A line of decendency. Branches leave the main trunk of the tree different from each other.
And you know what Darwin thought about different races, right? You agree with him?
I don't know what he thought about races. Tell me.
Those scholars who are interested in the history of the eugenics movement will find some arresting quotes in the fifth chapter of The Descent of Man. This chapter, titled "On the development of the intellectual and moral faculties during primeval and civilized times" provides source material for many of the eugenicist arguments. For example:
With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health. We civilized men, on the other hand, do our utmost to check the process of elimination; we build asylums for the imbecile, the maimed, and the sick; we institute poor-laws and our medical men exert their utmost skill to save the life of every one to the last moment…Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man. It is surprising how soon a want of care, or care wrongly directed, leads to the degeneration of a domestic race; but excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed. (Darwin, 1871, 1896, p. 133-134)