led up with new bones,
which interlock by perfect serrated sutures.[723] But most
physiologists, especially on the Continent, have now given up the
belief in plastic lymph or blastema, and Virchow[724] maintains that
every structure, new or old, is formed by the proliferation of
pre-existing cells. On this view false membranes, like cancerous or
other tumours, are merely abnormal developments of normal growths; and
we can thus understand how it is that they resemble adjoining
structures; for instance, that "false membrane in the serous cavities
acquires a covering of epithelium exactly like that which covers the
original serous membrane; adhesions of the iris may become black
apparently from the production of pigment-cells like those of the
uvea."[725]

No doubt the power of reparation, though not always quite perfect, is
an admirable provision, ready for various emergencies, even for those
which occur only at long intervals of time.[726] Yet this power is not
more wonderful than the growth and development of every single
creature, more especially of those which are propagated by fissiparous
generation. This subject has been here noticed, because we may infer
that, when any part or organ is either greatly increased in size or
wholly suppressed through variation and continued selection, the
co-ordinating power of the organisation will continually tend to bring
all the parts again into harmony with each other.

_On the Effects of the Increased Use and Disuse of Organs._

It is notorious, and we shall immediately adduce proofs, that increased use
or action strengthens muscles, glands, sense-organs, &c.; and that disuse,
on the other hand, weakens them. I have not met with any clear explanation
of this fact in works on Physiology. Mr. Herbert Spencer[727] maintains
that when muscles are much used, or when intermittent pressure is applied
to the epidermis, an excess of nutritive matter exudes from the vessels,
and that this gives additional development to the adjoining parts. That an
increased flow of blood towards an organ leads to its greater development
is probable, if not certain. Mr. Paget[728] thus accounts for the long,
thick, and dark-coloured hair which occasionally grows, even in young
children, near old-standing inflamed surfaces or fractured bones. When
Hunter {296} inserted the spur of a cock into the comb, which is well
su