Though there are many histories of the different branches of science and of science itself, a general survey of the progress of natural knowledge in its relation to other fields of human thought seems not previously to have been written. This attempt to supply the need does not pretend to give a detailed account of the growth of the various sciences. It is evident that almost every section could be expanded into a volume, and each chapter heading could appropriately become the title for an exhaustive treatise. We have deliberately constrained ourselves to produce an outline, rather than the fuller study towards which we were frequently tempted. We have set out to tell in plain language the story of the separation of science from the association with theology and philosophy by which, of necessity, its origins were beset. We have tried to recount the marvelous extension of natural knowledge, following on the liberation of science; to trace and to justify the rise of a mechanical theory of life, and to explain the recent tendency once more to recognize its limitations.