horny
hoofs affected by the union of the two forms, and the offspring from two
birds with differently coloured tails have their tails and not their whole
plumage affected.

The various tissues of the body plainly show, as many physiologists have
insisted,[909] an affinity for special organic substances, whether natural
or foreign to the body. We see this in the cells of the kidneys attracting
urea from the blood; in the worrara poison affecting the nerves; upas and
digitalis the muscles; the Lytta vesicatoria the kidneys; and in the
poisonous matter of many diseases, as small-pox, scarlet-fever,
hooping-cough, glanders, cancer, and hydrophobia, affecting certain
definite parts of the body or certain tissues or glands.

The affinity of various parts of the body for each other during {381} their
early development was shown in the last chapter, when discussing the
tendency to fusion in homologous parts. This affinity displays itself in
the normal fusion of organs which are separate at an early embryonic age,
and still more plainly in those marvellous cases of double monsters in
which each bone, muscle, vessel, and nerve in the one embryo, blends with
the corresponding part in the other. The affinity between homologous organs
may come into action with single parts, or with the entire individual, as
in the case of flowers or fruits which are symmetrically blended together
with all their parts doubled, but without any other trace of fusion.

It has also been assumed that the development of each gemmule depends on
its union with another cell or unit which has just commenced its
development, and which, from preceding it in order of growth, is of a
somewhat different nature. Nor is it a very improbable assumption that the
development of a gemmule is determined by its union with a cell slightly
different in nature, for abundant evidence was given in the seventeenth
chapter, showing that a slight degree of differentiation in the male and
female sexual elements favours in a marked manner their union and
subsequent development. But what determines the development of the gemmules
of the first-formed or primordial cell in the unimpregnated ovule, is
beyond conjecture.

It must also be admitted that analogy fails to guide us towards any
determination on several other points: for instance, whether cells, derived
from the same parent-cell, may, in the regular course of growth, become
developed into different structures, from absorbing