His work on the development of sea urchin eggs was fundamental and lasting; his work on evolution was not. He was opposed to 'chance', as he thought was Charles Darwin's theory. He wrote: "Das Werden der Organismen, eine Widerlegung der Darwinschen Zufallslehre" (Jena, 1916) (translation: "The Origin of Organisms: a refutation of Darwin's theory of chance").
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