petencies, which exist in every gland, and in every moving organ of
the body, and are as essential to living organism as chemical affinities
are to certain combinations of inanimate matter.

"If I might be indulged to make a simile in a philosophical work, I
should say that the animal appetencies are not only perhaps less
numerous originally than the chemical affinities, but that, like these
latter, they change with every fresh combination; thus vital air and
azote, when combined, produce nitrous acid, which now acquires the
property of dissolving silver; so that with every new additional part to
the embryon, as of the throat or lungs, I suppose a new animal appetency
to be produced."[178]

* * * * *

Here, again, it should be insisted on that neither can the "additional
part" precede "the appetency," nor the appetency precede the additional
part for long together--the two advance nearly _pari passu_; sometimes
the power a little ahead of the desire, stimulates the desire to an
activity it would not otherwise have known; as those who have more money
than they once had, feel new wants which they would not have known if
they had not obtained the power to gratify them; sometimes, on the other
hand, the desire is a little more active than the power, and pulls the
power up to itself by means of the effort made to gratify the desire--as
those who want a little more of this or that than they have money to pay
for, will try all manner of shifts to earn the additional money they
want, unless it is so much in excess of their present means that they
give up the endeavour as hopeless; but whichever gets ahead, immediately
sets to work to pull the other level with it, the getting ahead either
of power or desire being exclusively the work of external agencies,
while the coming up level of the other is due to agencies that are
incorporate with the organism itself. Thus an unusually abundant supply
of food, due to causes entirely beyond the control of the individual, is
an external agency; it will immediately set power a little ahead of
desire. On this the individual will eat as much as it can--thus learning
_pro tanto_ to be able to eat more, and to want more under ordinary
circumstances--and will also breed rapidly up to the balance of the
abundance. This is the work of the agencies incorporate in the organism,
and will bring desire level with power again. Famine, on the other hand,
puts desire ahead of power,