We have yet to add the investigations in regard to the history of evolution of the single organisms, as well as those in comparative anatomy, which in former centuries were begun by scientists like Swammerdam and Boerhave and carried more nearly to completion by K. E. von Baer, Carus, and others. In reducing all the tissues of plants and animals to one cell, and tracing back also their individual developments to the first differentiation of the simplest cell, they followed out the unity of the plan of the organic kingdoms--which hitherto had been maintained only ideally and proclaimed as a philosophic postulate--farther and deeper into the sphere of empiric reality. We must mention, moreover, the great palaeontological discoveries which, from the first foraminifera of the Cambrian formations up to the historical period of man, showed a great progressive scale in the appearance of the organisms and a very wide relationship between this scale and the natural systems of botany and zooelogy; and, finally, the principles of geology, which, under the leadership of Sir Charles Lyell, starting from the idea of an identity of the powers which were active in former times with those of the present, attempted to explain the most violent of the changes in the earth's crust in former times by causes active to-day. This often explains prodigious effects--such as the elevation and settling of entire mountains {37} and continents--by the constant and related action of the slightest causes and most gradual steps; it opens the perspective into vast epochs of long and numerous geological periods; and sometimes, where scientists like Cuvier and Agassiz have supposed the most complete cataclysms and the most universal revolutions of the globe, there prove to have been only gradual changes with revolutions very partially and locally limited. Finally, if we take into consideration the grand discoveries which strikingly illustrate the connection in extent and quality between the universe and all its agencies and powers--such as Robert von Mayer's discovery of the conservation of force and of the mechanical equivalent of heat, or the spectrum analysis and the information which it gives us by ever-increasing evidences of the identity of the cosmic and telluric substances--we may venture to say that the scientific and intellectual ground was well prepared for a theory which takes the origin of organisms into this common relationship of the essential unity and development of the universe.