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.