e
two outer toes are connected by membrane; for the whole leg tends to assume
the structure of the wing. There is a manifest relation between a crest of
feathers on the head and a marvellous amount of change in the skull of
various fowls; and in a lesser degree, between the greatly elongated,
lopping ears of rabbits and the structure of their skulls. With plants, the
leaves, various parts of the flower, and the fruit, often vary together in
a correlated manner.

In some cases we find correlation without being able even to conjecture
what is the nature of the connexion, as with various correlated
monstrosities and diseases. This is likewise the case with the colour of
the adult pigeon, in connexion with the presence of down on the young bird.
Numerous curious instances have been given of peculiarities of
constitution, in correlation with colour, as shown by the immunity of
individuals of some one colour from certain diseases, from the attacks of
parasites, and from the action of certain vegetable poisons.

Correlation is an important subject; for with species, and in a lesser
degree with domestic races, we continually find that {355} certain parts
have been greatly modified to serve some useful purpose; but we almost
invariably find that other parts have likewise been more or less modified,
without our being able to discover any advantage in the change. No doubt
great caution is necessary in coming to this conclusion, for it is
difficult to overrate our ignorance on the use of various parts of the
organisation; but from what we have now seen, we may believe that many
modifications are of no direct service, having arisen in correlation with
other and useful changes.

Homologous parts during their early development evince an affinity for each
other,--that is, they tend to cohere and fuse together much more readily
than other parts. This tendency to fusion explains a multitude of normal
structures. Multiple and homologous organs are especially liable to vary in
number and probably in form. As the supply of organised matter is not
unlimited, the principle of compensation sometimes comes into action; so
that, when one part is greatly developed, adjoining parts or functions are
apt to be reduced; but this principle is probably of much less importance
than the more general one of the economy of growth. Through mere mechanical
pressure hard parts occasionally affect soft adjoining parts. With plants
the position of the flowers