The historical development of a field of human knowledge progresses like the solution of a jig-saw puzzle, the full extent of which is completely unknown. What begins as an ocean may become only a lake; what starts as a grove of trees may develop into a forest. As study advances through the decades, the situation is repeatedly surveyed and the interpretation of the whole is modified to accord with the added information. For these reasons, conceptions and generalizations periodically undergo alteration, (...) as effectively stated and discussed by Cooper. Universal conformity in such conceptions is of course hardly to be expected in matters that deal largely with speculation rather than with simple experimental data. (shrink)