132 LAMARCK, HIS LIFE AND WORK 
Hence there have been changes of climate since 
these forms flourished, and, he adds, the intervals 
between these changes of climate were stationary 
periods, whose duration was practically without 
limit. He assigns a duration to these station- 
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 Tus- 
cany 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 petre- 
factions 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 fossilammonites, 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 fecultar 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 Prtzciples). 
Geikie estimates very highly Guettard’s labors in paleontology, say- 
ing that ‘‘ his descriptions and excellent drawings entitle him to rank 
as the first great leader of the paleontological school of France.” He 
published many long and elaborate memoirs containing brief de- 
scriptions, but without specific names, and figured some hundreds of 
fossil shells. He was the first to recognize trilobites (Illaenus) in the 
Silurian slates of Angers, in a memoir published in 1762. Some of 
his generic names, says Geikie, ‘‘ have passed into the languages of 
modern paleontology, and one of the genera of chalk sponges which 
he described has been named after him, Guetfardia. In his memoir 
‘*On the accidents that have befallen fossil shells compared with those 
which are found to happen to shells now living in the sea” (Trans. 
Acad. Roy. Sciences, 1765, pp. 189, 329, 399) he shows that the 
