to find out, from the relations of living languages to the dead and living ones, the pedigree of each idiom, just as the zoölogist is trying to discover the origin of present animals from their relations to the fossils and to each other. Everywhere thinking naturalists are tacitly or confessedly influenced by ideas closely resembling the conception of evolution. All are anxious to ascertain the past and future conditions from existing ones, which is the very essence of evolution. The nature of the transition of one condition into another, its laws, its velocity, its consequences, all these differ in the special departments, not the general fact of the change of condition itself. When, at the same time, the sunbeams warm the human body and delight the eye with glowing colors, when they dye the young plant green and the sensitive glass plate violet; when they move the radiometer, make the telephone resound, allure millions of tiny winged insects into the air, banish millions of other beings which shun the light under ground and to the depths of the waters, close night-blowing flowers while opening others bedecked with dew at the dawn of day—it is ever the one immense sun who, with the same life-giving and life-destroying rays, is working such different wonders. In like manner it is, with all natural sciences, the evolution theory which is producing the different conceptions of Nature. Every one is anxious to comprehend the true sequence of phenomena. All stand firmly on the impregnable basis of the "principle of sufficient reason," which states that every change must be preceded by a change and be followed by another. But when we ask. Which was the first, which the following? difference of opinion will arise: Is an animal, which has little capability, endowed with a simple organism, because it has as yet not been differentiated, or because it has retrograded in its descent from more highly developed beings? Such questions sometimes are extremely difficult to answer, and in these cases it devolves upon physiology to decide or at least to pronounce its weighty judgments. For whenever an organ retrogrades, function has disappeared much sooner than the rudimentary organ; but when, on the other hand, an organ is continuing to develop, function has appeared much sooner than the perfected organ. It would then have to be ascertained whether the organ in question is still possessed of a function, or has already lost it. If, for example, a perfected eye shows little or no sensitiveness to light, it must be in a state of retrogression; if it is very imperfect, and yet extremely sensitive to light, it is developing, while an eye of very simple structure which is not sensitive to light can only have become so through retrogression.
Embryonic eyes, of course, are in a state of progressive but individual development, while here we are only speaking of phyletic evolution. The inorganic sciences have similar questions to answer, though with them the conception of sensitiveness to light has a different meaning, and merely signifies receptibility, the capability of as-
