The Influence of Habit

Habit also has a deciding influence, especially in animals.

For instance:

• the wing bones of the domestic duck weigh less than the leg bones compared to the same bones in the wild-duck.
  • I presume that this is because the domestic duck flies much less and walks more.
• the udders in milked cows and goats are greatly developed compared to those that are not so milked.

All countries have a domestic animal with drooping ears.

• Some suggest that the drooping is from the disuse of the ear muscles because the animals are not alarmed by danger.

The laws regulating variation: correlation of growth

Any change in the embryo or larva will entail changes in the mature animal.

In monstrosities, there are correlations between quite distinct parts.

• Isidore Geoffroy St. Hilaire has a 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:

• Cats with blue eyes are deaf.

Heusinger writes 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 tend to have long or many horns
• pigeons with feathered feet have skin between their outer toes
• pigeons with short beaks have small feet
• pigeons with long beaks large feet

Hence, if we change any peculiarity, we will unconsciously modify other parts due to the mysterious laws of the correlation of growth.

There are 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
• depart somewhat 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 long treatise is the fullest and the best on this subject.

• He believes that like produces like.

This has beem doubted by some, since deviations appear frequently.

We see it in the father and child. We cannot tell if it is due to the same original cause acting on both.

Assume that a parents gets a rare deviation of 1 in 1 million which then its child also gets.

• The mere doctrine of chances almost compels us to attribute its reappearance to inheritance.

Cases of albinism, prickly skin, hairy bodies, &c., appear in several members of the same family.

If strange and rare deviations of structure are truly inherited, commoner deviations may be admitted to be inheritable.

We should look at:

• the inheritance of every character as the rule
• non-inheritance as the anomaly.

The laws governing inheritance are unknown.

No one can say:

• why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not
• why the child often gets certain traits from his grandparents or other ancestors
• why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more commonly but not exclusively to the like sex.

Peculiarities appearing in the males of our domestic breeds are often transmitted either exclusively, or in a much greater degree, to males alone.

A more important and trustworthy rule is that, at whatever period of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes earlier.

In many cases, this could not be otherwise.

Thus, the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage.

But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology.

These remarks are of course confined to the first appearance of the peculiarity, and not to its primary cause, which may have acted on the ovules or male element; in nearly the same manner as in the crossed offspring from ashort-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is clearly due to the male element.