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The idea that humans can be classified into sub-groups based on their physical characteristics, and often simply on physical appearance. Now no longer considered to be useful for scientific purposes, it was all the rage not too long ago. To give you an idea of how bad it was, here's a list of races picked out (and simplified, believe it of not) form an encyclopedia from the 1960s.
Races of Man: Basic types. Caucaoid Northern Chinese American Indian Filipino-Malaysian African Negro Australoid Sub-types:
And...
The Three Great Stocks of Man
The World Book Encyclopedia, 1966