like the horse, and those with a split hoof, like the ox and goat. Those with split hoofs he divided again according as they chewed the cud, like the ox, or did not, like the pig. Then came the animals whose hoofs are split into many parts, as the hippopotamus and rhinoceros; then those which have nails only in place of toes, as the elephant; then those which have toes but no separation between the fourth and fifth toes, as the cat, dog, and mole; and lastly, those which have the fifth finger, or toe, quite separate, as the monkeys. After this he divided them more fully, by their teeth, and thus made a very fair classification of quadrupeds.

The book upon Birds, which comes next in order, had already been published by Ray in 1677, four years after Willughby's death. In it birds were divided first into landbirds and water-birds, and then were classified by the shape of their beak and claws, and according as they fed upon flesh like the vulture, or upon fruit and seeds like the parrot. The water-birds were also divided into those which were longlegged, as the flamingo, or short-legged, as the duck, and according as the web between their toes was more or less complete.

The History of Fishes' is given as the joint work of Ray and Willughby; the groups into which they divided them are nearly the same as those now used, but they are too difficult to explain here.

The History of Insects' was Ray's work, and was published by friends after his death, in the same way as he had published Willughby's. He divided insects into-first, those which undergo metamorphosis (that is, turn from the caterpillar into the moth), as the silkworm, and all moths and butterflies; and second, those which do not change their form; and then he sub-divided them according to the number of their feet, the shape of their wings, and many other characters.

CHI. XVII. RAY'S CLASSIFICATION OF PLANTS.

145

But Ray's greatest work was upon Plants, which he classified much more perfectly than Casalpinus had done. He divided them first into imperfect plants, or those whose flowers are invisible, as mosses and mushrooms; and perfect plants, or those having visible flowers. The perfect plants he divided into two classes-first, the dicotyledons, or those whose seeds open out into two seed-leaves, like the wallflower or the bean, in which last you can see the two cotyledons very clearly if you take off the outer skin; and secondly, the monocotyledons, or those whose seeds have only one large seed-leaf, like a grain of wheat. The dicotyledons he again divided into those having simple flowers, like the buttercup, and those whose flowers are compound, like the daisy; for if you pick a daisy to pieces you will find that the centre is made up of a number of little flowers, each of them perfect in itself. It will have its own green calyx and coloured corolla, and its own stamens and seed-vessel; therefore each daisy is a branch of little flowerets, or a compound flower. Ray went on next to class the simple flowers according to the number of seeds they bore, and the way in which the seeds were arranged in the seed-vessel. In this way he made a rough but complete classification of all the known. plants. Linnæus, the great botanist of the eighteenth century, adopted many of Ray's divisions, which had meanwhile been made more perfect by Joseph Tournefort, a Frenchman, born at Aix, in Provence, in 1656.

Ray outlived his friend Willughby more than thirty years, and died in 1705 at the age of seventy-seven. His death brings us to the end of the Natural History of the seventeenth century, so far as we have been able to notice it. But I cannot too often remind you that these four men, Malpighi, Grew, Ray, and Willughby, are merely a few among an

immense number of observers in the same line of study. I have picked out those whose work you could best understand, and whose names ought to be known to you; but I could have selected not four but forty others who ought to have been mentioned, if my book and your knowledge had been greater. We must be content to catch here and there a glimpse of the advance that was being made, always remembering that an almost inexhaustible store of further information remains behind when we have opportunity to seek for it.

Chief Works consulted.—Cuvier, Hist. des Sciences Naturelles ;' Carpenter's Physiology;' Sprengel, 'Histoire de la Médecine;' Whewell's History of Inductive Sciences;' Carpenter, 'On the Microscope;' 'Memorial of John Ray,' E. Lankester, 1846; 'Life of Ray and Willughby,' Naturalists' Library, vol. xxxv.; Lardner's 'Encyclopædia'-Classification of Animals.

CH. XVIII.

SIR ISAAC NEWTON.

147

CHAPTER XVIII.

SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 1642, Birth of Newton-His Education-1666, His three great Discoveries first occur to him-Method of Fluxions and Differential Calculus-First thought of the Theory of Gravitation-Failure of his Results in consequence of the Faulty Measurement of the size of the Earth-1682, Hears of Picart's new Measurement-Works out the result correctly, and proves the Theory of Gravitation-Explanation of this Theory-Establishes the Law that Attraction varies inversely as the squares of the distance-1687, Publishes the 'Principia'-Some of the Problems dealt with in this Work.

Newton, 1642.-We must now leave the living creation to return to physical science, for, during all those years with which we have been occupied since the time of Galileo and Kepler, a boy had been growing up into manhood, who was to become one of the greatest men of science that England has ever known. In 1642, the same year in which Galileo died, a child was born at Woolsthorpe, near Grantham in Lincolnshire, who was so tiny that his mother said 'she could put him into a quart mug.' This tiny delicate baby was to become the great philosopher Newton.

We hear of him that he was at first very idle and inattentive at school, but, having been one day passed in the class by. one of his schoolfellows, he determined to regain his place, and soon succeeded in rising to the head of them all. In his play hours, when the other boys were romping, he amused himself by making little mechanical toys, such as a

vances.

water clock, a mill turned by a mouse, a carriage moved by the person who sat in it, and many other ingenious contriWhen he was fifteen his mother sent for him home to manage the farm which belonged to their estate; but it was soon clear that he was of no use as a farmer, for though he tried hard to do his work, his mind was not in it, and he was only happy when he could settle down under a hedge with his book to study some difficult problem. At last one of his uncles, seeing how bent the boy was upon study, persuaded his mother to send him back to school and to college, where he soon passed all his companions in mathematics, and became a Fellow of Trinity College, Cambridge, in 1667. But even before this, in the year 1666, his busy mind had already begun to work out the three greatest discoveries of his life. In that year he invented the remarkable mathematical process called the 'Method of Fluxions,' which is almost the same as that now called the 'Differential Calculus,' invented about the same time by Leibnitz, a great German mathematician. In that year he also made the discoveries about Light and Colour, which we shall speak of by-and-by; and again in that year he first thought out the great Theory of Gravitation, which we must now consider.

Theory of Gravitation, 1666.-In the course of his astronomical studies, Newton had come across a problem which he could not solve. The problem was this. Why does the moon always move round the earth, and the planets round the sun? The natural thing is for a body to go straight on. If you roll a marble along the floor it moves on in a straight line, and if it were not stopped by the air and the floor, it would roll on for ever. Why, then, should the bodies in the sky go round and round, and not straight forward?

While Newton was still pondering over this question, the