THE EVOLUTION OF PLANTS 6ll 
related to the Ginkgo another living fossil, ranking next 
below the modem cone-bearing trees. We thus ascend 
from the ferns to the conifers by a series of transitional 
forms as follows (reading from the bottom, up) : 
6. Coniferales (modern cone-bearing trees). 
5. Ginkgoales (primitive gymnosperms) . 
4. Cordaitales (transitional conifers). 
3. Cycadales (true cycads). 
2. Cycadofilicales (cycad-like ferns). 
i. Filicales (true ferns). 
524. Relation of the Above Groups. It must not be 
inferred that the above groups were derived one from the 
other by descent from lower to higher. They should be 
interpreted rather as samples remaining to show us, not 
the steps, but the kinds of steps through which the plant 
kingdom has passed in developing the more highly organ- 
ized, modern cone-bearing trees from more primitive forms 
like the ferns. As stated above, it is doubtful if the actual 
transitional forms have been preserved, so that the entire 
history of development can probably never be written. 
525. A Late Paleozoic Landscape. Fig. 432 illustrates 
the kind of landscape that must have been common in the 
latter part of the Paleozoic era along sluggish streams in 
certain regions such as Texas and New Mexico. Of the 
primitive vertebrates then abounding, only a few larger 
types are shown. The dragon-flies of that time are 
known to have had a spread of wing amounting, in some 
cases, to as much as 2 feet. In the foreground, at the 
left, are representatives of the Cycadofilicales, some of 
them bushy, and others resembling our modern tree ferns. 
At the right are dense thickets of Calamites, the ancient 
representatives of our modern scouring rushes (Equisetum). 
