There are many laws regulating variation, some few of which can be dimly seen, and will be hereafter 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 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 cause acting on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent—­say, once amongst several million individuals—­and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance.