We may add, that the law of scientific progress also makes it certain that no advance of science will ever deliver us from the necessity of using our faculties. The valuable part of every science is its theory--the mental part. Facts and fossils are of no value, except as being the materials for thought. No geologist would care much for a discovery of fossils in agreement with an established theory, but if the theory were still debated, then every discovery that tended to prove or disprove it, would be canvassed with intelligent interest. The pure sciences can grow, I am well aware, only by additions. But then they are simply instrumental. They are to the mixed sciences what arithmetic is to the ordinary business of life. Logarithms, algebra, the integral and differential calculuses, are simply easy ways of doing difficult sums. It is a great thing, no doubt, for science to perfect its instruments and processes, but scientific progress lies in the mixed sciences themselves, and these are constantly undergoing modification. The spectrum analysis is largely modifying the science of astronomy. Deep sea dredging, and other fresh means of information, have so modified geology, that no one holds now that similar strata are necessarily of the same date. A vast cretaceous formation is probably going on at this very day in the bed of the Atlantic. (Huxley, "Lay Sermons," p. 206.) The law, then, of scientific progress is constant modification; fresh facts are discovered, new theories started, old theories revived, existing theories altered, recast, newly shaped. Should a science become, practically, complete and perfect, scientific men would care for it no longer. The manufacturer and merchant would then seize upon it. In this way what was once a problem in the mind of the student, becomes an article of use, comfort, and enjoyment in our daily lives. Meanwhile, new sciences spring up, and old sciences take new shape, and, as a matter of fact, so large has become the scientific domain, that no one man can master it. Division of labour has become as necessary here as in the manual crafts. We are no longer encyclopædists, but each one must stick to his own page in the great book of learning.