Marxism and Darwinism

Marx and Engels express enthusiasm for Darwin's works, but there are reasons to curb it, as well.  On the one hand, he, too, is what they call a 'Materialist', but, on the other, his various Species-Principles diverge from theirs, to lesser and greater degrees.  For example, Evolution does not quite coincide with Dialectical Materialism, since Mutation is not mere Antithesis.  Furthermore, Marxism offers no argument as to why Socialism is a better means of Adaptation than is Capitalism, or any other system.  Finally, and perhaps most hostile to Socialism, is the Survival of the Fittest principle, which can easily be construed as Capitalistic, as 'Social Darwinists' have exploited.  At minimum, aligning the two doctrines requires a demonstration that a society based on cooperation inherently has greater adaptive and survival potentials than does one based on competition.