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N his work on the Principles of Science, Jevons described with great clearness the logical phases of scientific theories and illustrated them by a wealth of instances drawn from the sciences of mathematics, physics, astronomy, and chemistry. While he accords to the theory of evolution an importance equal to that of any other theory, he says but little about its evidence or logical history. He practically leaves the biological sciences and geology untouched, except in the chapter on classification, where he says, in closing the subject: "Natural classification in the animal and vegetable kingdoms is a special problem, and . . . the particular methods and difficulties to which it gives rise are not those common to all cases of classification, as so many physicists have supposed. Genealogical resemblances are only a special case of resemblances in general."

The sciences of chemistry, physics, and astronomy, based as they are on mathematics, allow precise statement and accurate experiment. In geology and biology, on the other hand, the factors are so complex that these sciences take on the nature of historical sciences, with all the difficulties which such a statement implies. The difference can be easily illustrated. Certain perturbations of the planets indicated the presence of another one as yet unseen. The amount of the disturbances could be accurately determined. Adams and Le Verrier almost simultaneously predicted the presence of Neptune at a definite point in the heavens, and the prediction was verified by the immediate discovery of the planet. The human race must have appeared at a definite time in some definite part of the earth; but biological science lacks all the factors with which to parallel the case of Neptune by pointing out by a prediction the time and place of the appearance of the race. It knows that the event occurred, but must wait for accident to reveal the place and depend on the broadest generalizations to reveal even the relative age of man.

The utter lack of rigidity in the relations of living things puts quantitative statement almost entirely out of question; except that in some cases it is possible to work out a valuable system of averages. Individual beings can be measured, but the laws of biology can not be put in mathematical form; hence the lack of mathematical precision, without which the history of a science does not lend itself very easily to logical treatment. Evolution is pre-eminently a historical law, but its relations to the evidence