Why does Justice Harlan say that segregation cannot be justified upon any legal grounds?

It cannot be justified upon any legal grounds. If evils will result from the commingling of the two races upon public highways established for the benefit of all, they will be infinitely less than those that will surely come from state legislation regulating the enjoyment of civil rights upon the basis of race.Explanation:

president thomas jefferson commissioned the expedition shortly after the louisiana purchase in 1803 to explore and to map the newly acquired territory, to find a practical route across the western half of the continent, and to establish an american presence in this territory before britain and other european powers