Long before this, however, at the age of twenty-eight, Darwin had begun his career as a Darwinian. In July, 1837, he began his notes on the transmutation of species, based on purely Baconian principles, on the rigid collection of facts which would bear in any way on the variations of animals and plants under domestication and in nature. Rare as was his reasoning power, his powers of observation were of a still more unique order. He persistently and doggedly followed every clue; he noticed little things which escaped others; he always noted exceptions and at once jotted down facts opposed to his theories. On the voyage the marvelous adaptations of animals and plants had been his greatest puzzle. Fifteen months later, in October, 1838, in reading the work of Malthus, on "Population," there flashed across his mind the three-fold clue of the struggle for existence, of constant variability, and of the selection of variations which happen to be adaptive.
The three memorable features of-Darwin's greatest work, "The Origin of Species," are, that he was twenty-one years in preparing it, that, although by 1844 he was a strongly convinced evolutionist and natural selectionist, he kept on with his observations for fifteen years, and the volume even then would have been still longer postponed but for a wonderful coincidence, which constitutes the third and not the least memorable feature. This coincidence was that Wallace had also become an evolutionist and had also discovered the principle of natural selection through the reading of the same essay of Malthus. It is further remarkable that of all persons Wallace selected Darwin as the one to whom to send his paper. It was then through the persuasion of the great botanist Hooker, who had known Darwin's views for thirteen years that these independent discoveries were published jointly on July 1, 1858. All the finest points of Darwin's personal character were displayed at this time; in fact, the entire Darwin-Wallace history up to and including Wallace's noble and self-depreciatory tribute to Darwin on July 1, of last summer, is one of the brightest chapters in the history of science. Wallace himself pointed out the very important distinction that while the theories contained in the two papers published fifty years ago were nearly identical, Wallace had deliberated only three days after coming across the passage in Malthus, while Darwin had deliberated for fifteen years. He modestly declared that the respective credit should be in the ratio of fifteen years to three days.
Several months past the age of fifty Darwin published his epoch-making work (November, 1859), and despite ill health, between fifty and seventy-three, he produced the nine great volumes which expand and illustrate the views expressed in "The Origin of Species."
A parallel to this remarkable late productiveness is that of Kant, who also put forth his greatest work after fifty. Let those past the five