Development and Distribution of Vegetation. 43 
earliest existing sedimentary rocks, and therefore the age of the earliest known 
fossils is about 28,000,000 years. These great discrepancies in the estimated 
age of the crust of the Earth arise, not from the untrustworthiness of the tes- 
timony, as one might suppose, but from the different estimates in years of the 
era taken as the unit of the measure for the greater periods. The discrepan- 
cies are to my mind reconcilable, and the number last named is herein taken 
as something like the proximate age of the stratified rocks in the Earth’s 
crust. At all events it cannot be much less. The oldest of these rocks have 
imbedded in them the remains of organic beings. Though fossilized early 
plants have not been found in such abundance as have the remains of certain 
animals, we know that plants must have existed in sufficient numbers before 
animals could live, since the food of the latter depends upon the former. 
Still enough plant fossils have been found and studied to directly furnish infor- 
mation of the existence, and to identify many of the kinds of plants in the 
earliest times. We may therefore assume that more than 20,000,000 years 
have elapsed since plants commenced their growth on the earth. 
THE GLACIAL PERIOD. 
Abundant evidence is left of the former existence of great ice masses over 
the region we now inhabit. Winter reigned from New Jersey to Kansas and 
northward during 160,000 years. Sometimes during this immense period the 
sun shone with illuminating splendor and ferveut heat upon the snow-clad 
hills, but such was the accumulated thickness—1000 or more feet — of the 
ice and snow that the bare ground was seldom exposed and more rarely quick- 
enced with germinating life. The icy masses, pushing from the north in 
great glaciers, crushed and ground the rocky earth which, imbedded in the 
frozen sheet, was finally distributed by the waters from the melting southern 
extremity. The “ drift,” as we call this debris, is 100 and more feet thick in 
my own locality. The boulders with which we are all familiar attest the car- 
rying power and hence the bulk of this prodigious ice-covering. This is what 
is called the glacial period. It is not to be assumed that it is the only occur- 
rence of the kind that has happened to the earth; but it is the one great 
period of low temperature, in the northern hemisphere, which is recent enough 
to materially affect the living vegetable and animal population of which we 
gain acquaintance, through their imbedded remains. 
We are better able to approximately fix the date of this glacial epoch in 
terms of years than of any other geological period; for astronomical calcula- 
tions have been brought to bear upon it in such way. as to acceptably deter- 
mine the time. It is known that the axis of the earth does not remain in a 
fixed direction, continually pointing to the same north and south spots in the 
heavens. Its prolonged northern extremity gradually describes a circle 
which includes within it but not at the center, the ‘“‘north star.’’ This circuit 
is swept by the pole in about 21,000 years and this, er-mbined with the eccen- 
tricities of the earth’s orbit, gives the data for calculating the recurrence of 
alternating colder and warmer winters every 10,500 years for each hemisphere. 
Taking this as a unit of measure and comparing the known variation of the 
earth’s orbit, it is determined that the glacial epoch must have commenced 
about 240,000 years ago and continued at least 160,000 years. Eighty thou- 
sand years ago the glaciers began to dissolve into floods of water and to grad- 
ually recede, leaving the earth again suitable for the growth of vegetation. 
Plants, previously pushed to the southward by the ice, followed again its 
retreat, and at length spread over the desolated area. 
Now we want especially. to note that during this 70 to 80,000 years vegeta- 
tion has been substantially what we now know it to be. During the whole 
stretch of this later time species have not greatly changed. The white pine, 
