40 CORRELATION OP GEOLOGICAL FAUNAS. [bull. 210. 
strongly contrasted with the other) has come from outside the par- 
ticular region in which it is introduced and is not the immediate 
evolutional successor of the underlying fauna. 
The supposition that two faunas will evolve separately if placed in 
two different regions implies simply the fact that no two actually 
distinct regions can he supposed to have exactly the same conditions 
of environment or the same actual set of species. Such conditions 
are frequently observed, as on two sides of an ocean, or, again, along 
the same coast, where we may find northern and southern faunas. 
When we cross from one ocean to another, under similar climates, it 
is familiarly observed that the composition of faunas living under 
similar physical conditions is different. Supposing, in this way, that 
we have a set of similar conditions in different parts of a basin which 
are separated one from another by barriers sufficient to prevent easy 
intercourse between the two parts, although not necessarily prohibit- 
ing migration, here we have all the conditions for the development of 
special faunas. With the breaking up of the geological conditions of 
such a general basin — as, for instance, by the rising of the bottom in 
relation to the surface of the ocean, or by the sinking of another part 
of the basin so as to bring deeper and purer waters where h?d been 
prevailing the accumulation of shore sediment — we may suppose the 
conditions of environment so completely changed for a particular 
part as to force the organisms to shift their position. In shifting, 
those which are able to shift and migrate would migrate, whereas 
those which are less capable of migration must necessarily be cut off, 
or at least be removed from the migrating fauna to such an extent as 
to change the equilibrium of the species. Coincident with such move- 
ment of the fauna due to geological changes in the province, it is 
assumed that the evolution of the species finding favorable conditions 
for life would be more rapid than it was during their existence in the 
conditions from which they came, the biological equilibrium of which 
had for a long period of time been approximately fixed and rigid. 
Migration as a stimulus to variation. — It is inferred from what has 
been already said that the more rapid changes in the contents of a 
geological fauna have been caused, or certainly stimulated, by the 
forced geographical change of place of residence of the fauna itself. 
This may be formulated under the term modification by migration. 
When it is attempted to explain how such effects are produced it 
becomes evident that the principle of variation must be conceived of 
as affecting the species of the fauna more intensely when the environ- 
mental conditions are forcibly modified than during the periods, how- 
ever long, in which the biological equilibrium of the fauna maintains 
its integrity. Throughout the whole geological column there are 
illustrations of this fact which will occur to paleontologists. It is a 
common observation that so long as that integrity of the fauna suffi- 
cient to lead to regarding the stage as the same continues through a 
