VEGETATION ERAS. 287 



fications of vegetation seem to be the primary causes of changes in the nature 

 and distribution of animal Hfe. This must have been, as it now is, the funda- 

 mental sequence throughout the earth's history. This basic relation of cause 

 and effect is supplemented by the fact that climatic changes are also due to 

 changes in the emanations of energy from the sun, but it is not seriously 

 disturbed thereby. It is also complicated by the reactions of each effect upon 

 its immediate or more remote cause, such as those of vegetation upon both 

 climate and topography. As a basic rule, however, it is as universal as it is 

 absolute. Deformation is the first cause, climatic change its effect, or a first 

 cause itself, and the cause of changes in the flora, and this effect upon the vege- 

 tation becomes the primary cause of changes of fauna, though the latter may 

 be acted upon directly by climatic or topographic change also. 



It has already been pointed out that vegetation occupies the critical position 

 in this sequence. The effects of deformation and climate converge upon it, 

 and are revealed not only in the vegetation changes, but also in the consequent 

 changes of animal life. Thus, vegetation is an epitome of physical causes and 

 biological effects. The evidences drawn from it have not only their proper 

 paleobotanic value, but they point in both directions, toward the originating 

 physical causes and the resulting faunal effects. Deformation looks in but 

 one direction, i. e., forward to its effects, and animal life can only point back- 

 ward to its causes. Thus, if the primary divisions of geological time are to be 

 based upon life relations, as is clearly indicated by the terms Eozoic, Paleozoic, 

 Mesozoic, andCfenozoic, it seems evident that the flora should take precedence 

 over the faima. This view is supported by the fact that geological effects 

 necessarily lag behind their causes. Deformation appears to have been a 

 slow process, often requiring a period or more for its completion. While the 

 associated climatic change doubtless accompanied it in some degree, the full 

 climatic effect must have followed the completion of the deformative processes. 

 Not until then would the full force of the modifying action of climate be felt 

 by the vegetation. Likewise, deformation and climate must have had some 

 effect, especially a destructive one, upon the animal life, but the primary 

 changes of dominance and distribution, i. e., of evolution, must have followed 

 a change in the vegetation dominance. In the case of animals, this lagging of 

 the effect behind the cause must have been especially marked. This fact has 

 been recognized by Chamberhn and Salisbury (1906 : 3 : 158, 174) : 



"The great physical changes which inaugurated the changes in life appear 

 to have taken place before the Arapahoe formation was deposited. Their 

 effects had distinctly modified plant hfe by the time the Denver beds were 

 deposited, but they appear to have had less effect upon the vertebrate life of 

 the west, perhaps because conditions were not yet favorable for the incoming 

 of mammaUan life from the regions where it originated. . . The intro- 

 duction of the dicotyledons and of the monocotyledons was the groundwork 

 for a profound evolution of herbivorous and frugivorous land animals, and 

 these in turn, for the development of the animals that prey upon them. A 

 zoological revolution, as extraordinary as the phytological one, might natu- 

 rally be anticipated, but it did not immediately follow as the record shows. 

 The zoological transformation may have been delayed because animals suited 

 to the proper evolution had not then come in contact with the new vegetable 

 realm; but with the opening Tertiary, the anticipated revolution appeared, 

 and swept forward with prodigious rapidity." 



