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 Coniferse is somewhat doubtful. 



THE CORDAITE^: 



Probably allied to the Conifers is a peculiar group of 

 fossils, the Cordaiteae. 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 



