SCIENTIFIC THOUGHT IN THE NINETEENTH CENTURY. 399 



tion of his experiments, "Any tiling- which any insulated body or sys- 

 tem of bodies can continue to furnish without limitation can not 

 possibly be a material substance," the fate of the supposed imponder- 

 able fluid heat was sealed; but it was not till near the middle of our 

 century that Joule completed the work of Rumford by the determina- 

 tion of the mechanical equivalent of heat. About the same time 

 Foucault's measurement of the velocity of light in air and in water 

 aft'orded conclusive proof of the undulatory theory of light. In these 

 great discoveries was laid the strong foundation for the magnificent 

 generalization of the conservation of energy— a generalization which 

 the sagacious intuition of Mayer and Carpenter and Le Conte at once 

 extended beyond the realm of inorganic nature to the more subtile 

 processes of vegetable and animal life. In this connection I may be 

 permitted to refer to .the work of some of my colleagues with the 

 Atwater-Rosa calorimeter, which has given more complete experi- 

 mental proof than had previously been given of the conservation of 

 energy in the human body. 



But by far the greatest of the intellectual achievements of our age 

 has been the development of the idea of the unity of process pervading 

 the whole history of nature. The word which sums up in itself the 

 expression of the most characteristic and fruitful intellectual life of our 

 age is the word "evolution." The latter half of our century has been 

 so dominated by that idea in all its thinking that it may well be named 

 the Age of Evolution. We may give as the date of the beginning of 

 the new epoch the year 1858; and the Wittenberg theses of the intel- 

 lectual reformation of our time were the twin papers of Darwin and 

 Wallace, wherein was promulgated the theory of natural selection. 



And 3"et, of course, the idea of evolution was not new wIkmi these 

 papers were presented to the Linna\an Society. Consciously or uncon- 

 sciously, the aim of science at all times must have l)een to bring 

 events that seemed isolated into a continuous development. To (wclude 

 the idea of evolution from any class of phenomena is to exclude that 

 class of phenomena from the realm of science. In the former half 

 of our century evolutionary conceptions of the history of inorganic 

 nature had l)ecome pretty well established. The- nebular hypothesis 

 was ol)viously a theory of planetary evolution. The Lyellian geology, 

 which tt)ok the place of the catastrophism of the last century, was 

 the conception of evolution applied to the physical history of the earth. 



Nor had there been wanting anticipations of evolution svilhin the 

 realm of biology. The author of that sublime Hel)rew psalm of cre- 

 ation, preserved to us as the first chapter of Genesis, was in his way 

 a good deal of an evolutionist. "Let the earth bring forth," "let the 

 '. waters bring forth," are words that point to a process of growth 

 rather than to a process of manufacture in the origination of living 

 beings. In crude and vague forms the idea of evolution was held by 



