges’ that were believed to have formerly linked continents, which would have provided an obvious mechanism of species dispersal, Darwin the geologist is honest enough to admit he is not a great supporter of that theory, so believes alternative mechanisms need to be identified.
Before summing up, Darwin discusses a number of phenomena that are easily explained by his theory of descent with modification, but which would make no sense at all had species been uniquely created from scratch. How is it even possible to classify species into groups if they bear no physical relationship with each other? Why do species belonging to particular groups have the same basic design or archetype? Why are different animal embryos so hard to tell apart? Why do certain species bear apparently useless, or near-useless organs? Creationists might try explain these away by appealing to the aesthetic whim of a creator moving in mysterious ways, but Darwin rightly recognised all of these apparent enigmas were easily explained by realising all species are related to each other geneologically.
In a magnificent final chapter, Darwin recaps what has gone before, but with increasing confidence. In places, he even manages to overcome his almost painful modesty and strut a little. His theory’s merit, he fully appreciates, is that it explains so much, and that it will open up many new fields of research:
when we regard every production of nature as one which has had a history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, nearly in the same way as when we look at any great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting, I speak from experience, will the study of natural history become!
Well said, Mr D! You did indeed made natural history more interesting. You gave future biologists a theory by which to work. You revolutionised an entire science. There was grandeur in your view of life—and you knew it!