Williams] FAUNAL AGGREGATES. 31 
This power of endurance is undoubtedly an exceedingly complex 
fact, but it is recorded simply by the continued appearance of fossils 
with the same morphological characters. If the characters are of 
specific rank their endurance is of relatively short geological time; if 
the characters are generic they are repeated for a longer period of time, 
etc. These endurance values of the characters of organisms were 
spoken of as bionic. The general term chron was proposed as a desig- 
nation for a division of geological time, and thus one is enabled to 
speak of geochron as the time duration expressed by formations, and 
biochron as the duration expressed bj r the life history of organisms. 
A definite and independent value (i. e., independent of the forma- 
tion scale) was given to the chronological terms hemera, epoch, period, 
era, eon by using the bionic or endurance quality of organisms as 
the measure of them. Thus hemera was to be measured by the endur- 
ance of the bionic equilibrium of a local faunule; epoch, by the 
endurance of species; period, by the endurance of genera; era, by the 
endurance of families; eon, by the endurance of orders. 
One other set of terms applies peculiarly to faunas. Fossil faunas 
express evidence of a certain amount of migration or shifting of place 
of habitation during their life history. Barrande spoke of colonies. 
Recurrence of faunas has been described. In case a marine fauna 
shifts upon the sea bottom during differential movements of the crust 
of the earth two results are possible — either the bionic equilibrium 
of the fauna will be disturbed and thus the faunal composition will 
be modified, with more or less mutation of the species, or the faunal 
equilibrium will be retained and the fauna in its integrity will appear 
at a higher stratigraphical position in the region to which it migrates 
than in the region from which it has shifted. This will be expressed 
by a transgression of the fauna in relation to the formation. It may 
be expressed by a mingling of the species of two faunas; then it is 
defined as transitional. It is possible to have such oscillation of 
orogenic movements that a region may be reoccupied by a fauna 
which has shifted out of it temporarily. In such cases there will 
appear in the stratigraphical section evidence of recurrence of faunas, 
and the "colonies" of Barrande may be thus explained, in so far as 
they are not explained by disturbance of the strata after sedimentation. 
As orogenic movements presumably cover long periods of time in 
one direction for a given area, the direction of the induced migrations 
of organisms w T ould also be in one general direction, thus furnishing 
no occasion for recurrence of faunas. In such cases the order of the 
faunas would be correctly expressed, though in two sections the time 
represented would differ at top and bottom. 
Mingling of faunas would also be expressed by the arrival of migrat- 
ing species into the midst of a native fauna before the shifting was 
general. 
