 My doubts as to the validity of Darwin’s theory were for a long time not confined to this point alone: the assumption of the existence of preformed constituents of all parts of the body seemed to me far too easy a solution of the difficult, besides entailing an impossibility in the shape of an absolutely inconceivable aggregation of primary constituents. I therefore endeavoured to see if it were not possible to imagine that the germ-plasm, though of complex structure, was not composed of such an immense number of particles, and that its further complication arose subsequently in the course of development. In other words, what I sought was a substance from which the whole organism might arise by epigenesis, and not by evolution. After repeated attempts in which I more than once imagined myself successful, but all of which broke down when further tested by facts. I finally became convinced that an epigenetic development is an impossibility. Moreover, I found an actual proof of the reality of evolution, which … is so simple that I can scarcely understand how it was possible that it should have escaped my notice so long. (Weismann 1892 [1893: xiii–xiv])