Marx Engels Correspondence 1881
Source: Karl Marx and Frederick Engels, Selected Correspondence (Progress Publishers, Moscow, 1975). Scanned and prepared for the Marxist Internet Archive by Paul Flewers.
... Even though the Katheder-Socialists  persistently call upon us proletarian Socialists to tell them how we can prevent overpopulation and the consequent threat to the existence of the new social order, I see no reason at all why I should do them the favour. I consider it a sheer waste of time to dispel all the scruples and doubts of these people which arise from their muddled superwisdom, or even to refute, for instance, the awful twaddle which Schäffle  alone has compiled in his numerous big volumes. It would require a fair-sized book merely to correct all the passages set in inverted commas which these gentlemen have misquoted from Capital. They should first learn to read and to copy before demanding that one should answer their questions...
There is of course the abstract possibility that the human population will become so numerous that its further increase will have to be checked. If it should become necessary for communist society to regulate the production of men, just as it will have already regulated the production of things, then it, and it alone, will be able to do this without difficulties. It seems to me that it should not be too difficult for such a society to achieve in a planned way what has already come about naturally, without planning, in France and Lower Austria. In any case it will be for those people to decide if, when and what they want to do about it, and what means to employ. I don’t feel qualified to offer them any advice or counsel in this matter. They will presumably be at least as clever as we are.
Incidentally, I wrote as early as 1844 (Deutsch-Französische Jahrbücher, page 109):
... even if Malthus were absolutely right, this (socialist) transformation would have to be undertaken on the spot; for only this transformation, and the education of the masses which it alone provides, makes it possible to place that moral restraint of the propagative instinct which Malthus himself presents as the most effective and easiest remedy for over-population. 
This is enough for now, the other points we can discuss when we meet...
1. Katheder Socialists – representatives of a trend in bourgeois economics and sociology which arose towards the end of the nineteenth century. They were in the main German professors who under the guise of socialism advocated bourgeois reformism from their university chairs (Katheder in German) – Progress Publishers.
2. Albert Eberhard Friedrich Schäffle (1831-1903) – German vulgar bourgeois economist and sociologist, after publication of Marx’s Capital, Volume 1, advocated class peace and cooperation between bourgeoisie and proletariat – Progress Publishers.
3. Frederick Engels, Outlines of a Critique of Political Economy (see Karl Marx, Economic and Philosophic Manuscripts of 1844 (Moscow, 1961), pp. 203-04) – Progress Publishers.