See if you can guess who wrote the following:
[A]lthough each species must have passed through numerous transitional stages, it is probable that the periods, during which each underwent modification, though many and long as measured by years, have been short in comparison with the periods during which each remained in an unchanged condition.
Gould? Eldredge? Sounds like a textbook description of punctuated equilibrium, doesn't it?
But no, the awkward comma after the word periods is the real and potentially surprising give-away: our punctuationally challenged punctuationist is none other than Charles Darwin, writing in the fourth edition of On the Origin of Species!
What's that? Darwin a punctationist? Some mistake, surely! Wasn't he the one who kept banging on about how Natura non facit saltum (Nature does not make leaps)?
In January 2008, Evolving Thoughts' John S. Wilkins wrote a typically thought-provoking post explaining how Linnaeus's phrase Natura non facit saltum doesn't really mean what you probably thought it meant. The phrase, Wilkins explained, was originally intended to describe how Nature exists in small gradations; not how Nature comes about (i.e. evolves) gradually, in small, continuous steps. The distinction is a subtle one, and the two interpretations of the phrase are not necessarily contradictory—but neither does the fact that living and extinct species can be grouped into a continuum necessarily imply that they must have evolved at a uniform rate.
The only objections that have occurred to me are 1st that you have loaded yourself with an unnecessary difficulty in adopting 'Natura non facit saltum' so unreservedly. I believe she does make small jumps—
Huxley's comment seems to have put Darwin slightly on the defensive. By the second edition of Origin, published less than a year later, he had inserted a qualifier into his description of Linnaeus's dictum, describing it as 'that old, but somewhat exaggerated, canon in natural history of "Natura non facit saltum"' (my emphasis added).
I would advise you to be cautious about stating so broadly (I thought that you perhaps knew of distinct cases unknown to me) about species not varying for many generations & then suddenly varying. To a certain extent I quite believe it; ie that a plant will not vary until after some few generations (perhaps dozen or so) & then will begin to vary possibly suddenly, more likely gradually. But even my belief in this is grounded on very few facts.— I believe another & very distinct explanation may be given of a sort of current belief in the doctrine, viz that variations are often not attended to, & till they are attended to & accumulated, they make no show.—
In other words, although Darwin was prepared to believe that some variations could appear relatively suddenly, he also suspected that variations were happening all of the time, but not always being attended to (i.e. selected) and accumulated. Again, these viewpoints are not necessarily incompatible.
But, whereas Darwin seemed to be hedging his bets somewhat in his 1859 letter to Hooker, as we have seen from the passage quoted at the beginning of this article, he was clearly less hesitant of expressing what would now be described as punctuationist views seven years later in the fourth edition of Origin.
Had something happened between the publication of the third and fourth editions of Origin (1861 and 1866 respectively) to cause Darwin to insert the passage? I believe a clue comes earlier in the fourth edition, where Darwin is again describing the punctuated nature of evolution:
It is a more important consideration, clearly leading to the same result, as lately insisted on by Dr. Falconer, namely, that the periods during which species have been undergoing modification, though very long as measured by years, have probably been short in comparison with the periods during which these same species remained without undergoing any change. We may infer that this has been the case, from there being no inherent tendency in organic beings to become modified or to progress in structure, and from all modifications depending, firstly on long-continued variability, and secondly on changes in the physical conditions of life, or on changes in the habits and structure of competing species, or on the immigration of new forms; and such contingencies will supervene in most cases only after long intervals of time and at a slow rate. These changes, moreover, in the organic and inorganic conditions of life will affect only a limited number of the inhabitants of any one area or country.
As in his 1859 letter to Hooker, Darwin is again saying that variations are happening all of the time, but their selection is down to life's contingencies (a favourite word of punctuationists). The clue I referred to is the name Dr Hugh Falconer. In 1863, Darwin and Falconer had corresponded briefly on the subject of species formation. In one letter, Darwin wrote:
I should rather like to see it rendered highly probable that the process of formation of a new species was short compared to its duration; that is if the process was allowed to be slow and long: the idea is new to me.— Heer's view that new species are suddenly formed like monsters, I feel a conviction from many reasons is false.
Here, I suspect, we come to the real reason for Darwin's insertion of the passage quoted at the beginning of this article into the fourth edition of Origin. In a footnote to this letter, the Darwin Correspondence Project explains:
Working mostly with Tertiary plants and insects, the Swiss palaeontologist Oswald Heer maintained that species were generally constant, but that during occasional periods of creation, existing types underwent abrupt variation and gave rise to new species […]. By 1863, Heer's view of new species formation was being presented in the international literature as a rival to CD's theory of slow evolution by natural selection.
It seems to me that, by inserting the punctationist passage into the fourth edition of Origin, Darwin was trying to make it quite clear that his own theory of evolution by means of Natural Selection was quite adequate for explaining the apparently punctuated nature of species formation—without recourse to Oswald Heer's monsters.
As usual, Darwin was correct. He understood that it was perfectly possible for evolution to be punctuated, without the need for Nature to make monstrous leaps.
Postscript (27-Feb-2009): The aforementioned John S. Wilkins has written again on this subject in a post entitled Myth 4: Darwin was a gradualist.