Baseleveling and Organic Evolution. — Woodworth. '2'29 
in the Jurassic and probably existed in considerable numbers 
then, yet not one has been found in the ( Jretaceous," to which 
he adds, "It is probable, therefore, that during- the Cretaceous 
the marsupials which doubtless existed had been driven to 
some other portion of the earth, where we shall yet find their 
remains when our knowledge of the geology of the globe is 
more complete. "'* Professor Marsh, with the aid of Mr. J. B. 
Hatcher, has found, since this statement was made, the abun- 
dant mammalian remains of the Laramie beds in Dakota and 
Montana.f thus fulfilling a long made conjecture ; but these 
mammals, though living at the close of the Cretaceous, are of 
old, lowly types, and with them were found the bones of the still 
dominant dinosaurs. On the east coast of North America 
mammalian remains are as yet unknown in the Cretaceous. 
If these animals existed in the region at that time their range 
was doubtless limited to the monadnocks. These cradles for 
the mammalia offered an escape from the lowland life, but 
presented no opportunities for preserving fossils, except 
where great interior lakes existed. 
The close of the Mesozoic attended by the extinction of the di- 
nosaur group and the uplift <ni<l erosion of the peneplain. 
The break in organic life which occurs in the interim be- 
tween the ( retaceous and the lower Tertiary is generally wide- 
spread : in a few areas such as that of the interior district of 
North America the passage is well recorded. There the di- 
nosaur group passes out and gives place to new life which came 
in as by a bridge from some land before separated from the 
habitat of the reptilia. It has long been recognized that the 
geographic changes of this time were concerned in this or- 
ganic revolution. Professor Verrill, in a lecture at Yale col 
lege, regarded lack of parental care as a probable cause of the 
extinction of the large and powerful reptiles of the Mesozoic 
age and of the large mammals of the Tertiary:! and Prof. 
Marsh has pointed out the fact, true of these reptiles as of 
the gigantic brutes of the American Tertiaries of which he 
particularly speaks, that they had \ brains of diminutive pro- 
portions. "The small brain, highly specialized characters and 
*Elementsof Geology, third ed., 1891, p. 192. 
fAm. Jour.Sci., III. vol. xxxviii. 1889, pp. 81-83. 
tQuoted by E. \Y. Morse, Science, vol. v 1887, p. ;.">. 
