The correct answer is Social Equality.
Mark Twain was an american Writer who supported various civilian causes related to the equality of rights of all people.
He was an unconditional supporter of the emancipation of women and active defender of women's suffrage, a strong supporter of abolitionism and emancipation. Being an admirer of Helen Keller, he supported her financially in his studies and wrote in praise of the labor and union movement.
He had lived both in America and in Europe so he knew the differences between the conditions of the immigrants in both continents. Cities in America had a great revolutionary time helped by a lot of inventions which were accelerated growing and changing fastly the lifestyle of the people.
The immigrant contributed absolutely clarely to the growing and developing. The went to America for economic opportunities and they found also liberty and equality of rights more than their precedence countries.
In that way and context Mark Twain said that quote.