THE EVOLUTION OF PLANTS 6ll 



related to the Ginkgo — another living fossil, ranking next 

 below the modern 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 (cy cad-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), 



