[83] It should be stated that the first observer to inaugurate the comparative method was that remarkable forerunner of modern palæontologists, Steno the Dane, who was for a while a professor at Padua. In 1669, in his treatise entitled _De Solido intra Solidum naturaliter contento_, which Lyell translates "On gems, crystals, and organic petrefactions inclosed within solid rocks," he showed, by dissecting a shark from the Mediterranean, that certain fossil teeth found in Tuscany were also those of some shark. "He had also compared the shells discovered in the Italian strata with living species, pointed out their resemblance, and traced the various gradations from shells merely calcined, or which had only lost their animal gluten, to those petrefactions in which there was a perfect substitution of stony matter" (Lyell's _Principles_, p. 25). About twenty years afterwards, the English philosopher Robert Hooke, in a discourse on earthquakes, written in 1688, but published posthumously in 1705, was aware that the fossil ammonites, nautili, and many other shells and fossil skeletons found in England, were of different species from any then known; but he doubted whether the species had become extinct, observing that the knowledge of naturalists of all the marine species, especially those inhabiting the deep sea, was very deficient. In some parts of his writings, however, he leans to the opinion that species had been lost. Some species, he observes with great sagacity, "are _peculiar to certain places_, and not to be found elsewhere." Turtles and such large ammonites as are found in Portland seem to have been the productions of hotter countries, and he thought that England once lay under the sea within the torrid zone (Lyell's _Principles_). Gesner the botanist, of Zurich, also published in 1758 an excellent treatise on petrefactions and the changes of the earth which they testify. He observed that some fossils, "such as ammonites, gryphites, belemnites, and other shells, are either of unknown species or found only in the Indian and other distant seas" (Lyell's _Principles_).