ed and more heterogeneous sloth. But the admission
that the several aspects under which this increasing contrast shows
itself bear variable ratios to one another, does not negative the
general truth enunciated. Looking at the facts in the mass, it cannot be
denied that the successively higher groups of organisms are severally
characterized, not only by greater differentiation of parts, but also by
greater differentiation from the surrounding medium in sundry other
physical attributes. It would seem that this peculiarity has some
necessary connexion with superior vital manifestations. One of those
lowly gelatinous forms which are some of them so transparent and
colourless as to be with difficulty distinguished from the water they
float in, is not more like its medium in chemical, mechanical, optical,
thermal, and other properties, than it is in the passivity with which it
submits to all the actions brought to bear on it; while the mammal does
not more widely differ from inanimate things in these properties than it
does in the activity with which it meets surrounding changes by
compensating changes in itself. Between these two extremes, we see a
tolerably constant ratio between these two kinds of contrast. In
proportion as an organism is physically like its environment it remains
a passive partaker of the changes going on in its environment; while in
proportion as it is endowed with powers of counteracting such changes,
it exhibits greater unlikeness to its environment.

* * * * *

Thus far we have proceeded inductively, in conformity with established
usage; but it seems to us that much may be done in this and other
departments of biologic inquiry by pursuing the deductive method. The
generalizations at present constituting the science of physiology, both
general and special, have been reached _a posteriori_; but certain
fundamental data have now been discovered, starting from which we may
reason our way _a priori_, not only to some of the truths that have been
ascertained by observation and experiment, but also to some others. The
possibility of such _a priori_ conclusions will be at once recognized on
considering some familiar cases.

Chemists have shown that a necessary condition to vital activity in
animals is oxidation of certain matters contained in the body either as
components or as waste products. The oxygen requisite for this oxidation
is contained in the surrounding medium--air