AISE ETER Ae P ERAS VENAT TIT AIRERA Ns 
1894.] The Numerical Intensity of Faunas. 757 
tion, and after a length of time, bring about a low vitality 
which in conjunction with the uncongenial surroundings 
would wind up the species. 
Of course the term unfavorable is here used comparatively, 
not meaning inimical, because a new species upon the doctrine 
of adaptation could not arise in hostile circumstances, but 
meaning less favorable than the most auspicious surroundings. 
The result as measured in numerical estimates would be a low 
mean, which perhaps as the environment improved might 
increase. Itis only likely that such conditions are present 
when a species migrates, or is invaded by a change of physical 
conditions less advantageous than those it has previously en- 
joyed. A new species with high vitality is hardly consistent 
with unfavorable environment at the beginning, and the 
category we are considering would only be exemplified in the 
numerical exhibit of species whose habitat has been affected 
unfavorably. The repression of great numbers of individuals 
at any one time would tend to lengthen the life of the species, 
inasmuch as it would relieve it from struggle in its own midst, 
and this would have a tendency to extend its days. 
In the paleontological record the case of Atrypa reticularis 
seems to illustrate this numerical constancy. From the Upper 
Silurian in the Niagara through the Lower Helderberg, Oris- 
kany Schoharie and Upper Helderberg it keeps up a more or 
less uniform though not excessive representation until diverg- 
ing in the Devonian into A. vexata and A. spinosa it becomes 
itself more numerous seeming then to pass under the condi- 
tions of the first category—high vitality and favorable environ- 
ment—and declining rapidly terminates in the Upper Hamil- 
ton. Atrypa reticularis, as is well known, does not attain a 
large size in the Silurian, but, according to Hall, exhibits con- 
siderable variety of form. It is in the stage of “ oscillation,” 
not yet having attained specific fixity and this fact of formal 
instability points to a lack of congruity between itself and its 
environment and leads us to consider it an example under this 
heading. : 
Favorable Environment and Low Vitality.—By * Low Vitality " 
we here designate a certain sluggishness in fecundity in cer- 
