44 ANIMAL ENVIRONMENT 
changes it is obvious that an area of sterile soil will support more animals 
as time goes on, than at the outset, when the conditions were such that 
only a few hardy species could live. Here again, then, time is the impor- 
tant factor in determining the change of the area, so as to be suitable 
for more species (because more species are adapted to live in the result- 
ing than in the initial conditions). The length of time which has 
elapsed since a given set of surface and physiographic conditions became 
exposed to the atmosphere is very important in governing the number, 
kind, and distribution of animals in a given area. 
c) The value of physiographic form.—Physiographic features are 
classified according to their form and their mode of origin. What is the 
importance of their forms and modes of origin to the animal ecologist ? 
Has a kame or an esker or a valley train any significance so far as animals 
are concerned? So far as anyone has been able to observe, the fact 
that they possess their particular form is of no significance whatever. 
Their relations to present ground-water level, their slope, relation to the 
sun, etc., are significant. The amount of surface soil and the denseness 
of vegetation are also of very great importance, and conditions in these 
respects are usually closely correlated with the length of time that the 
structures have been exposed to the atmosphere. 
Since age is important, we turn at once to the history of an area in 
order to learn the relative age of the various features present. We have 
parted company with the physiographer and his discussion of mode of 
. origin, and are interested in origins only in point of time. 
III. History oF THE REGION ABOUT LAKE MICHIGAN (59) 
I. PHYSIOGRAPHIC HISTORY 
We will give the briefest possible account of the history of the Chicago 
area, following Leverett (59), Salisbury (57, 60), Alden (61), Atwood 
(62), Goldthwait (62, 63, 64), and Lane (65). 
The most important features of our area were shaped during and 
since the glacial epoch. To us, the only important movement of the 
ice was that of the last Wisconsin ice sheet. This came to us mainly 
from the east and north. It spread out over the great basins now 
occupied by the Great Lakes and thence pushed on to the higher rock to 
the south of them and reached its southernmost extent in Southern 
Illinois. 
In retiring from here (Fig. 1) one of the positions in which the 
edge of the ice halted corresponded to the present Valparaiso Moraine. 
The crest of this moraine extends from the Fox Lake region (see map) 
