150 years ago today

It is one thing to remember the date of an anniversary and quite another to truly recognize the significance of it. When it comes to Charles Darwin it seems that we have too much of the former and not enough of the latter, especially concerning what transpired 150 years ago today. Many are saying that today is the 150th birthday of natural selection, yet this is not really true. William Wells, Patrick Matthew, and Edward Blyth all preceded both Darwin and A.R. Wallace in print, each scratching the surface of the idea of natural selection but either misconstuing it as a preservative mechanism (like Blyth) or burying the concept where almost no one would find it (as Matthew did in a book about growing trees for shipbuilding). These false starts make what transpired at the Linnean Society on this day in 1858 all the more important; although it had generally been ignored for 45 years Darwin and Wallace both recognized the significance of the "secondary law" that could explain the origin of species.

Presenting such information and then walking away from the subject would do a disservice to my readers, however, and I would be a hypocrite if I did not go back to the source material. How can I recognize the importance of what happened on this date if I have never read the words of the men I wish to honor? Fortunately I own a copy of the book Adam or Ape which presents the papers read before the Linnean Society (along with many other invaluable works), although even if I did not have a hard copy at hand the papers are available for free online. (I do not wish to be smug, but I am troubled that many people have posted tidbits about what today is but have not linked back to the original material. If we cease to read the works that make this date important we are only going to contribute to the ever-pernicious mess of textbook cardboard that continues to confuse so many.)

The papers, bearing the title "On the Tendency of Species to Form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection" was prefaced with a letter dated June 30th, 1858 by Charles Lyell and Joseph Hooker. This letter of introduction, noting Wallace's recent discovery of natural selection and Darwin's longer history of work on the subject, is interesting for several reasons. First is that Darwin is favored from the outset, his longer history of work on natural selection giving him pride of place. Indeed, the introduction closes;

... we feel it to be desirable that views founded on a wide deduction from facts, and matured by years of reflection, should constitute at once a goal from which others may start, and that, while the scientific world is waiting for the appearance of Mr Darwin's complete work, some of the leading results of his labours, as well as those of his able correspondent, should together be laid before the public.

In this context Wallace is essentially placed as a catalyst, an "able correspondent" that sparked Darwin into action. Darwin's more complete work might have been anxiously awaited but it seems there is no such buzz surrounding Wallace. This makes an earlier excerpt (if true) all the more interesting. Writing that Darwin was spurred into action by Wallace's essay on natural selection, asking for Lyell to get Wallace's permission to print the work (which Wallace did not actually give; he did not know of the reading when it took place), Lyell and Hooker comment that Darwin was reluctant to have his papers read with Wallace's;

Of this step [obtaining Wallace's permission to read the essay] we highly approved, provided Mr Darwin did not withold from the public, as he was strongly inclined to do (in favour of Mr Wallace), the memoir which he had himself written on the same subject, and which, as before stated, one of us had perused in 1844, and the contents of which we had both of us been privy to for many years.

Why would Darwin want to hold back his work even as Wallace's essay threatened to beat him to natural selection? The answer may have been that Darwin did not feel ready to present anything on the subject, least of all excerpts from a short manuscript he had written in 1839. Although the textbook cardboard surrounding this event often claims that Darwin and Wallace were each represented by one paper explaining natural selection this is not the case. For Darwin the 1839 sketch and excerpts from an 1857 letter to Asa Gray were presented, this slap-dash collection of thoughts being contrasted with Wallace's work. It is then little wonder that Darwin did not feel ready, and as a footnote to the mention of the 1839 sketch he wrote;

This MS. work was never intended for publication, and therefore was not written with care. C.D. 1858.

This has the ring of an apology to it and it is apparent that Darwin may have felt like he had been caught with his trousers down. Be that as it may, the sketch does encompass a number of the main theoretical pillars of Darwin's idea, particularly competition and the struggle for existence. Using an island as a model (and perhaps thinking of the diversity he saw on the Galapagos), Darwin writes;

But let the external conditions of a country alter. If in a small degree, the relative proportions of the inhabitants will in most cases simply be slightly changed; but let the number of inhabitants be small, as on an island,1 and free access to it from other countries be circumscribed, and let the change of conditions continue progressing (forming new stations), in such a case the original inhabitants must cease to be as perfectly adapted to the changed conditions as they were originally. It has been shown in a former part of this work, that such changes of external conditions would, from their acting on the reproductive system, probably cause the organization of those beings which were most affected to become, as under domestication, plastic. Now, can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits, or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would also have a better chance. Yearly more are bred than can survive; the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive. Let this work of selection on the one hand, and death on the other, go on for a thousand generations, who will pretend to affirm that it would produce no effect, when we remember what, in a few years, Bakewell effected in cattle, and Western in sheep, by this identical principle of selection?

