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Rh 'Anthropogenie oder Entwicklungs-geschichte des Menschen,' 1874; 4th edition, 1891.

'Natuerliche Schoepfungs-Geschichte.' 2 vols. Berlin. 1st edition, 1868; 9th edition, 1898. This work has been translated into most European languages (the first edition in English, under the title 'Natural History of Creation' in 1873; the eighth in 1892).

'Monographie der Kalkschwaemme.' 3 vols. Berlin, 1872 (out of print). With the subtitle, 'An Attempt to solve analytically the Problem of the Origin of Species.' In this work, illustrated by sixty plates, he showed that the Calcispongia are individually so yielding, so adaptive to external influences, that it is practically impossible to break up the whole group into anything like satisfactory species or genera. According to predilection, we can distinguish either 1 genus with only 3 species, or 3, 21, 43 genera, with 21, 111, 181, or 289 species respectively.

In this work, in 1872, Haeckel established