In this chapter Darwin discusses how variations occur in species. He admits that earlier in the book he has attributed this to chance but that this is not true. He attributes variation to the conditions that species have been exposed to under several successive generations. The two factors that effect this are the nature of the organism and the nature of the conditions. Darwin admits that we have limited data regarding the change climate, food, and other environmental factors over time but believes that they are "greater than can be proved by clear evidence". Giving examples of birds, shells and plants Darwin makes a parallel to the variations found in natural and domesticated conditions where the environments are consistent.
In the next section Darwin discusses the enlargement and diminishing of certain parts due to it's use or lack thereof. Though this has not been definitively proven Darwin says that there are enough instances where it seems to occur and be passed on to future generations that it cannot be dismissed as a possibility. Natural selection may also play a role as in the case of the Spanish mole where skin and fir have built up over generations likely aiding it's survival, protecting from disease as it lives underground, but causing it to go blind from the disuse of it's eyes.
Next Darwin discusses acclimatization. The degree of adaptation is often overstated though in cases it does exist. One reason why we may see more acclimatization in domesticated species is the crossing of distinct wild stocks.
A number of examples are given of correlated variation where variation in one part of a plant or animal corresponds to variation in another. In some cases the relations between these tow may be readily understood, in others they are not.
Darwin notes that in more advanced animals, variation seems to occur in the specialized organs that animals use for survival, such as the wing of a bird. In more rudimentary organisms such as some plants there may be individual variation but are limited in what they pass on as a result of "natural selection having had no power to check the deviation in their structure."
Where a integral part of an organism shows little variation within a species, such as in the wing of a bat, it is a sign that the variation occurred long ago. Conversely, where a great deal of variation is seen it can be attributed to a more recent occurrence in the development of a species. "For in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner and degree, and by the continued rejection of those tending to revert to a former and less-modified condition. (p114)"
Reversion can be seen when sub species of different environments show similar variation. This can often be traced back to a trait of a common ancestor.
In closing the chapter Darwin notes the limits of our understanding of the laws of variation.
