“My visceral perception of brotherhood harmonizes with our best modern biological knowledge. […] Many people think (or fear) that equality of human races represents a hope of liberal sentimentality probably squashed by the hard realities of history. They are wrong. This essay can be summarized in a single phrase, a motto if you will: Human equality is a contingent fact of history. Equality is not true by definition; it is neither an ethical principle (though equal treatment may be) nor a statement about norms of social action. It just worked out that way. A hundred different and plausible scenarios for human history would have yielded other results (and moral dilemmas of enormous magnitude). They didn't happen.”

Veto message to the House of Representatives (22 February 1869).
Quote

2000s, Is Diversity Good? (2003)
Context: To allow slavery to be introduced into free territories, where it had not hitherto existed, was, Abraham Lincoln held, a very bad thing. His opponent, Stephen A. Douglas, held that it was a sacred right, belonging to the people of each territory, to decide for themselves whether or not to have slavery among their domestic institutions. According to Douglas, Lincoln wanted to destroy the diversity upon which the union had subsisted, by insisting that all the states ought to be free. But for Douglas himself, the principle of 'popular sovereignty' did not admit of exceptions. There was to be no diversity, no deviation from the right of the people to decide. For Lincoln the wrongness of slavery meant that no one, and no people, had the right to decide in its favor. For Lincoln, the principle of human equality, "that all men are created equal", did not admit exceptions.

As quoted in His Brother's Blood: Speeches and Writings, 1838–64 https://web.archive.org/web/20160319090912/https://books.google.com/books?id=qMEv8DNXVbIC&pg=PA333#v=onepage&q&f=false (2004), edited by William Frederick Moore and Jane Ann Moore, p. 333
1860s, Speech (June 1862)

1910s, The Progressives, Past and Present (1910)
Context: Fundamentally, our chief problem may be summed up as the effort to make men as nearly as they can be made, both free and equal; the freedom and equality necessarily resting on a basis of justice and brotherhood. It is not possible, with the imperfections of mankind, ever wholly to achieve such an ideal, if only for the reason that the shortcomings of men are such that complete and unrestricted individual liberty would mean the negation of even approximate equality, while a rigid and absolute equality would imply the destruction of every shred of liberty. Our business is to secure a practical working combination between the two. This combination should aim, on the one hand to secure to each man the largest measure of individual liberty that is compatible with his fellows getting from life a just share of the good things to which they are legitimately entitled; while, on the other hand, it should aim to bring about among well-behaved, hardworking people a measure of equality which shall be substantial, and which shall yet permit to the individual the personal liberty of achievement and reward without which life would not be worth living, without which all progress would stop, and civilization first stagnate and then go backwards. Such a combination cannot be completely realized. It can be realized at all only by the application of the spirit of fraternity, the spirit of brotherhood. This spirit demands that each man shall learn and apply the principle that his liberty must be used not only for his own benefit but for the interest of the community as a whole, while the community in its turn, acting as a whole, shall understand that while it must insist on its own rights as against the individual, it must also scrupulously safeguard these same rights of the individual.

Undated entry of December 1863 or early 1864, as translated by Humphry Ward (1893), p. 215
Journal Intime (1882), Journal entries

as quoted by Dr. Javed Laghari in Monograph titled "Leaders of Pakistan" published by SZABIST, Pages 2-10 ( Vol.1 June-2009 http://www.szabist.edu.pk/Publications/Books/LeadershipBK-CP-09.pdf/). Retrieved on July 21, 2016

In Defense of Emma Goldman and the Right of Expropriation (1893)
Context: Humanity can not be made equal by declarations on paper. Unless the material conditions for equality exist, it is worse than mockery to pronounce men equal. And unless there is equality (and by equality I mean equal chances for every one to make the most of himself) unless, I say, these equal chances exist, freedom, either of thought, speech, or action, is equally a mockery.

Steppin' off the Edge http://steppinofftheedge.com/podcast/philosophy-of-open-source/, Podcast Interview, January 2011

Harijan, 30-1-1937, p. 407; In: My God (1962), Chapter 13. Pathways of God http://www.mkgandhi.org/god/mygod/pathwaystogod.html, Printed and Published by: Jitendra T. Desai, Navajivan Mudranalaya, Ahemadabad-380014 India
Posthumous publications (1950s and later)