BEEDE AND ROGERS.] Coal Measures Faunal Studies. 327 
their range in such cases than to chance an error in doing so. 
The spaces on the chart may be filled in easily enough when 
the additional range is certainly known, but to ascribe a spe- 
cies, in a chart of this kind, to a horizon where it does not 
occur, is apt to become a permanent error. 
Another point of some significance which may be determined 
by the chart is the time at which migrations of individual 
species is most apt to occur. It is well understood that the 
most vigorous species, represented by large numbers of in- 
dividuals, are the ones that migrate most readily, other things 
being equal, and form the cosmopolitan elements of the faunas 
of the various periods. They are frequently characterized by 
their great stratigraphic range, which is in accord with their 
hardiness. 7 
In the life-history of many species there is a time when it 
is more vigorous and found in much greater abundance than 
at any other time, even though it may be found throughout a 
very considerable range of strata. This period in the exist- 
ence of a species is usually referred to as its culmination. 
Other things being equal, it is believed that a species is more 
apt to withstand the vicissitudes of migration at this time 
than at any other on account of its greater numbers and re- 
productive ability and greater vitality. Of course its mi- 
gration at any time is governed largely by its opportunities 
to do so, as the reduction of barriers and extent of con- 
tinental shelf, currents and food supply. It is believed that 
this feature is of sufficient importance to be taken into ac- 
count in making the broader correlations, especially if there 
is evidence that there was a possibility of migration from the 
home basin at a time within such culmination. It is further 
believed that the influx of a considerable faunal element into 
a region is sufficient evidence of the equal opportunity for the 
inhabitants of a basin to emigrate. 
On the continental shelves of such oceans as the Pacifie, 
especially where there is great instability of conditions and 
opportunity for great intermigrations, the chance for a spe- 
cies to complete a life-cycle in any geographical province is 
small, as has been pointed out by Prof. J. P. Smith.22 In 
such cases species appear and disappear rapidly, according as 
they are forced out by the arrival of more vigorous species 
from other regions or by orographic movements. On the other 
272. Jour. Geol., vol. III, pp. 486, 487, 1895. 
