The known universe thus becomes still more one. Later researches again, especially those of Maxwell, tend to the identification of light and heat with electricity, and in the last stage matter as a whole seems to be swallowed up in motion. It is found that similar equations will express all kinds of motion; that all are really various forms of the motion of something which the mind postulates as the thing in motion; we have in each case to deal with wave-movements of different length. The broad change, therefore, which has taken place since the mechanics of Newton is the advance from the consideration of masses to that of molecules of smaller and smaller size, and the truth of the former is not thereby invalidated. Newton, Descartes, Fresnel, Carnot, Joule, Mayer, Faraday, Helmholtz, Maxwell appear as one great succession of unifiers. All have been engaged in the same work of consolidating thought at the same time that they extended it. Their conceptions of force, mass, matter, ether, atom, molecule have provisional validity as the imagined objective substratum of our experience, and the fact that we analyse these conceptions still further and sometimes discard them, does not in any way invalidate the law or general form in which they have enabled us to sum up our experience and predict the future. But now we turn to the other side. In spite of the continued progress noted on the mechanical side, it is true that the predominant scientific interest changed in the nineteenth century from mechanics to biology, from matter to life, from Newton to Darwin. Darwin was born in 1809, the year in which Lamarck, who invented the term biology, published his _Philosophie Zoologique_. The _Origin of Species_ appeared in 1858 after the conservation of energy had been established, and the range and influence of evolutionary biology have grown ever since.