d surface,
as the early paleontologists supposed. Speaking broadly, the same
general stages have attended the evolution of organic forms everywhere,
but there is nothing to show that equal periods of time witnessed
corresponding changes in diverse regions, but quite the contrary.
To cite but a single illustration, the marsupial order, which is the
dominant mammalian type of the living fauna of Australia to-day,
existed in Europe and died out there in the tertiary age. Hence a future
geologist might think the Australia of to-day contemporaneous with a
period in Europe which in reality antedated it by perhaps millions of
years.

All these puzzling features unite to render the subject of historical
geology anything but the simple matter the fathers of the science
esteemed it. No one would now attempt to trace the exact sequence of
formation of all the mountains of the globe, as Elie de Beaumont did
a half-century ago. Even within the limits of a single continent, the
geologist must proceed with much caution in attempting to chronicle the
order in which its various parts rose from the matrix of the sea. The
key to this story is found in the identification of the strata that
are the surface feature in each territory. If Devonian rocks are at
the surface in any given region, for example, it would appear that this
region became a land surface in the Devonian age, or just afterwards.
But a moment's consideration shows that there is an element of
uncertainty about this, due to the steady denudation that all land
surfaces undergo. The Devonian rocks may lie at the surface simply
because the thousands of feet of carboniferous strata that once lay
above them have been worn away. All that the cautious geologist dare
assert, therefore, is that the region in question did not become
permanent land surface earlier than the Devonian age.

But to know even this is much--sufficient, indeed, to establish the
chronological order of elevation, if not its exact period, for all parts
of any continent that have been geologically explored--understanding
always that there must be no scrupling about a latitude of a few
millions or perhaps tens of millions of years here and there.

Regarding our own continent, for example, we learn through the
researches of a multitude of workers that in the early day it was a mere
archipelago. Its chief island--the backbone of the future continent--was
a great V-shaped area surrounding what is now Hudson Bay, an