wili.tams] SHIFTING OF FAUNAS. 107 
On the other hand, it is assumed that changes of conditions of environ- 
ment, which may have been very slight but which necessitate a shift- 
ing or migration of the faunas, may produce some and even consider- 
able changes in a short time in the faunas concerned. The changes 
may be produced in the following ways: If the forced migration be 
sudden, the ability of the different species to migrate will, in the first 
place, be very unequal; some species can migrate and. some can not; 
some can migrate easily and others with difficulty, and the sudden 
necessity of migrating, as a fauna, must necessarily break up what I 
have called the biological equilibrium of the fauna. In every shifl 
some species will be forced to drop out, because they can not migrate or 
because they can not adjust themselves to the new conditions. 1 1* such 
a dropping out of species from the faunas takes place, there results at 
once a new condition of affairs in the faunal life. Competition is dif- 
ferent; the means of livelihood have changed; the necessity of new 
habits of life is forced upon the remaining species. In the process of 
adjustment of one to another, irrespective of the changing conditions, 
we may suppose that the species which remain in the fauna will, some 
of them, be reduced in rank and some of them increased, which will 
be indicated by change in abundance or rarity. The increased or 
decreased abundance of species in the fauna is one of the evidences 
of this shifting process. Where a species is abundant, I have fre- 
quently observed that variability also is increased. 
Relatively speaking, the variability is almost in proportion to 1 lie 
vigor and abundance of reproduction of the individuals. Here at 
once we see a means of rapid evolution. If a species varies and the 
variation is augmented by favorable conditions of livelihood, the 
change from one environment to another necessitates 1 he modifical ion 
of some of the species almost immediately, and the variability of the 
fauna will be strongly expressed when migration of the species lakes 
place. The adjustment of the fauna to its changed conditions is a 
matter of slower accomplishment, but it maybe supposed that migra- 
tion from one region to another will result in more or less modifica- 
tion and readjustment of the proportionate fertility and abundance of 
the species, unless the change of environmental conditions be so slow- 
as to enable the whole fauna to move its center of distribution with- 
out disturbance of its bionic equilibrium. Such cases would be rare 
and the distances not great. 
The investigations of Grabau and Cleland, already referred to, illus- 
trate this principle. The study of the Cayuga Lake section was made 
for the purpose of furnishing a minute comparison with the Eighteen- 
mile Creek section, as well as to determine the exact composition of 
the temporary combination of specie's found in each stratum. 'Hie 
result was very clear. The general fauna was found to he very much 
alike from the bottom of the Hamilton up to its lop in both sections. 
The difference between the several zones was constantly fluctuating, 
