234 HEREDITY AND EVOLUTION IN PLANTS 



6. Coniferales (modern cone-bearing trees). 



5. Ginkgoales (primitive gymnosperms). 



4. Cordai tales (transitional conifers). 



3. Cycadales (true cycads). 



2. Cycadofilicales (cycad-like ferns). 



I. Filicales (true ferns). 



149. 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 

 hke 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. 



150. A Late Paleozoic Landscape. — The frontispiece 

 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 two feet. In the foreground, 

 at the left, are representatives of the Cycadofilicales, some 

 of them bushy, and others resembUng our modern tree ferns. 

 At the right are dense thickets of Calamites, the ancient 

 representatives of our modern scouring rushes {Equisetum). 

 In the background, at the left, are the unbranched, 

 Sigillarias, and the branched Lepidodendrons. The Cor- 

 daitales, which formed the Devonian forests, were not yet 

 extinct, but none is shown in the figure. Other forms, 



