Variation Under Nature

When confronted with two populations of plants that look similar to one another, I do not have the foggiest idea as to whether or not these are different species, different varieties of the same species, or merely two groups of the same plant that just happen to have some immediately visible differentiation. This, I always surmised, is because I'm a layperson, and I don't know anything about biology. It turns out that my guess is nearly as good as anybody else's.

Mr. Darwin takes great pains, and a good number of pages, to make the point that when dealing with closely related forms, the definitions of "species" and "variety" are more or less arbitrary, and very much dependent on who you're speaking to. Related plants and animals lie on a continuum of variation. At one end are individual differences, like those between plants grown from different seeds. At the other end are the differences that mark the dividing line between species, chimpanzees and bonobos, for instance. Between these two extremes are varieties, groups recognized as distinct populations, but close enough that they're still the same species.

(Included, briefly, in all of this, are "monstrosities." Interestingly, Darwin presumes that a monstrosity exhibits "some considerable deviation of structure, generally injurious, or not useful to the species;" one wonders if he would find the many breeds of modern dogs that have had pretty severe defects bred into them as falling under that definition.)

In the end, where individual differences stop and varieties begin, and how far varieties go before transitioning into distinct species was, in Darwin's time (and perhaps to the present) essentially arbitrary, driven more by convention and preference than anything tangible. Where one naturalist sees several different species, another sees only a few, and some number of varieties. Some went so far as to dispense with varieties altogether - any variation more important than individual differences marked a new species, including some that weren't related to physiology at all, such as location of habitat. If Darwin is to be believed, and if I understand him correctly, these sorts would consider the Canadian Geese one finds in Seattle to be a different species than those found in London. According to Darwin, at this level, the term species loses its meaning, and "comes to be a mere useless abstraction, implying and assuming a separate act of creation." I'm curious as to the reasoning behind that conclusion, as Darwin does not explain any further.

It is in this chapter where Darwin begins to draw distinctions between evolution by natural selection, and the existence of species due to special acts of creation through hypothesizing what sorts of results one might find if some agency had directly created life on earth. Whether any idea of the immutability of species is always connected with this is unclear, although we may surmise that there were adherents in the creationist camp that held so - Darwin himself admitted to once having believed so, earlier in the book. In any event, the second part of the chapter deals with some observations about the relationships between genera, and their subordinate species. Its dry and fairly obvious stuff - a genera that has a broader range will have species that show more variation than a genera with a narrower range. (Although genera is the plural of genus, Darwin seems to use it as both the singular and the plural - it seems logical to assume that such was the common usage at the time.) But I suspect that it will come in handy in the next chapter: Struggle For Existence.