Nutrition not a simple process. (1) Need of subsidiary
organs for the various stages of alteration, _e.g._, of
bread into blood, of that into bone, etc. (2) Need also
of organs for excreting the non-utilizable portions of
the food, _e.g._, much vegetable matter is superfluous.
(3) Need of organs of a third kind, for distributing the
pabulum through the body.