WAKD.J DISCUSSION OF DIAGRAMS. 451 



iferous ; next, the second and still greater age of extensive marsh, 

 vast low plains cut by shallow estuaries or partially or wholly cut off 

 from the sea and forming brackish or fresh water deposits, which culmi- 

 nated in the Miocene ; then, the two intermediate periods of only less 

 favorable conditions occurring in the Brown Jura and the Oenomanian, 

 respectively; and, finally, the long intermediate ages of less favor- 

 able or wholly unfavorable conditions and the abrupt termination of 

 the entire period of plant deposition which accompanied the age of 

 mountain building towards the close of the Tertiary. The almost com- 

 plete absence of vegetable remains in the Trias, the lower Cretaceous, 

 and the Taronian of both continents points to the probable general sub- 

 sidence of land areas at those epochs, at least for such portions of the 

 earth's surface as have been explored by paleontologists. But the great 

 relative abundance of such life in the middle and again in the extreme 

 upper Cretaceous shows that those must have been great land areas at 

 all times — areas which are now either under the sea or belong to some 

 of the still scientifically " unexplored regions" of the globe. The proof 

 of this is made conclusive by the fact that new and higher types come 

 forth abruptly iu these floras which must have required ages of most 

 favorable conditions for their prior development. 



Discussion of Diagram 2^o. III. — This diagram is simply the application 

 of the rational scientific method to the incomplete facts afforded by the 

 present infantile state of the science of fossil plants. It does not pre ten d 

 to give the exact history of plant development, but only to constitute 

 a certain advance in this direction beyond what the fragmentary data 

 out of which it is constructed can alone furnish. For example, it is 

 certain that the earliest record discovered by man of the existence of 

 any type of vegetation cannot mark the absolute origin of that type, 

 and it is therefore necessary in every case to project the type down- 

 ward to an unknown distance. If the real facts could be indicated we 

 should see during these unrecorded periods the actual transformations 

 which must also be assumed to have taken place in each case before 

 the fully-developed type could appear. This we are unable to repre- 

 sent, and must merely indicate the early history of each type by its 

 downward projection to an assumed point of origin. Neither can it be 

 supposed that the great fluctuations shown in the diagram last con- 

 sidered are due altogether or chiefly to fluctuations in the degree of 

 vigor, territorial expansion, or local prominence of the given form of 

 vegetable life. They are the results of varying geological conditions or 

 of human good fortune, while the modifications in the forms themselves 

 take place slowly and at uniform rates either in the ascending or the 

 descending scale. Recognizing this law of uniformity, no fluctuations 

 in any homogeneous type have been admitted, but simply a more or 

 less regular development in each from its assumed point of origin to its 

 supposed period of maximum predominance, followed by an equally uni- 

 form decline to the present epoch when its condition relative to past 



