36 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
does not necessarily involve the institution of a new combination of 
conditions, but rather causes a transfer from place to place of exist- 
ing conditions of environment. Such movement of the earth's sur- 
face, resulting in the geographical changing of the conditions of 
environment for each particular spot on the surface, would necessitate 
the movement of the faunas living under particular conditions or else 
their destruction. They must either shift their place of habitation 
as the conditions favorable to their existence are changed, or, if they 
attempt to stay on the same spot, they must adjust themselves to new 
conditions of environment. This principle of migration necessarily 
involves a change in the geographical distribution of the living faunas; 
that the species should be modified as such migration takes place is 
a natural conclusion to be drawn from the facts. 
The other kind of change which organisms undergo during the lapse 
of geological time may occur without any disturbance of the physical 
conditions of the province in which they live, and is coincident with 
the passage of time alone. The ordinary theory of evolution contem- 
plates a modification of species under such conditions, a gradual 
variation of form coincident with the continuance of the species 
under like conditions during their "struggle for existence." The 
modification they suffer is then due to "natural selection" and the 
"survival of the fittest." I say this is the prevalent hypothesis to 
account for the modification of species by evolution. It is altogether 
probable that both these methods of modification have been effective 
to a greater or less extent in producing the total results which go 
under the name of evolution of species. 
But the paleontologist, as lie studies the succession of species, will 
have his attention more closely called to the modifications which are 
coordinate with the geological movements of the surface and are 
expressed in changes of local conditions Avithin the whole province in 
which the organisms live. This modification by forced migration has 
to do with the breaking up and reinstituting of biological equilibrium 
of the faunas, and in less measure and with less effect with the prin- 
ciple of struggle for existence among common competitors. 
In order to discuss the subject of the migration of species and the 
effects of forced migration upon faunas, it is necessary to discriminate 
two distinct sets of facts as under discussion at the same time. In 
the first place, there are the geological formations in which the fos- 
sils are preserved, which are made of fragmental particles of sand or 
mud or limestone, massed together into sheets called strata, piled 
one upon another, forming geological columns. These are the forma- 
tions of the geological "time scale." These are local, from the fact 
that the materials of which they are composed are sediments which 
have been deposited under water and have necessarily been brought 
from some contiguous lands to the place of their deposit. Geological 
formations are thus, from the nature of things, local deposits, having 
