In this first chapter Darwin begins his investigation by looking at domesticated plants and animals. Darwin notes that there is greater variation observed in domesticated species than would be seen in the wild. There is the belief that this could be caused by variations in climate, treatment or an excess of food, but Darwin sees these factors as being insufficient. He notes that if this were true would have to be exposed to these factors for several generations to cause any amount of variation and even our oldest domesticated plants and animals may show signs of variation even when the environmental conditions have remained constant.
If environment were the primary cause of variation then definite changes in environment would cause definite changes in organisms which would proceed in a linear fashion through successive generations if the environmental factors remained constant. What is seen much more often as a result of changed conditions is “indefinite variability”, such as the variation of the young from the same litter or the seeds from the same capsule. Some of these variations may be strong and many may be slight, but if they are passed down to future generations these variations may grow and become more clearly articulated.
Darwin admits that the laws governing inheritance are not yet fully understood, why some traits might skip generations while others only appear in one sex or the other. He reviews a number of domesticated species, in particular various breeds of dogs where it remains unclear if they derive from a single parent species or several.
After looking at this through a number of different types of animals he has focused his study on domestic pigeons. Darwin goes into great detail describing the differences in various types of domesticated pigeons from their beaks, to bone structure from feathers to coloring and other features. As great as these differences appear he believes the common opinion of naturalists to be correct, that all pigeons are descendants of the rock pigeon. If we were to try to see these extreme variations as proof that pigeons were descended from multiple original species, all of those would have become extinct since they do not still exist in the wild. A conclusion that seems unlikely. Additionally these differing pigeons would not be able to breed as the offspring of two distinct species are sterile. In the case of pigeons and other domesticated species nature gives successive variation and man selects the traits that are useful for him when he breeds them. Darwin looks at how far breeders have modified their cattle and sheep in a single lifetime by consciously selecting the best qualities for breeding. This is an example of conscious selection in domesticated animals.
Domesticated species are also effected by unconscious selection where small changes, often unnoticed at first accumulate over generations. Some changes may be cosmetic, while others may be physical, but the effect of those changes in times of hardship or famine will help some of the species survive and pass on those traits to subsequent generations. In this way variations that at first may be unnoticeable may grow to the point of a new breed or subspecies.
The cumulative effect of conscious and unconscious selection on species has led to more variation in domestic species than would be seen in the wild due to active and passive rolls in breeding to encourage or discourage certain traits.