This chapter is entitled "Laws of Variation", which implies that it will discuss genetics, the subject which Darwin famously got wrong. That having been said, Darwin is also very forthright in admitting his ignorance, that his understanding of genetics is in effect a hunch. Surprisingly, he actually noticed the proportions expected of dominant allele inheritance that Mendel pursued further to propose the first viable model of particulate inheritance. Had he only investigated that further himself! But he did not; and it is a testament to his model of evolution that it did not matter to his main point. Darwin assumed a "blending" model of inheritance, in which -- on average -- the character states of any set of offspring will represent the mean between their parents. This does not happen with particulate inheritance, in which a state is either present or not, and can persist for many generations in spite of individuals having it mating with those having its corresponding opposite state. How could Darwin's model work to preserve useful mutations? Remember that I said that the character states of individuals were on average the mean between those of their parents: variation will always occur in Darwin's model, and may offset the tendency for extreme characters to "average out" into the less remarkable norm.
To put it another way, suppose that a natural amount of variation exists in a population, such that some trait can be measured as being anywhere from x to -x, and all values have an even distribution. The average value of the trait across the entire population will then be 0. Now, suppose that we remove all individuals with a negative value of that trait (we could suppose that environmental circumstances changed such that positive values of the trait were extremely advantageous). The range of the trait is now x to 0, and its average value is x/2. Now, if we suppose that this trait arose by a process of variation that tends to extend the extremes of the range of values by an additional degree that we will call y. Even if no further selection acts upon this population, so long as y is large enough to counter the blending of a trait towards the average, the extreme values will persist; if y is greater than that, the range of variation will increase, and when y=x, we will have the full range of variation that existed in the original population, except in our selected sample with a mean at x/2 rather than at 0. Obviously, further selection can drive the average value ot this trait further in the same direction. Thus, Darwin's model of blending inheritance can be made to work.
Now, of course, this is all ultimately irrelevant: inheritance does not work that way. But I feel it important to spell out how it could have worked were it true, so as to illustrate how Darwin's theory was not significantly threatened by his defective model of genetics. I do not know whether Darwin himself thought things through this far, or whether any of his contemporaries did -- or indeed whether anyone has -- but I believe this to be a reasonable model.
That having been discussed, on with the chapter! Darwin starts out with observations on the effect of environment on the adult form, which he correctly infers to influence succeeding generations only when it affects the gametes (produced by what he calls the "generative organs" or gonads, and upon which he focuses his attention). It is in fact known that an individual's development can be altered, sometimes significantly, by the environmental conditions under which it was brought up: the resulting differences delineate what are called ecotypes. It is important to note that the capacity to produce all of the forms present in the population is still (potentially, at least) inherited, even if it is not displayed in the population. To give an example, a high-altitude ecotype of a plant might have leaves of a different size or shape than those of a low-altitude ecotype, but seeds from either ecotype will produce plants that match their environment rather than their parents in those features characteristic of the ecotype. Confusingly, the capacity to adapt in the course of development (necessary to produce different ecotypes) can itself be subject to genetic variation, such that one population might have a more extreme range of ecotypes than another. Darwin is aware of this distinction, although the terminology had not yet been invented, and correctly dismisses it in his discussion, since it only confuses matters. Evolution can only act upon heritable traits, and if a trait can be shown to be influenced by its environment, it becomes overly complicated for use as a model of evolution.
The next section of this chapter covers another aspect of biology that Darwin got wrong: he assumed that the effects of use or disuse were heritable. This is similar to Lamarck's concept of evolution, in which an individual improving some faculty or structure in the course of its life passes on such improvements to its offspring. (The famous example of this is the giraffe, which Lamarck supposed originally had a neck no longer than that of any other hoofed mammal, but which over successive generations kept straining to reach ever higher leaves, and thereby managed to lengthen its neck.) However, Darwin does not long dwell upon this, and does not suppose it to be as important an evolutionary force as natural selection. He supposes the reduction of eyes in subterranean animals, and the wings of island birds and beetles, to be due to the continued effects of disuse, and yet if one looks at the level of the population rather than of the individual, this is not an unreasonable shorthand. In discussing such examples, interestingly, this section deals less with the inheritance of acquired characteristics (to use the proper terminology for Lamarckian evolution) than it does with biogeography: the tendency of organisms in a given region to be related to one another, rather than to their counterparts from similar ecosystems in more distant regions.
The next section, on acclimatisation, touches on these ideas as well, but quickly returns to the concept of the ecotype. Darwin was astute to discern the difference between these phenomena, even as he was ignorant of the facts behind them. He even goes so far as to propose experiments to test the limits of acclimatisation, in order to suggest that the boundaries between ecotype and inherited variation were not yet then known.
Darwin next addresses "Correlation of Growth", which he ties quickly to Richard Owen's concept of serial homology, developed more or less to its present form about a decade previously. Perhaps the most significant aspect of this, understated by just about everybody, is the homology inherent in symmetry. In other words, the structures on the left side of the body are homologous to their counterparts on the right; and the developmental processes that influence one side will also influence the other. Similarly, processes that affect an animal's forelimbs will tend as well to affect its hind limbs. This is not absolute, of course; the dissimilarly sized claws of fiddler crabs are one clear example of symmetry being broken, and one of the most striking differences between humans and our closest relatives is the much greater length of our legs and of their arms. In any event, the importance of development to evolution is immense, and Darwin had clearly grasped this. He also pointed out that correlation between structures need not involve any developmental linkage: closely related species will tend to have traits in common due to simple inheritance, and developmental correlation need not be invoked.
Darwin proceeds from there to speculate upon an idea popular in his time, of a sort of balance across all parts of the developing body, such that overdevelopment in one part draws resources from elsewhere and results in underdevelopment in another part. He is circumspect about this, though, and unwilling to draw any lines between this effect and that of a combination of natural selection and inherited reduction of unusued body parts. He streamlines this further, with the adage that "natural selection is continually trying to economise in every part of the organisation." This is not a bad personification of the process of evolution, although now we would see developmental correlation and genetic drift as more significant players in the reduction of unused characters. As a result, less-derived organisms (or, as Darwin would put it, "those lower on the scale") show a greater degree of similarity in serially homologous parts: they have had less evolutionary pressure to specialise individual organs, which renders all of them more similar to one another. He concludes this section by pointing out that unused structures are prone to vary more. In other words, if there is no natural selection on a trait, it is free to vary. All that remains to bring this to the modern concept of genetic drift is the notion of fixation, that in a sufficiently small population some nonselected character states will be lost simply by chance.
This brings us to the halfway point in this chapter; I shall tackle the remaining half tomorrow.