in whether
functionally-produced modifications are inherited; yet certain apparent
instances of such inheritance have forced themselves on observation
without being sought for. In addition to other indications of a less
conspicuous kind, is the one I have given above--the fact that the
apparatus for tearing and mastication has decreased with decrease of its
function, alike in civilized man and in some varieties of dogs which
lead protected and pampered lives. Of the numerous cases named by Mr.
Darwin, it is observable that they are yielded not by one class of parts
only, but by most if not all classes--by the dermal system, the muscular
system, the osseous system, the nervous system, the viscera; and that
among parts liable to be functionally modified, the most numerous
observed cases of inheritance are furnished by those which admit of
preservation and easy comparison--the bones: these cases, moreover,
being specially significant as showing how, in sundry unallied species,
parallel changes of structure have occurred along with parallel changes
of habit.

What, then, shall we say of the general implication? Are we to stop
short with the admission that inheritance of functionally-produced
modifications takes place only in cases in which there is evidence of
it? May we properly assume that these many instances of changes of
structure caused by changes of function, occurring in various tissues
and various organs, are merely special and exceptional instances having
no general significance? Shall we suppose that though the evidence which
already exists has come to light without aid from a body of inquirers,
there would be no great increase were due attention devoted to the
collection of evidence? This is, I think, not a reasonable supposition.
To me the _ensemble_ of the facts suggests the belief, scarcely to be
resisted, that the inheritance of functionally-produced modifications
takes place universally. Looking at physiological phenomena as
conforming to physical principles, it is difficult to conceive that a
changed play of organic forces which in many cases of different kinds
produces an inherited change of structure, does not do this in all
cases. The implication, very strong I think, is that the action of every
organ produces on it a reaction which, usually not altering its rate of
nutrition, sometimes leaves it with diminished nutrition consequent on
diminished action, and at other times increases its nutrition in
pr