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
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, etc.;
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 a
deviation appears not unfrequently, and we see it in the father and
child, we cannot tell whether it may not be due to the same original
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