fter briefly mentioned. I will here only allude to
what may be called correlation of growth. Any change in the embryo or larva
will almost certainly entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire's
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation are
quite whimsical: thus {12} cats with blue eyes are invariably deaf; colour
and constitutional peculiarities go together, of which many remarkable
cases could be given amongst animals and plants. From the facts collected
by Heusinger, it appears that white sheep and pigs are differently affected
from coloured individuals by certain vegetable poisons. Hairless dogs have
imperfect teeth: long-haired and coarse-haired animals are apt to have, as
is asserted, long or many horns; pigeons with feathered feet have skin
between their outer toes; pigeons with short beaks have small feet, and
those with long beaks large feet. Hence, if man goes on selecting, and thus
augmenting, any peculiarity, he will almost certainly unconsciously modify
other parts of the structure, owing to the mysterious laws of the
correlation of growth.

The result of the various, quite unknown, or dimly seen laws of variation
is infinitely complex and diversified. It is well worth while carefully to
study the several treatises published on some of our old cultivated plants,
as on the hyacinth, potato, even the dahlia, &c.; and it is really
surprising to note the endless points in structure and constitution in
which the varieties and sub-varieties differ slightly from each other. The
whole organisation seems to have become plastic, and tends to depart in
some small degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the number
and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, is endless. Dr. Prosper
Lucas's treatise, in two large volumes, is the fullest and the best on this
subject. No breeder doubts how strong is the tendency to inheritance: like
produces like is his fundamental belief: doubts have been thrown on this
principle by theoretical writers alone. When any deviation of structure
often appears, and we see it in the {13} father and child, we cannot tell