On leaving Cambridge he resolved to go to the Continent, with the view of extending his knowledge of natural history, to which he had long been devoted. Accordingly, in the spring of 1663, accompanied by Mr Willughby, Mr Skippon, and Mr Bacon, his pupils, he crossed to Calais, and traversing the Low Countries, visited Germany, Italy, and several islands in the Mediterranean. In returning homewards he directed his way through Switzerland and France, and arrived in his own country in the spring of 1666, with a rich store of materials for the cultivation of his favourite science. He now occupied himself in reading the works that had been published during his absence; in reviewing and arranging Mr Willughby's collections; and in making a catalogue of such plants as were natives of the English soil. He was also employed during the winter in forming a table of plants and quadrupeds to illustrate the famous work of Dr Wilkins on a "Real or Universal Character." In the summer of 1667, he made a journey into the west, accompanied by his favourite pupil. While on this excursion the two friends described many natural objects, and in particular examined the Cornish mines, and the methods employed for smelting ores. His fame as a naturalist being now fully established, he was solicited to become a member of the Royal Society, which he accordingly entered in 1667. The remainder of this year he spent with his friends in Sussex and Warwickshire. In 1668, he made a journey into Yorkshire and Westmoreland, where he assiduously exerted himself in collecting plants and animals. The greater part of the winter was passed in Warwickshire, with Mr Willughby, who in the following spring engaged with him in a series of experiments on the ascent and descent of the sap in trees, the results of which were published in the Philosophical Transactions. Although botany seems to have been his principal study, his attention was by no means confined to it, for, like most naturalists of the time, he was a general collector. The materials which he had accumulated in the course of his journeys having now increased to a great extent, he began to digest his observations; commencing, rather oddly, with a set of proverbs, which he made ready for the press, although they were not published till 1672. In 1669, he also prepared his Catalogue of English Plants, which was printed in the following year.