decades take heart, for it appears that while there are inborn differences between men in this regard, imagination, observation, reasoning and production do not necessarily dim with age. Darwin's mind remained young and plastic to the end; his latest and one of his most characteristic works, "The Formation of Vegetable Mould through the Action of Earth Worms" was published at the age of seventy-two, after forty-four years of observation. It contained another and perhaps the most extreme demonstration of Lyell's principle that vast changes in nature are brought about by the slow operation of infinitesimal causes.
Three of Darwin's succeeding volumes are a filling out of the "Origin." "The Variation of Animals and Plants under Domestication" (2 vols., 1868) presents the entire fabric of the notes begun twenty-one years before on the transmutation of species. "The Descent of Man" (1871) was another logical outcome of the "Origin," yet it was only faintly adumbrated by a single allusion in that work to the fact that the transmutation of species necessarily led to the evolution of man. The "Descent" marks the third of the great dates in the history of thought, as the "Origin" marks the second, because it is the final step in the development of ideas which began with Copernicus in 1543. The world-wide sensation, the mighty storm produced by this bold climax of Darwin's work, is so fresh in the memory of all that a mere allusion suffices. The evolutionary or genetic basis for modern psychology as stated in "The Descent of Man" was given still more concrete form in Darwin's succeeding and most delightful volume "The Expression of the Emotions" (1872).
The knowledge of zoology and anatomy displayed in these four evolutionary volumes came from direct observation, vast and systematic reading and note-taking from the simple materials which Darwin could collect at Down. Always penetrating as these observations are, they are still, in my opinion, surpassed in beauty and ingenuity by hiswork on plants, published between 1862 and 1880. Here the principles of coadaptation of plants and insects in cross-and self-fertilization, in climbing plants and insectivorous plants, in forms of flowers, in movements of plants, are all brought forth in support of the theory of natural selection and the operation of unknown laws. Darwin's most precise observations and some of his most brilliant discoveries recorded in these volumes laid the foundations of modern experimental botany.
Of his method Darwin writes:
The only work which Darwin wrote deductively was his "Coral