Question

In the 19th century, which trait was used to label a person as Caucasoid?

Answer 1

In the 19th century,SKIN COLOR  trait was  used  to label a person as Caucasoid.

Answer 2

The trait or characteristic used to label a person as Caucasoidin the 19th century is the skin color.

In the early half of the nineteenth century, theories about the origins of human lineage depended on Linnean models. These groupings defined types of men by outward physical characteristics, and the special emphasis was placed on skin pigmentation.  

The Caucasian race refers to a grouping of human beings considered as a biological taxon, which, depending on which of the historical race classifications is used, has included some or all of the ancient and modern populations of Europe, Western Asia, Central Asia, South Asia, North Africa, and the Horn of Africa.

In the United States, term Caucasian has also often been used as a synonym for white or of European, Middle Eastern, or North African ancestry.