210 The American Geologist. October, 1894 
deeply dissected plains. The contrast between the old and 
the new views when carried to their consequences in the or- 
ganic realm is so striking that it is thought the following 
discussion will be acceptable both to those who do and to 
those who do not grant the competency or opportunity of me- 
teoric denudation to account for these baseleveled areas or 
peneplains. 
Synopsis. 
The object of the paper is to discuss the effect, on organ- 
isms, of the changes in physical geography which arise in the 
process of baseleveling. Part I presents a cursory review of 
the history of opinion regarding land erosion, including the 
speculations of some ancient writers. Part II deals with the 
general effects of river changes on organisms, and aims to set 
forth the manner in which these changes have influenced the 
dispersion of freshwater species. Part III has to do with the 
Jura-Cretaceous peneplain of North America in particular, 
with the determination of the time of development of the 
peneplain, and with the development of the fauna and flora 
of the Mesozoic era which lived upon its surface. It is in- 
ferred that the production of this lowland was favorable to 
the reptilian class, and that it was a factor in securing to 
them the dominance which they enjoyed even after the mam- 
malia had made their appearance. In conclusion, baseleveled 
lands or peneplains and the periods of their elaboration are 
compared with glacial epochs and marine invasion in their 
effect as the environment of organic life. 
PART I. HISTORICAL SKETCH. 
In reviewing the rapid strides which the new school of to- 
pographic geologists have made in interpreting the history of 
erosion alone, we are in danger of overlooking the fact that 
the idea of a land reduced to an essential plain by erosion is 
far older than anything like a demonstration of the existence 
of a surface of this nature. In a work entitled, "The Scrip- 
ture Theory of the Earth,"* published in London in 1773, the 
anonymous author, after instancing numerous cases of the fall 
of mountains, erosion by snowslides, ice. rains and rivers, pre- 
Bents the following conclusion: 
*The lull till.' reads: "The Scripture Theory of the Earth throughout 
all its revolutions and all periods of its existence, from the Creation in 
tin' final Renovation of all things." Sec pp. :!i>.")-(i. 
