Although the theory of evolution is now accepted as an established fact by almost all naturalists, this general agree ment does not extend beyond the point of believing that the present organic world has arisen by descent from simpler forms. The application of the theory to concrete cases is beset with grave difficulties, and gives rise to the most divergent views. The uninitiated reader who takes up a monograph upon some animal group may well be surprised to see the minuteness and accuracy with which the genealogy of the series is set forth, and the relationships of the various genera and species mar shalled in orderly array. But another treatise upon the same animals will contain an equally complete family tree, which contradicts the first in almost every particular. To some extent this almost hopeless divergence is inherent in the very nature of the problem, it being largely a question of the value of evi dence and of the balance of probabilities, as to which men must be expected to difler; but there is another potent cause of the discrepancy. When the contradictory phylogenetic schemes are analyzed, it is frequently found to be the case that the discus sion rests upon certain postulates and assumptions, sometimes.

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