384 
ELEMENTS OF GEOLOGY. 
conical incisors — the latter being divided by short inter¬ 
vals.” 
Low Grade of Early Mammals favorable to the Theory of 
Progressive Development. —There is every reason to believe 
that this fossil quadruped is at least as ancient as the Micro- 
lestes of the European Trias above described, p. 368 ; and 
the fact is highly important, as proving that a certain low 
grade of marsupials had not only a wide range in time, from 
the Trias to the Purbeck, or uppermost oolitic strata of Eu¬ 
rope, but had also a wide range in space, namely, from Europe 
to North America, in an east and west direction, and, in re¬ 
gard to latitude, from Stonesfield, in 52° N., to that of North 
Carolina, 35° N. 
If the three localities in Europe where the most ancient 
mammalia have been found—Purbeck, Stonesfield, and Stutt¬ 
gart—had belonged all of them to formations of the same 
age, we might well have imagined so limited an area to have 
been peopled exclusively with pouched quadrupeds, just as 
Australia now is, while other parts of the globe were inhabit¬ 
ed by placentals; for Australia now supports one hundred 
and sixty species of marsupials, while the rest of the conti¬ 
nents and islands are tenanted by about seventeen hundred 
species of mammalia, of which only forty-six are marsupial, 
namely, the opossums of North and South America. But 
the great difference of age of the strata in each of these three 
localities seems to indicate the predominance throughout a 
vast lapse of time (from the era of the Upper Trias to that 
of the Purbeck beds) of a low grade of quadrupeds; and this 
persistency of similar generic and ordinal types in Europe 
while the species were changing, and while the fish, reptiles, 
and mollusca were undergoing great ^modifications, would 
naturally lead us to suspect that there must also have been 
a vast extension in space of the same marsupial forms during 
that portion of the Secondary or Mesozoic epoch which has 
been termed “ the age of reptiles.” Such an inference as to 
the wide geographical range of the ancient marsupials has 
been confirmed by the discovery in the Trias of North Amer¬ 
ica of the above-mentioned Dromatherium. The predomi¬ 
nance in earlier ages of these mammalia of a low grade, and 
the absence, so far as oiir investigations have yet gone, of 
species of higher organization, whether aquatic or terrestrial, 
is certainly in favor of the theory of progressive development. 
