kind or
other, is common to the whole order of testacea, but it would be absurd to
define the _vis vitae_ of testaceous animals as existing in the shell,
though we know it to be the constant accompaniment, and have every reason
to believe the constant effect, of the specific life that acts in those
animals. Were we (_argumenti __ causa_) to imagine shell coextensive with
the organized creation, this would produce no abatement in the falsity of
the reasoning. Nor does the flaw stop here; for a physiological, that is a
real, definition, as distinguished from the verbal definitions of
lexicography, must consist neither in any single property or function of
the thing to be defined, nor yet in all collectively, which latter,
indeed, would be a history, not a definition. It must consist, therefore,
in the _law_ of the thing, or in such an _idea_ of it, as, being admitted,
all the properties and functions are admitted by implication. It must
likewise be so far _causal_, that a full insight having been obtained of
the law, we derive from it a progressive insight into the necessity and
_generation_ of the phenomena of which it is the law. Suppose a disease in
question, which appeared always accompanied with certain symptoms in
certain stages, and with some one or more symptoms in all stages--say
deranged digestion, capricious alternation of vivacity and languor,
headache, dilated pupil, diminished sensibility to light, &c.--Neither the
man who selected the one constant symptom, nor he who enumerated all the
symptoms, would give the scientific definition _talem scilicet, quali
scientia fit vel datur_, but the man who at once named and defined the
disease hydrocephalus, producing pressure on the brain. For it is the
essence of a scientific definition to be causative, not by introduction of
imaginary somewhats, natural or supernatural under the name of causes, but
by announcing the law of action in the particular case, in subordination
to the common law of which all the phenomena are modifications or results.

Now in the definition on which, as the representative of a whole class, we
are _now_ animadverting, a single effect is given as constituting the
cause. For nutrition by digestion is certainly necessary to life, only
under certain circumstances, but that life is previously necessary to
digestion is absolutely certain under all circumstances. Besides, what
other phenomenon of Life would the conception of assimilation, _per se_,