Cretaceous and Tertiary Floras. 173 
lower Cretaceous period was one of rapid physiographic change. ALFRED 
RussELL WALLACE') follows BaLr’s hint as to the cause of the late appear- 
ance of the exogens. The suddenness of their appearance, he notes must be 
only apparent, being “due to unknown conditions which have prevented their 
preservation, or their discovery in earlier formations”?).. The erosion of mate- 
rial and its transport supposes rapidly moving water in which the stone and 
soil particles are being ground against each other and rounded off by con- 
stant attrition. Suppose a luxuriant vegetation of monocotyledons and dico- 
tyledons to be in process of evolution in such a region, what would be the 
influence of the physical conditions of a region undergoing base-leveling upon 
the preservation of plant remains? The preservation of plant remains woul 
be very precarious. The soil of the hillsides might have been either too dry, 
the flow of the streams too rapid, the grinding together of the rocks and 
gravel too great to permit of the fossilization of the vegetal remains, which 
require sedimentation of the material transported. 
Naturally this sedimentation did not take place in regions of rapid base- 
leveling, or in a region where lakes were and are absent, and we must, there- 
fore, appeal to these facts in order to explain the absence of fossil remains 
from large districts of the earth’s surface. According to DARWIN’), it is the 
“imperfection of the geological record”, which precludes the possibility of 
deciding definitely, as to the exact past vegetal covering of any particular 
geographic_ district. 
Evolutionary Principle. From the paleontologic evidence, it seems probable 
that a rapid and extensive development of new plant forms took place during 
the Cretaceous period. The theories of DE VRIES on the origin of species 
enable us to suggest the reasons for this evolution of new forms. DE VRIES 
has recourse to a periodicity of variation. Mutability occurs only at ‚eelaın 
Periods, and a species might continue existence. indefinitely without giving rise 
to new forms. If this periodieity of mutation is recognized as an evolutionary 
principle, we have a reasonable explanation for the sudden appearance ofso many 
new forms during the Cretaceous period for during this stage of the development 
ofthe vegetable kingdom, through causes yet unknown, the progenitors of the 
existing phanerogams were in a high state of mutability giving rise to new 
elementary species. DE VRIES emphasizes this as the method of the origin of 
Species, for in his words: new elementary species may appear suddenly without 
transition, or intermediate forms between them and their immediate ancestors; 
and therefore the new species were constant from the moment of their orıgin h 
1) WALLACE, Darwinism. Humboldt Library edition, p- 270- 
x WoopworTH, American Geologist. Oct. 189 Vp. 231. 
3) Darwın, Origi : i ter X. 
‚ Origin of Bpecen, heading Chapte Alıy; Vene, Jay 106. — 
This has not been the only method of the origin of new species, which may have ee 7 
n and by adaptation to environment, 
Or acelimatizati 
acclimatization. (HARSHBERGER.) 
