"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."
in Engels' letter to Lavrov 12 November 1875
http://www.marxists.org/archive/marx/works/1875/letters/75_11_12.htm
"(1) Of the Darwinian theory I accept the theory of evolution but only take Darwin’s method of proof (struggle for life, natural selection)[3] 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."