Williams] GEOLOGICAL EXPRESSION OF FAUNAL MIGRATION. 35 
environment in the province, have been living at the same time, as is 
clearly known to be the fact in the case of geographical distribution 
of living faunas at the present time on the face of the earth. 
The term fades has been applied to the peculiar combination of 
species of a fauna characteristic of particular, restricted conditions 
of environment. So that two sets of species, living simply under 
dijferent conditions of enviroment, are said to express different facies 
of the fauna of the period in which they lived. In attempting to 
make correlations and classifications of stratigraphical formations, 
geologists have found difficulty in distinguishing between the differ- 
ent facies of the fauna of the same period and the successive muta- 
tions of the fauna consequent upon geological succession. To put 
this in a word, difference in faunas may be due either to geographical 
distribution or to geological range. 
Geographical distribution furnishes the basis of classifying living 
faunas existing on the earth at the same time, and the facts con- 
cerning it are so well known that no one need hesitate to explain 
difference of living faunas by difference of geographical distribution. 
The principal fact in the case is that environments of different kinds 
are occupied by different species. This is a matter of fact, irrespec- 
tive of any theory as to how such relation of the faunas to their 
environment has come about. 
When, however, we are led to ask how the adjustments came about 
in geological time, we have to choose an answer from these two possi- 
bilities, viz, either (a) slowly progressing and relatively constant 
evolution has taken place among organisms constantly struggling 
together and varying, or (b) faunas become rapidly adjusted to new 
conditions, attaining a biological equilibrium, and then maintain 
that equilibrium with extremety slight variation for great periods of 
time, under like conditions, but quickly and rapidly suffer specific 
modification whenever the environment changes and the equilibrium 
is thus disturbed. Such a disturbance, it is assumed, has taken place 
whenever a sudden change occurs in the sequence of sediments from 
one formation to another with change of sediments and corresponding 
change of fossils. 
Instead of assuming that the fossils were destroyed at such points 
and recreated in the following period, the theory here proposed is 
that the faunas have shifted over the ocean bottom. The uppermost 
of two successive faunules in a single continuous section is presumed 
to have lived synchronously with the underlying faunule, but in a 
separate region ; and at the point where the faunal change occurred the 
second fauna migrated into the region, expelling and replacing the first. 
Such cases are not universal, but it is assumed that the shifting of 
faunas is more or less common. In other words, the elevation or 
depression of continents in relation to ocean level, which involves 
"the shifting of the position of deep or shallow or shore conditions, 
