This letter is part of correspondence between F. Engels and P. L. Lavrov, a Russian Narodnik.  The original lushly annotated edition — with extensive footnotes provided by British Marxist Historian Dona Torr — is available at the Marxists Internet Archive. 
I find it a very interesting short document: it illustrates Engels’ rhetorical style of coupling harsh and precise judgments with warm regards, it expresses a decisive opposition to Malthusianism and sentimental pleading for primitivist asceticism (we might consider today’s critics of “developmentalism” in Bolivia and “consumerism” in China), and it showcases his sheer passion for and understanding of the natural sciences. 
Many of the ideas sketched out here would soon be further developed in Engels’ Socialism: Utopian and Scientific (1880), a key text in propelling Marxism to world-historical importance.
One amusing detail regarding the original: The first and last paragraphs of the letter were written in French. Two quotations from Lavrov’s article and a few phrases were in Russian, “struggle for life, natural selection” in the first parenthetical was in English, and the rest of the letter was in German! — R. D.
My dear Monsieur Lavrov,
Now that I have returned from a visit to Germany I have at last got to your article, which I have just read with much interest. Here are my observations upon it, written in German, as this enables me to be more concise.
(1) Of the Darwinian theory I accept the theory of evolution but only take Darwin’s method of proof (struggle for life, natural selection) as the first, provisional, and incomplete expression of a newly-discovered fact. Before Darwin, the very people (Vogt, Buchner, Moleschott, etc.)  who now see nothing but the struggle for existence everywhere were stressing precisely the co-operation in organic nature — how the vegetable kingdom supplies the animal kingdom with oxygen and foodstuffs while the animal kingdom in turn supplies the vegetable kingdom with carbonic acid and manures, as Liebig, in particular, had emphasised. Both conceptions have a certain justification within certain limits, but each is as one-sided and narrow as the other. The interaction of natural bodies — whether animate or inanimate — includes alike harmony and collision, struggle and co-operation. If, therefore, a so-called natural scientist permits himself to subsume the whole manifold wealth of historical development under the one-sided and meagre phrase, “struggle for existence,” a phrase which even in the sphere of nature can only be taken with a grain of salt, such a proceeding is its own condemnation.
(2) Of the three convinced Darwinists cited, Hellwald alone seems to be worth mentioning. Seidlitz is only a lesser light at best, and Robert Byr is a novelist, whose novel Three Times is appearing at the moment in By Land and Sea — just the right place for his whole rodomontade too.
(3) Without disputing the merits of your method of attack, which I might call a psychological one, I should myself have chosen a different method. Each of us is more or less influenced by the intellectual medium in which he chiefly moves. For Russia, where you know your public better than I do, and for a propagandist journal appealing to the bond of sentiment, to moral feeling, your method is probably the better one. For Germany, where false sentimentality has done and is still doing such enormous harm, it would be unsuitable, and would be misunderstood and distorted sentimentally. What we need is hate rather than love — to begin with, at any rate — and, above all, to get rid of the last remnants of German idealism and instate material facts in their historic rights. I should, therefore, attack these bourgeois Darwinists something after this fashion (and shall perhaps do so in time):
The whole Darwinian theory of the struggle for existence is simply the transference from society to animate nature of Hobbes’ theory of the war of every man against every man and the bourgeois economic theory of competition, along with the Malthusian theory of population. This feat having been accomplished — (as indicated under 1 I dispute its unqualified justification, especially where the Malthusian theory is concerned) — the same theories are next transferred back again from organic nature to history and their validity as eternal laws of human society declared to have been proved. The childishness of this procedure is obvious, it is not worth wasting words over. But if I wanted to go into it further I should do it in such a way that I exposed them in the first place as bad economists and only in the second place as bad natural scientists and philosophers.
(4) The essential difference between human and animal society is that animals are at most gatherers whilst men are producers. This single but cardinal distinction alone makes it impossible simply to transfer the laws of animal societies to human societies. It makes it possible that, as you justly remark,
“Man waged a struggle not only for existence but for enjoyment and for the increase of his enjoyments … he was ready to renounce the lower enjoyments for the sake of the higher.”
Without contesting your further deductions from this, the further conclusions I should draw from my premises would be the following:
At a certain stage, therefore, human production reaches a level where not only essential necessities but also luxuries are produced, even if, for the time being, they are only produced for a minority. Hence the struggle for existence — if we allow this category as valid here for a moment — transforms itself into a struggle for enjoyments, a struggle no longer for the mere means of existence but for the means of development, socially produced means of development, and at this stage the categories of the animal kingdom are no longer applicable. But if, as has now come about, production in its capitalist form produces a far greater abundance of the means of existence and development than capitalist society can consume, because capitalist society keeps the great mass of the real producers artificially removed from the means of existence and development; if this society is forced, by the law of its own existence, continually to increase production already too great for it, and, therefore, periodically every ten years, reaches a point where it itself destroys a mass not only of products but of productive forces, what sense is there still left in the talk about the “struggle for existence”? The struggle for existence can then only consist in the producing class taking away the control of production and distribution from the class hitherto entrusted with it but now no longer capable of it; that, however, is the Socialist revolution.
Incidentally it is to be noted that the mere consideration of past history as a series of class struggles is enough to reveal all the superficiality of the conception of that same history as a slightly varied version of the “struggle for existence.” I should therefore never make that concession to these spurious natural scientists.
(5) For the same reason I should have given a different formulation to your statement, which is substantially quite correct,
“that the idea of solidarity, as a means of lightening the struggle, could ultimately expand to a point at which it embraces all humanity, counterposing it as a solidarised society of brothers to the rest of the world of minerals, vegetables and animals.”
(6) On the other hand I cannot agree with you that the war of every man against every man was the first phase of human development. In my opinion the social instinct was one of the most essential levers in the development of man from the ape. The first men must have lived gregariously and so far back as we can see we find that this was the case.
I have been interrupted afresh and take up these lines again to-day in order to send them to you. You will see that my remarks apply rather to the form, the method, of your attack than to its basis. I hope you will find them clear enough I have written them hurriedly and on re-reading them should like to change many words, but I am afraid of making the manuscript too illegible.
With cordial greetings,