Williams.] SHIFTING OF FAUNAS. 105 
then, giving the much needed statistics. With these statistics in 
hand it is possible now to express more clearly the laws involved in 
this shifting of the corporate faunas, as wholes, and their coincident 
modification. 
BIOLOGICAL CONSEQUENCES OF SHIFTING OF FAUNAS. 
The principles assumed to account for the change in the character 
of faunas are of two kinds, viz., (i) the geographical shifting of the 
faunas, and (ii) the evolution of organisms independent of change of 
environment. Only so long as the conditions of a marine basin 
remain constant, or differ so slightly and so slowly that the faunas 
living under them can preserve their integrity as a whole and pre- 
serve that balance of adjustment to each other which may be called 
biological equilibrium — only so long as these conditions remain can 
the fauna be supposed to retain its integrity as a fauna. This state 
of things is represented in many geological formations for a great 
period of time. Throughout strata of limestone, in some places 
reaching 1,000 feet or more in thickness, this integrity of the fauna is 
preserved. It is to be interpreted as due in some measure to the 
conditions of environment remaining constant, whether evolution 
takes place under such conditions or not. Attention is called in the 
present statement to the fact that the fauna as a whole does maintain 
a relative integrity, which permits the assumption of at least very 
slight evolution of the types. Some of the species may drop out, and 
occasionally a few new ones ma} f come in during the course of Hi is 
life period — if we might so call it — of the fauna. At the same time the 
variations, pure and simple, which are observed are very slight, and 
not to be compared with the differences which are often noted on 
passing across a very limited distance of sediments where the con- 
ditions have changed and the fauna is broken up. It is nol necessary 
to assume that a very great length of time has intervened between the 
embedding of the old and the appearance of the new fauna as we 
follow upward a strati graphical section. Throughout the geological 
column many cases are known where one fauna is i in mediately fol- 
lowed by another, without greater break of sedimentation than the 
passage between two strata and with perfect parallelism of the con- 
tiguous strata, yet the species are almost completely changed. The 
species of the same genera are often found to be quite different. 
The student of paleontology is not required to assume that in such 
cases the second fauna has been evolved directly Prom the species 
which underlie it in the strata below. The more natural assumption, 
and the one which is borne out by further investigations in other 
regions, is that the new fauna has come to be deposited in the second 
series of beds lying above the first fauna by the shift ingot' the faunas 
upon the ocean bottom itself. A migration from some other region 
into the region where it is recorded is made by the species. This 
