30 CORRELATION OF GEOLOGICAL FAUNAS. [bull 210. 
period of time may have a metropolis or center of distribution. The 
species of the fauna may migrate, and the whole fauna with its metrop- 
olis may shift. The composition of the fauna may he described in 
terms of the species, to each one of which degrees of relative and 
actual abundance or rarity of individuals, and smallness or largeness 
of size of specimens, may be applied. 
The integrity of the fauna may be defined as the preservation of 
equilibrium of dominance of some species over others, and the life 
period of the fauna may be recognized by the corporate integrity of 
the fauna. Geographical distribution and geological range are terms 
apptying to the species of a fauna. 
Adaptation to conditions of environment, plasticity, variability, 
permanency of characters, and evolutional mutation are qualities of 
species of the same or successive faunas, and may be detected by 
comparison of specimens from different geographical or geological 
positions. 
From such analyses of species and aggregates of species in corporate 
faunas may be framed conceptions of their chronological relations; 
and thus evidence of time duration may be gathered in terms of geo- 
graphical area or thickness of strata occupied by the fossil remains of 
the once living races of organisms. An individual specimen of a spe- 
cies does not express an appreciable length of time duration, but only 
a point of time during the life period of the species. Species vary 
greatly in the lengths of their life periods. The life period of a large 
number of known fossil species is greater than the average duration 
of most of the named formational divisions of smaller size. 
The life period of genera is in many cases greater than the dura- 
tion represented by formational systems. Nevertheless, an approx- 
imation to those formational divisions which have been found con- 
venient in actual usage is presented by the life periods of species, 
genera, and orders of marine organisms, as has been shown by a ten- 
tative scheme of classification on a bionic basis, 05 already published. 
In the paper presenting this scheme it was pointed out that in the 
Paleozoic is recorded the total life period of trilobites and that such 
genera as Olenellus, Asaphus, Phacops, have a life endurance at least 
of the same order of length as the grander subdivisions called systems 
or series in common usage. Again, it may be pointed out that the 
life history of such species as Spirifer radiatus, arenosus, disjuncfus, 
or cameraius is of the same order of magnitude as the geological 
divisions of the formation scale called Niagara, Oriskany, Chemung, 
and Coal Measures. In the paper just cited it was shown that these 
portions of time duration are the measure of an actual power of 
endurance expressed by the organisms themselves. 
" Jour. Geol., Vol. IX, p. 587. See also p. 133 of this bulletin. 
