760 The American Naturalist. [September, 
of these lines wherein, however, the factors of environment 
and vitality are oppositely related. There would then be two 
stages of equal numerical efficiency, opposite in conditions 
but equivalent in effects, favorable environment and low 
vitality, and unfavorable environment and high vitality, and 
succeeding these as an inevitable sequence comes at the end of 
either road of retreat, the final stage of unfavorable environ- 
ment and low vitality and the extinction of the species. Along 
either of the avenues of deterioration the numerical intensity 
is supposed to decline similarly but this superficial resemblance 
covers a radical contrast of agencies and we are brought to 
consider two kinds of strain; the strain of internal weakness, 
and the strain of external disparity. This introducesa crucial 
question we think in reference to the Darwinian hypothesis. 
That hypothesis assumes that species are perpetuated by the 
concordance declared between them and their surroundings, 
and it seems enclosed in this wide opening statement, that the 
Darwinian must allow a certain power of provocation upon 
organisms from exterior conditions, viz., that the inherent 
variability (fully emphasized by Darwin) of organisms is 
stimulated by changing environment while it should be more 
quiescent under unchanged circumstances of life. Without at 
present pressing this question the inference, we think, is rea- 
sonable. Therefore, in establishing a line of numerical decline 
for a species we have in this suggestion a form of test as to 
whether that decline arises from changing environment or 
changing vitality. If it proceeds from changing environment 
it will be, upon the Darwinian theory, accompanied by specific 
offshoots, and the disappearing species will sink from sight 
amidst the emergence of related species; but, if it proceeds 
from devitalization it will display a species dying as it were 
alone, unattended by the growth of related varieties, and pass- 
ing away without those bequests of derivative forms which, in 
the other instance, represent the yet internally vigorous species 
struggling to maintain its empire under the guise of modified 
offspring. These propositions will, it may perhaps be con- 
ceded, repay more careful and detailed application to zoological 
history, as it has been written in the successive ages of geology. 
