Chapters Two and Three will be discussed this week together. They lay the foundation for the principle of Natural Selection, covering respectively variation in nature and the "struggle for existence", the principle wherein with each generation many more offspring are generated than can survive.
Chapter Two is only twenty pages long in the online facsimile, and alludes to Darwin's never-written follow-up to the Origin, in which he planned on expanding on the Origin to his satisfaction. As it happened, the degree of detail provided in this work proved sufficient, and the "long catalogue of dry facts" that would have constituted much of the successor unnecessary.
The question of variation within species brings up the question of what constitutes a species, which is a contentious issue today. Back in Darwin's day, the question was just as open, although the candidate answers were very different. One of the more popular notions was that each species was created independently, and that "varieties" (sub-species and so on, a term used here with some scientific force, and somewhat interchangeable with "race") had obviously descended from and were members of the same species. At the same time, then as now, "every naturalist knows vaguely what he means when he speaks of a species." It is odd how some things never change!
Again anticipating the study of ecology, Darwin notes that features found to be useful in one individual of a species may not be so useful in another. He alludes to thickness of coat as an example: this is a more important feature to an animal in an extreme environment than to one in a more temperate locale, although the capacity to vary its thickness seasonally may be more important to the resident of the temperate zone. What varies in a species has traditionally not been considered important, but Darwin notes the circularity of the argument in which "important organs never vary", thereby removing such variation from consideration by using it to divide organisms into different species. Yet, substantial variation in unambiguously important organs can be found within otherwise-defined species: what we now know to be developmental variation in insects is noted by Darwin to produce very different morphologies in the paths taken by individual nerves.
The problem is most severe in what Darwin calls "protean" or "polymorphic" species. Such species (or indeed genera), claims Darwin, tend to be highly variable everywhere that they have been found, and in the case of fossils, in every period. This suggests that plasticity in development is itself an inheritable trait, at least inasmuch as that the corrective forces keeping development on a particular path are stronger in less-variable species, and weaker in more-variable ones. Darwin notes that the parts of organisms that vary are likely of less value to their survival than are the parts that do not, which is a necessary observation of any system in which natural selection is in operation.
Darwin next delves into the question of terminology, addressing the question of how much variation can be tolerated within a given species. Essentially, he says, the question cannot easily be settled; every specialist has a different list of valid species. He brushes by the distinction between allopatric and sympatric species, which would become important to evolutionary theorists in years to come. Here he merely notes that geography can be as characteristic as is form to determining species boundaries. Ultimately, the question of dividing individual species must be done using objective criteria, something that still has not been settled today; to do otherwise is "vainly to beat the air." Darwin briefly mentions what are now called "ring species", in which neighbouring forms are obviously continuous species, but members of the extremes of the range are distinct and do not easily interbreed.
The degree of known variation within a group of organisms is in no small part a function of the degree to which it has been studied. This is, of course, obvious, but it has important implications to the distinction of individual species, and Darwin goes from there to point out that it is much easier to learn a new group of organisms by taking their representatives from a particular locale in isolation; to consider initially those from other locations as well tends to confuse things. Once familiarity with a particular flora or fauna is attained, though, extending one's range of observations will challange one's sense of "species", both usefully and otherwise, as what could be diagnostic for a species in one region may ultimately prove highly variable between regions.
Having shown how variable individual species may be, and having suggested almost in passing that varieties within a species may in fact be incipient species, Darwin makes the important point that such varieties need not ultimately become different species. He goes on to point out that larger genera (those with more species) are both more widely distributed and more dominant than smaller genera, a point which I am not sure holds today. There are, at the very least, certainly exceptions, and Darwin points to a few of them himself. In particular, those species adapted to unusual environments (aquatic plants, for instance), tend to be more uniform. The other case that he gives, "plants low in the scale of organisation" being more widely distributed than their "higher" counterparts, I find definitely doubtful, on a similar principle to Darwin's own above. While individual knowledge of variation increases (at the very least) one's capacity to erect species distinctions, the features which vary in "lowly" organisms do so less markedly, and so the important distinctions are all the more subtle. To make a modern point of this, while they may not have evolved as far from the common ancestor as "higher" forms, they have nevertheless been evolving for the same length of time, and often for the same number of generations, and have had every opportunity to vary in ways that we may not appreciate.
An unwritten generalisation follows: Darwin believes nature to be essentially uniformitarian. This was a controversial geological principle when Darwin left on the Beagle, stating that the processes which shaped the Earth in the past are still in effect today. There are too many known exceptions for this to be considered strictly true today, and many of the processes which we now understand to be in effect (plate tectonics, for instance) were unsuspected in the early nineteenth century, but the essential notion is correct. Darwin's application of this to biology is a cornerstone to the science. In other words, evolution has happened, and it is continuing to happen today: it can be made an experimental science. These notions are not explicit in the Origin (at least not at this point in the work), but they are very much implied.
In summary, this chapter appears to make two important main points. First, the larger genera (by which is meant the more speciose genera), the more varieties can be found within each of its species. A related though distinct point is that the threshold of variation characterising each of those species and varieties is lower than that for the smaller genera, within which species and varieties tend to be more distinct from one another. As I said above, I am not sure that this is actually the case, and in any event, I do not see how it is critical to Darwin's argument. Far more important is this chapter's other point, that variation is a continuum: the more closely one looks, the more similarities one finds between different groups, and the more individual differences one sees within individual groups. Species are artificial constructs, conveniences for human classification, and are not always unambiguously delineated. Furthermore, "little groups of species are generally clustered like satellites around certain other species", by which Darwin means that genera may be divided into subgenera, the species of which resemble one another to a greater degree than they do members of other subgenera, and which tend to be found in closer proximity to one another geographically as well as morphologically. This is not an unimportant point: the continuity between species is not evenly distributed, and the fuzziness of the species definition can be extended to higher levels as well.