Explain the purpose of extended metaphor in paragraphs 3 and 4 of i have a dream

What is most likely the speaker’s reason to open with this? in approaching this problem, we cannot turn the clock back to 1868, when the [fourteenth] amendment was adopted, or even to 1896, when plessy v. ferguson was written. we must consider public education in the light of its full development and its present place in american life throughout the nation. only in this way can it be determined if segregation in public schools deprives these plaintiffs of the equal protection of the laws.