Williams. I GEOLOGICAL EXPRESSION OE FAUNAL MIGRATION. 41 
series of sediments, the individual species suffer but slight change, 
and this has been observed through hundreds of feet of limestones, 
binning up into the thousands, and not confined to only a single case. 
The interpretation of this fact is that so long as the equilibrium of the 
species composing a fauna is preserved they may continue to reproduce 
and live on without any considerable modification of their specific 
characteristics. Interpreting this into the principles of evolution, it 
means that natural selection having attained a relative equilibrium, 
evolution will stand still, in so far as the modification of organisms is 
concerned, for great periods of time. On the theory of modification 
by migration it is assumed that this equilibrium is an equilibrium of 
active forces residing in the organism, which are held in the state of 
equilibrium by the combination of circumstances going under the 
name of "natural selection." There is also implied, however, the idea 
that the species are in a plastic state, ready for modification, and that 
those which survive vigorously are in a more plastic state than those 
which succumb and are lost in the fight. 
That species vary so soon as they are subjected to new conditions 
of environment implies that the variation is an expression of special 
vigor in the organism and not a sign of weakness — that variation is 
the expression of vitality (if we may use that term in a general sense) 
and is not a consequence of competition among the individuals them- 
selves. Darwin has spoken of such variation as "spontaneous varia- 
tion;" that is, variation which is not accounted for on the principle 
of natural selection, but which is presumed to be present before natu- 
ral selection is capable of acting upon the morphological characters 
of the organisms. 
This interpretation also explains another fact which paleontologists 
have frequently observed — the fact that succession of faunas of the 
same general facies is rarely traceable to gradual modification of a 
subjacent fauna. In such a case the metropolis, or center of distri- 
bution, of the new fauna is generally (and it may be universally) 
found in a different geographical area from that of the old fauna 
which it replaces. 
