or water, as the case may
be. If the organism be minute, mere contact of its external surface with
the oxygenated medium achieves the requisite oxidation; but if the
organism is bulky, and so exposes a surface which is small in
proportion to its mass, any considerable oxidation cannot be thus
achieved. One of two things is therefore implied. Either this bulky
organism, receiving no oxygen but that absorbed through its integument,
must possess but little vital activity; or else, if it possesses much
vital activity, there must be some extensive ramified surface, internal
or external, through which adequate aeration may take place--a
respiratory apparatus. That is to say, lungs, or gills, or branchiae, or
their equivalents, are predicable _a priori_ as possessed by all active
creatures of any size.

Similarly with respect to nutriment. There are _entozoa_ which, living
in the insides of other animals, and being constantly bathed by
nutritive fluids, absorb a sufficiency through their outer surfaces; and
so have no need of stomachs, and do not possess them. But all other
animals, inhabiting media that are not in themselves nutritive, but only
contain masses of food here and there, must have appliances by which
these masses of food may be utilized. Evidently mere external contact of
a solid organism with a solid portion of nutriment, could not result in
the absorption of it in any moderate time, if at all. To effect
absorption, there must be both a solvent or macerating action, and an
extended surface fit for containing and imbibing the dissolved products:
there must be a digestive cavity. Thus, given the ordinary conditions of
animal life, and the possession of stomachs by all creatures living
under these conditions may be deductively known.

Carrying out the train of reasoning still further, we may infer the
existence of a vascular system or something equivalent to it, in all
creatures of any size and activity. In a comparatively small inert
animal, such as the hydra, which consists of little more than a sac
having a double wall--an outer layer of cells forming the skin, and an
inner layer forming the digestive and absorbent surface--there is no
need for a special apparatus to diffuse through the body the aliment
taken up; for the body is little more than a wrapper to the food it
encloses. But where the bulk is considerable, or where the activity is
such as to involve much waste and repair, or where both these
characteri