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260 a technical, almost domestic, problem. Any day that mystery may be solved. he discoveries of the last twenty-five years enable us for the first time to discuss these questions intelligently and on a basis of fact. That synthesis will follow on an analysis we do not and cannot doubt."

With this distinction between fact, course and causes clearly in mind, the significance of Darwin's work in the history of biological thought can be understood. Darwin's accomplishment was two-fold. In the first place he established organic evolution as the only reasonable explanation of the past history of living things. Secondly, he offered, in natural selection, what then appeared an adequate explanation for the origin of species, and, hence, for the causes of evolution. Darwin's evolutionary argument in his "Origin of Species" was that one species could give rise to another "by means" as he believed, "of natural selection or the preservation of favored races in the struggle for life." If one species could be shown to give rise to another, the same process could be continued. No limit could be set. The types thus produced could depart indefinitely from the parent form. Once the mutability of species be admitted the only reasonable conclusion is that evolution has taken place. His argument was supported by an immense collection of facts along observational and experimental lines. The total result was overwhelming, coming as it did, more than 100 years after setting forth of transmutation, and its repeated rejection by the main body of naturalists. Evolution was accepted so quickly by scientists that the world was startled. This sudden conversion gave rise to the impression, even among scientific workers, that no serious contribution to evolutionary theory had been made before the work of Darwin. Such an impression does not represent the facts and it does grave injustice to the pioneer thinkers of the eighteenth century, to whom we have alluded.

Darwin's second accomplishment, natural selection, was accepted by science as a causo-mechanical explanation of evolutionary change. The cogent statement and the simplicity of the principle of selection were of great importance for its aceeptance as the cause of evolution, along with the broader theory of evolution as the historic fact. Extended exposition of the selection process will not be attempted. It may be found in numerous elementary books, and in the early chapters of the "Origin of Species." The tabulation known as Wallace's chart, which is an admirable outline of the argument, may be cited in this connection:

Proved Facts—(a) Rapid increase of numbers; (b) total numbers stationary; (c) struggle for existence; (d) variation and heredity; (e) survival of the fittest; (f) change of environment.

Consequences—Struggle for existence; survival of the fittest (natural selection); structural modifications.

The importance of Darwin's work in the history of scientific thought is that it convinced science of the truth of organic evolution and proposed a then plausible theory of evolutionary causation. Since Darwin's time evolution as the historic fact has received confirmation on every hand. It is now regarded by competent scientists as the only rational explanation of an overwhelming mass of facts. Its strength lies in the extent to which it gives meaning to so many phenomena that would be meaningless without such an hypothesis.

But the case of natural selection is far different. Of recent years this theory of the causes of evolution has suffered a decline. No other hypothesis, however, has completely displaced it. It remains the most satisfactory explanation of the origin of adaptations, although its all-sufficiency is no longer accepted. The initial step in evolution is the appearance of individual variations which are perpetuated by heredity, rather than the selection of variations after