ON THE TRUE AIM OF PHYSIOLOGY. 827 



to find out, from the relations of living languages to the dead and 

 living ones, the pedigree of each idiom, just as the zoologist 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 hy ideas closely resembling the conception 

 of evolution. All are anxious to ascertain the past and future condi- 

 tions 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 dif- 

 ferent 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 ? differ- 

 ence of opinion will arise : Is an animal, which has little capability, 

 endowed with a simple organism, because it has as yet not been dif- 

 ferentiated, 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 con- 

 tinuing to develop, function has appeared much sooner than the per- 

 fected 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 indi- 

 vidual development, while here we are only speaking of phyletic evo- 

 lution. The inorganic sciences have similar questions to answer, 

 though with them the conception of sensitiveness to light has a differ- 

 ent meaning, and merely signifies receptibility, the capability of as- 



