174 EVOLUTION OF PLANTS 
Gulf States is represented in Tertiary deposits by an ap- 
parently identical species, which at that time had a wide 
range over nearly the whole northern hemisphere. The 
genus Sequoia is another striking example of the sur- 
vival, in a limited range, of a once widely distributed 
type. At present the two species, S. sempervirens, the 
coast redwood of California, and S. gigantea, the giant 
tree of the Sierra Nevada, are all that remain of a 
genus once represented by numerous widely distributed 
species. 
Besides the genera still existing there are a number 
known only as fossils, some of which extend back to 
the Carboniferous. The exact relation of these extinct 
forms to the existing Conifer is somewhat doubtful. 
THE CORDAITES 
Probably allied to the Conifers is a peculiar group of 
fossils, the Cordaitee. These first appear in very old 
formations, some writers claiming that they are found 
in the Silurian rocks. The occurrence of seed-bearing 
plants in such ancient formations is, to say the least, 
unexpected. They are most abundant in the coal meas- 
ures and disappear soon after. The flowers have been 
preserved in some instances in an astonishingly perfect 
condition, even the pollen-spores with an enclosed struct-. 
ure supposed to be the gametophyte being recogniza- 
ble. The latter is much more highly developed than in 
any living seed plants, and this shows the primitive 
nature of these plants. In regard to the structure of 
the flowers, they show certain resemblances to both 
