In the 19th century, scientists used a variety of physical traits to categorize people into races. These traits were often superficial and arbitrary, but they were used to justify the idea that different races were fundamentally different and unequal. Some of the most common traits used included:

* Skin color: This was perhaps the most obvious and widely used trait, with lighter skin being associated with superiority and darker skin with inferiority.

* Cranial capacity: Scientists believed that the size and shape of the skull could indicate intelligence and other mental abilities. They often used phrenology, the study of the bumps on the skull, to justify their claims.

* Facial features: Features like the nose, lips, and eye shape were also used to distinguish between races, with European features often considered "ideal."

* Hair texture: Straight, wavy, and curly hair were each associated with different races.

* Body shape and stature: Differences in height, build, and other physical proportions were also used to categorize people.

It's important to note that these biological classifications were often based on flawed science and prejudiced assumptions. They were used to support racist ideologies and justify the exploitation and oppression of people of color.

The concept of race as a biological reality has been disproven by modern science, which recognizes that there is more genetic variation within racial groups than between them. However, the legacy of 19th-century racial classifications continues to have a significant impact on society today, contributing to systemic racism and discrimination based on race.