If we suppose the _Micrographia_ of Hooke to be greatly enlarged, so as to become, instead of the passing occupation of a man busied with a hundred other interests, the main pursuit of a long and laborious life, we shall get a rough notion of the microscopic revelations of Leeuwenhoek. His researches were desultory, though not quite so desultory as Hooke's; he must have often spent months upon an investigation which Hooke would have dismissed in as many weeks. Both travelled over the whole realm of nature, and lacked that concentration which made the work of Swammerdam so productive and so lasting. [Illustration: ANTONY VAN LEEUWENHOEK. From the portrait by Verkolje, prefixed to the _Epistolæ ad Soc. Reg. Angl._, Leyden, 1719.] Leeuwenhoek worked with simple lenses, ground and mounted by his own hands. It was easy to make lenses of high magnifying power, but hard to correct their optical defects, to bring a sufficiently strong light to bear upon the object, and to focus the lens. When he wished to send out his preparations for examination by others, he found it best to fix the objects in the focus, and to provide each with a separate lens. With such microscopes he managed to study and figure very minute objects, such as blood-corpuscles, spermatozoa, and bacteria. The spermatozoa were brought under his notice by a young Dutch physician named Hamm; but it was Leeuwenhoek's account of them, and his daring theory of their physiological _rôle_, which gave them such celebrity. To Leeuwenhoek we owe the first discovery of the rotifers, the infusoria, Hydra, the yeast-cell, the bacteria, and the generation of aphids without male parents. The tradition of the minute anatomists has never been lost, though we shall be unable to pursue it in these pages. Lyonet (see p. 61) even surpassed Swammerdam in the elaborate finish of some of his insect-dissections. Early Notions about the Nature of Fossils. Throughout the sixteenth century naturalists held animated debates about the shells which are found far from the sea, and even on the top of high hills. Had they ever formed part of living animals or not? Such a question could hardly have been seriously discussed among simple-minded people; but the learned men of the sixteenth century were rarely simple-minded. They had been trained to argue, and argument could make it plausible that such shapes as these were generated by fermentation or by the influence of the stars. So prevalent were these doctrines that it entitles any early philosopher to the respect of later generations that he should have taken shells, bones, and teeth to be evidences of animal life. In this singular roll of honour we find the names of Cesalpini, Palissy, Scilla, Stenson, Hooke, and Woodward.