The emphasis on the word chance made things clear; this was not some orderly system where creatures "came into being" via some supernatural force to the pleasure of a Creator. As the earth changed so would creatures to match their new surroundings, all of nature running to stay in place. The second document, the letter to Asa Gray, provides an update on Darwin's general line of reasoning for natural selection. Most interesting, however, is a portion of Darwin's program that often goes overlooked today; the principle of divergence;

Another 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, and in the plants or insects on any little uniform islet, belonging almost invariably to as many genera and families as species. We can understand the meaning of this fact amongst the higher animals, whose habits we understand. We know that it has been experimentally shown that a plot of land will yield a greater weight if sown with several species and genera of grasses, than if sown with only two or three 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 is the "tangled bank" in which the spread of life is closely tied to death and extinction. Evolution was not a straight line process but a branching one, filling most every spot that it was possible to fill so that a particular place could support the most life. This is often seen on islands, the diversity and specialization of lemurs on Madagascar being an example of what competition can produce.

Wallace's essay stands in sharp contrast to the selections of Darwin's notes and starts off on an entirely different tack. Wallace begins by noting that varieties of domesticated animals often revert to the "wild type" when they become feral, their status being unstable and changes induced by artifical selection being temporary. Striking down the analogy between domestic varieties and natural varieties, Wallace sets out to explain how natural selection can produce both new natural varieties and cause domesticated animals to return to their "parent form" in nature.

With his thesis in place Wallace explains how there are checks on the increase of populations, using birds as primary examples (Wallace's passage being very similar to that of Darwin's 1839 sketch which also used birds in a thought experiment about increasing population). Predation, climate, migration, and most importantly food all affect population, thus there are pressures present on a population that keeps it in check. If all creatures were the same these pressures would keep populations in a state of stasis but, as Wallace next points out, organisms vary and these variations provide the raw material for those selective pressures to create change over generations;

Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. Even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they inhabit. It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers. These results must follow as surely as old age, intemperance, or scarcity of food produce an increased mortality. In both cases there may be many individual exceptions; but on the average the rule will invariably be found to hold good. All varieties will therefore fall into two classes--those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority. Now, let some alteration of physical conditions occur in the district--a long period of drought, a destruction of vegetation by locusts, the irruption of some new carnivorous animal seeking "pastures new"--any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination; it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organized variety would suffer first, and, were the pressure severe, must soon become extinct. The same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. The superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety.

The variety would now have replaced the species, of which it would be a more perfectly developed and more highly organized form. It would be in all respects better adapted to secure its safety, and to prolong its individual existence and that of the race. Such a variety could not return to the original form; for that form is an inferior one, and could never compete with it for existence. Granted, therefore, a "tendency" to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions again alone survive. But this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur.

As Wallace stated in his opening remarks the species produced by selection could not go backwards and return to a previous form, primarily because that earlier form would most likely be ill-adapted to the changing conditions driving natural selection (any creature going "backwards" would be wiped out). Also important is that Wallace recognizes the process as producing divergence, not a straight line of progress. Species produced divergent varieties that would be differently favored or disfavored by the changing conditions. This is in stark constrast to Wallace's thinking on domestic animals. While Darwin used artifical selection as an example of how animals can be changed (if they can be modified into so many different forms by people why can't nature do the same?) Wallace uses it to underline that selection in nature is permanent. In Wallace's view domestic animals will either go extinct or more closely resemble their parent stock if let loose into the wild, natural selection acting on the inferior domestic forms just as on native forms. Then returning to adaptation and survival, Wallace concludes;

We believe we have now shown that there is a tendency in nature to the continued progression of certain classes of varieties further and further from the original type--a progression to which there appears no reason to assign any definite limits--and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency to revert to the original type. This progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.

(Interestingly enough Wallace mentions natural selection for the long neck of the giraffe in his essay. He spends about as much time on the subject as Lamarck ever did, yet I've never seen Wallace's hypothesis show up in any textbook when the subject is broached.)

Although Darwin's excerpts were certainly informative they somewhat pale in comparison to the more detailed account given in Wallace's essay. After reading through all the documents I have to wonder if the reading of the papers was more an event to save Darwin's claim to priority and say "Don't mind the mess; things will be fixed up soon." Indeed, although Wallace's work more elegantly presented the case for natural selection Darwin was clearly favored in the introduction, the politics and inner machinations of science as a profession playing no small part in the events that transpired on July 1, 1858. The world did not wake up accepting evolution by natural selection on July 2nd, 1858, however; this date is but one major milestone in a string of many. Even the publication of On the Origin of Species is better understood as a beginning rather than an end. Just as life has evolved so has the evolution idea, and we would do well to remember the "struggle for existence" among the work of naturalists.