structural:summarization
Variation under domestication and under nature

Chapter I covers animal husbandry and plant breeding, going back to ancient Egypt. Darwin discusses contemporary opinions on the origins of different breeds under cultivation to argue that many have been produced from common ancestors by selective breeding. As an illustration of artificial selection, he describes fancy pigeon breeding, noting that "he diversity of the breeds is something astonishing", yet all were descended from one species, the rock dove. Darwin saw two distinct kinds of variation: (1) rare abrupt changes he called "sports" or "monstrosities" (example: Ancon sheep with short legs), and (2) ubiquitous small differences (example: slightly shorter or longer bill of pigeons). Both types of hereditary changes can be used by breeders. However, for Darwin the small changes were most important in evolution. In this chapter Darwin expresses his erroneous belief that environmental change is necessary to generate variation. The opening two sentences of On The Origin demonstrate this point and also show that Darwin did not see the role of natural selection in stabilizing selection:

When we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is, that they generally differ much more from each other, than do the individuals of any one species or variety in a state of nature. When we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, I think we are driven to conclude that this greater variability is simply due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species have been exposed under nature.

It can be seen here that Darwin attributes the greater variation amongst individuals of domestic varieties compared to their progenitor populations in nature as being due to their "conditions of life" (environment) being "not so uniform as", and "somewhat different from" those of the parent species". He later erroneously elaborates that changed "conditions of life" act on the reproductive organs to generate greater variability in the progeny. Even after 1860 when Darwin read the correct reason for greater variation in domestic varieties in Patrick Matthew 's book, On Naval Timber and Arboriculture published in 1831, future editions of On the Origin of Species contained these first two sentences unchanged and continued to omit the correct explanation of stabilizing selection.

In Chapter II, Darwin specifies that the distinction between species and varieties is arbitrary, with experts disagreeing and changing their decisions when new forms were found. He concludes that "a well-marked variety may be justly called an incipient species" and that "species are only strongly marked and permanent varieties". He argues for the ubiquity of variation in nature. Historians have noted that naturalists had long been aware that the individuals of a species differed from one another, but had generally considered such variations to be limited and unimportant deviations from the archetype of each species, that archetype being a fixed ideal in the mind of God. Darwin and Wallace made variation among individuals of the same species central to understanding the natural world.