294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



histology elucidated. Petrified woods, especially of coniferous trees, 

 either silicified, calcified, or ferruginized, are common at many 

 geological horizons. More delicate plant fragments are only rarely 

 preserved in this way. Such remains where the replacement has 

 been by calcium carbonate characterize certain horizons of the 

 English Coal Measures, and similar "coal balls" have been discov- 

 ered more recently in Moravia, Westphalia, and Russia. Silicified 

 plant fossils are found at certain horizons in the Permian of St. 

 Etienne and Autun in France, and often the most delicate thin- 

 walled tissue, such as cambium and phloem, is well preserved. The 

 stumps of Cycadeoidea from the American Lower Cretaceous arc 

 silicified, usually denoting a burial in sandy deposits, and range in 

 perfection from those of the Lower Cretaceous of Maryland, which 

 are little more than sandstone casts, to some of the wonderfully pre- 

 served stumps of the Black Hills in Wyoming, in which even the 

 embryos in the ovules are completely preserved. 



GENERAL PRINCIPLES. 



The general principles which paleobotany has elucidated or illus- 

 trated can be but briefly indicated. First among these is the fact, 

 roughly traced in the subsequent discussion, that the history of 

 plants shows them to have had a gradual transformation from early 

 simplicity to later complexity and progressive differentiation of both 

 structures and habits in the successively higher groups, thus exempli- 

 fying the universal principle of evolution. The original scene of 

 plant activity was in the water. Gradually the main theater of op- 

 erations was transferred to the land, and the distinction between 

 vascular and cellular plants in this respect is analogous to the dis- 

 tinction between vertebrate and invertebrate animals. 



Each successive group of plants that appeared upon the scene illus- 

 trates a second great principle — that of adaptive radiation; that is, 

 by progressive modifications (the mutations of Waagen), groups be- 

 came dominant, such as the Lepidophytes, Arthrophytes, and Pteri- 

 dosperms of the Carboniferous, the Cycadophytes of the Mesozoic, 

 or the Angiosperms of the Cenozoic, their members became adapted 

 for a great variety of environments and tended to occupy all of the 

 available situations on the land or in cases became secondarily 

 adapted for an aquatic existence, like the water ferns or the various 

 aquatic angiosperms. Paleobotany shows one group after another 

 thus dominating, becoming specialized during the process and then 

 waning or becoming entirely extinct. The accompanying diagram 

 illustrates the successive dominance of different plant types and the 

 increasing complexity of the vegetable kingdom as a whole. 



Another principle is illustrated by the progressive loss of plasticity 

 as organisms or organs became complex and specialized. It was the 



