Answer:
Based on the information provided in the question, the main idea expressed by Justice Harlan in the excerpt is:
A. The Constitution does not tolerate classes among its citizens based on the color of their skin.
Justice Harlan was a Supreme Court Justice who served during the late 19th century, and is known for his dissenting opinion in the landmark case of Plessy v. Ferguson. In that case, the Supreme Court upheld the constitutionality of racial segregation under the "separate but equal" doctrine, which allowed for segregation as long as the separate facilities were deemed equal.
Justice Harlan was a vocal opponent of this decision, and argued that the Constitution did not tolerate classes among its citizens based on the color of their skin. In his dissent, he argued that the idea of "separate but equal" was a fiction, and that segregation was inherently unequal and violated the principles of the Constitution.
Therefore, the main idea expressed by Justice Harlan in the given excerpt is that the Constitution does not allow for discrimination based on race, and that the idea of racial classes is incompatible with the principles of the Constitution.