most suggestive papers on the fossil vertebrate fauna of North America, and on classification especially of Amphibia and Reptiles.
Among works of a more general philosophical scope may be mentioned 'The Origin of the Fittest,' 1887, and his latest work, 'The Primary Factors of Organic Evolution,' 1896.
Albert von Koelliker, born in 1817, became Professor of Anatomy at Würzburg. His earlier studies and discoveries contributed considerably to the systematic development of the cell theory. In 1844 he observed the division and further multiplication of the original egg cell. Next year he showed the continuity between nerve cells and nerve fibres in the Vertebrata; later, that the non-striped or smooth muscular tissue is composed of cellular elements. He demonstrated that the Gregarinæ are unicellular creatures. In 1852 he went with his younger friend Gegenbaur to Messina, where he studied especially
