[Professor Geddes is Professor of Botany at University College, Dundee. He has written "Chapters in Modern Botany" and many botanical articles and papers. Professor Thomson is Regius Professor of Natural History, Aberdeen University. His works include "The Study of Animal Life," "Outlines of Zoölogy," "The Natural History of the Year" and "The Science of Life." "The Evolution of Sex" was written jointly by both these authors. The article from which extracts follow was published in the _Contemporary Review_, 1895: the editor's permission to reprint is gratefully acknowledged. The Life of Louis Pasteur by M. Radot, 2 vols., were published by McClure, Phillips & Co., New York, 1901.] The course of Pasteur's scientific work is one of remarkably natural and logical sequence. As the veteran M. Chevreuil long ago said in the Academy of Sciences, "It is by first examining in their chronological order the researches of M. Pasteur, and then considering them as a whole, that we appreciate the rigor of his conclusions, and the perspicacity of a mind which, strong in the truths which it has already discovered, sweeps forward to the establishment of what is new." We shall therefore summarize the record of his greatest achievements. As was natural in a pupil of Dumas, Balard, and Delafosse, Pasteur's first important piece of work was chemical and crystallographic, and we may best understand its spirit by recalling the work of Delafosse's master in mineralogy, the Abbé Hauy, who is still remembered for that bold attempt to visualize the ultimate structure of the crystal, to penetrate the inmost secret of its architecture, which also reappears in another way in the work of Mendelejeff. Pasteur's puzzle concerned the tartrates and paratartrates of soda and ammonia. These two salts are alike in chemical composition, in crystalline form, in specific gravity, and so on, but they differ in behaviour. Thus, as Biot had shown, a solution of tartrate deflects the plane of polarized light passed through it, while a solution of the paratartrate does not. The salts are the same, yet they behave differently. A note to the Academy from the famous chemist Mitscherlich emphasized the entire similarity of the two salts, and this acted as an additional stimulus to Pasteur. He succeeded in distinguishing the minute facets which even Mitscherlich had missed; he proved that the paratartrate is a combination of a left-handed and a right-handed tartrate, and did much else which only the expert chemist could duly explain. Biot was first doubtful, then delighted; Arago, who had also busied himself with these matters, moved that Pasteur's paper be printed in the memoirs of the Academy, and Mitscherlich himself congratulated the young discoverer who had tripped him up.