26 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210; ; 
succession the same dominant species. So long as this is the case it 
may be assumed that the same fauna is under examination. When 
the dominant species become replaced by others a change in the fauna 
is taking place, though it may be shown that a large majority of the 
species are identical. 
This maintenance by a fauna of the same relations of abundance 
and rarity among the component species may be called the bionic 
equilibrium of the fauna, since we can not assume that the whole 
fauna dies out and a new one comes in, but rather must believe that 
the fauna changes by an adjustment of equilibrium among its species. 
Some of the species may become extinct, some of them may be modi- 
fied, and some may be left behind or become separated from the main 
fauna in the course of its migration. 
The term bionic refers to the quality of persistence in transmitting 
the same characters from generation to generation, a quality that is 
recognized by the presence of the same species in the same relative 
abundance in the successive faunules. This relative abundance of 
individuals of the same species is thus taken as the evidence of the 
bionic rank of the species in the faunule at the particular time in 
which it lived. 
It has been observed that species having a high bionic rank are 
more variable than those with low bionic rank; therefore it is to be 
expected that the varietal forms which are destined to become the 
new species of later stages of the fauna will be found among the 
varietal forms of dominant species. On the other hand, the dominant 
species of a new fauna are likely to be the rare forms of an antecedent 
fauna which in the revolution of the conditions have gained in bionic 
vigor and replaced the old species which have lost their bionic domi- 
nance. It is to catch this replacement of the old fauna by a new one 
that the observer should watch with care the thin occasional inter- 
calated beds containing species either wholly or in part different from 
the prevailing fauna. 
It has been often observed that the first traces of the new over- 
lying fauna are to be detected almost pure in such little zones occur- 
ring in the midst of the normal rocks of a formation several feet 
or even tens of feet below its actual top. Much light is thrown 
upon the time relations of faunas and upon the shifting of sedi- 
ments and faunas (to be ultimately interpreted into elevation and 
depression of parts of the earth's surface in relation to other parts) 
by noting precisely the sequence of faunules, and particularly the 
first evidence of change in the faunal contents of the zones of a con- 
tinuous section. 
The question of bionic values may be discussed more satisfactorily 
farther on in this paper, after the presentation of concrete examples 
to be used as illustrations. The general conception of bionic relations 
and values is given in a paper first read before the Geological Society 
