It happened about this time that the Grand Duke of Tuscany visited Amsterdam. Accompanied by Thevenot, he examined the collections made by Swammerdam and his father, and was so struck by the wonderful dissections of insects that he offered 12,000 florins for the museum, on condition that its proprietor should accompany it to Florence, and take up his residence in the palace. But the young naturalist had been no much accustomed to roam about at will, that he could not relinquish his liberty, and therefore refused the offer. In 1669, he published his General History of Insects, which he dedicated to the senate of Amsterdam. The expense which he incurred in procuring specimens from all quarters, while no emoluments resulted from his labours, so displeased his father, that he earnestly urged him to relinquish his unprofitable pursuits, and engage in the practice of medicine. At length, finding him unwilling to follow his advice, he was obliged to threaten a total intermission of supplies; though by this time the ardent student had fallen into such a state of debility that he was totally unfit to undergo the fatigues of practice. He was, however, sensible of the propriety of the counsel which was administered to him, and retired to the country to recruit his strength; but he had scarcely arrived when he recommenced his studies, being wholly unable to resist the temptation offered by solitude and by the presence of the objects which invited his research. In the mean time, Thevenot, being made acquainted with these circumstances, urged him to return to France, generously offering him every thing necessary to enable him to follow the bent of his genius. His father, however, did not approve of this scheme, which was therefore relinquished; but the son did not the less continue to pursue his former occupations. In 1672, he published his Miraculum Naturae, seu Uteri Muliebris Fabrica. He soon afterwards entered upon an extensive examination of fishes, having reference chiefly to the pancreas. About this time he began to be impressed with religious ideas; becoming sensible of the vanity of human pursuits, as well as of the sinfulness of that inordinate ambition which impels men to aim at the highest place in the estimation of their fellows. He accordingly resolved to eradicate that base passion from his breast. In this state of mind he imbibed the mystical notions of the celebrated Antoinette Bourignon.