Perhaps a chapter in modern science may illustrate the intellectual relation of these great men.  The revolution produced by Copernicus in the doctrine of the heavens has often been likened to the revolution which the Darwinian theory produced in the views held by biologists as to life on this earth.  The Darwinian theory did not at first command universal assent even among those naturalists whose lives had been devoted with the greatest success to the study of organisms.  Take, for instance, that great naturalist, Professor Owen, by whose labours vast extension has been given to our knowledge of the fossil animals which dwelt on the earth in past ages.  Now, though Owens researches were intimately connected with the great labours of Darwin, and afforded the latter material for his epoch-making generalization, yet Owen deliberately refused to accept the new doctrines.  Like Tycho, he kept on rigidly accumulating his facts under the influence of a set of ideas as to the origin of living forms which are now universally admitted to be erroneous.  If, therefore, we liken Darwin to Copernicus, and Owen to Tycho, we may liken the biologists of the present day to Kepler, who interpreted the results of accurate observation upon sound theoretical principles.

In reading the works of Kepler in the light of our modern knowledge we are often struck by the extent to which his perception of the sublimest truths in nature was associated with the most extravagant errors and absurdities.  But, of course, it must be remembered that he wrote in an age in which even the rudiments of science, as we now understand it, were almost entirely unknown.

It may well be doubted whether any joy experienced by mortals is more genuine than that which rewards the successful searcher after natural truths.  Every science-worker, be his efforts ever so humble, will be able to sympathise with the enthusiastic delight of Kepler when at last, after years of toil, the glorious light broke forth, and that which he considered to be the greatest of his astonishing laws first dawned upon him.  Kepler rightly judged that the number of days which a planet required to perform its voyage round the sun must be connected in some manner with the distance from the planet to the sun; that is to say, with the radius of the planet’s orbit, inasmuch as we may for our present object regard the planet’s orbit as circular.

Here, again, in his search for the unknown law, Kepler had no accurate dynamical principles to guide his steps.  Of course, we now know not only what the connection between the planet’s distance and the planet’s periodic time actually is, but we also know that it is a necessary consequence of the law of universal gravitation.  Kepler, it is true, was not without certain surmises on the subject, but they were of the most fanciful description.  His notions of the planets, accurate as they were in certain important respects, were mixed up with