The collection you are looking for is *Marx and Engels on Malthus* ed. Ronald Meek (Lawrence and Wishart, 1953). Meek contributes a useful introduction and notes. The book was republished by Ramparts Press under the title *Marx and Engels on the Population Bomb* in 1971, with a foreword by Steve Weissman. You can find it in used bookstores. If you have difficulty let me know--I happen to have a spare copy.
The final section is called "Marx and Engels on Malthus and Darwinism". For the complex relation between Malthus and Darwin see S.J. Gould's essay "Darwin's Middle Road" in *The Panda's Thumb* (Norton, 1980) and the references cited therein.
There is also a good collection called *Marx and Engels on Ecology* ed. Howard Parsons (Greenwood, 1977).
Phil Gasper ptrg at sirius.com 415-522-1895