Also, the words of Lincoln with which Jones ends that same paragraph run not "If our republic ever dies, it will die from suicide, from degeneracy" but "As a nation of freemen, we must live through all time, or die by suicide." Indeed, the noun "degeneracy" occurs only in the letter to Joshua F. Speed dated 24 August 1855:I am not a Know-Nothing. That is certain. How could I be? How can any one who abhors the oppression of negroes, be in favor of degrading classes of white people? Our progress in degeneracy appears to me to be pretty rapid. As a nation, we began by declaring that 'all men are created equal.' We now practically read it 'all men are created equal, except negroes.' When the Know-Nothings get control, it will read 'all men are created equal, except negroes, and foreigners, and catholics.'
But from those words alone it should be obvious that, as I've already said, Lincoln gave voice to similar convictions. Indeed, the entire thrust of the speech under discussion here, the "Address to the Young Men's Lyceum of Springfield, Illinois" delivered on 27 January 1838, is perfectly consistent with the summary provided by Jones, the instrument of the death by suicide that Lincoln warns against—"if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher"—being, in that case, the "mobocratic spirit", i.e.
the increasing disregard for law which pervades the country; the growing disposition to substitute the wild and furious passions, in lieu of the sober judgement of Courts; and the worse than savage mobs, for the executive ministers of justice.
Abraham Lincoln: speeches and writings, 1832-1858: speeches, letters, and miscellaneous writings, the Lincoln-Douglas debates, [edited and] annotated by Don E. Fehrenbacher (New York: The Library of America, 1989), 31, 29; Letter to Joshua F. Speed: 363 (360-363)). (The Library of America edition is based on
the 1953 edition edited by Basler (above), but corrects it where necessary.)
Some low-hanging scholarship (articles) on the speech (most to least recent, in progress). Note—on, at least potentially, the broader theme of internal degeneration—especially Wilson and Howe (which I haven't yet read):