THE LAW OF SIMILARITY.
The first and most important of the laws to be considered in this
connection is that of SIMILARITY. It is by virtue of this law that the
peculiar characters, qualities and properties of the parents, whether
external or internal, good or bad, healthy or diseased, are
transmitted to their offspring. This is one of the plainest and most
certain of the laws of nature. Children resemble their parents, and
they do so because these are hereditary. The law is constant. Within
certain limits progeny always and every where resemble their parents.
If this were not so, there would be no constancy of species, and a
horse might beget a calf or a sow have a litter of puppies, which is
never the case,--for in all time we find repeated in the offspring the
structure, the instincts and all the general characteristics of the
parents, and never those of another species. Such is the law of nature
and hence the axiom that "like produces like." But while experience
teaches the constancy of hereditary transmission, it teaches just as
plainly that the constancy is not absolute and perfect, and this
introduces us to another law, viz: that of variation, which will be
considered by and by; our present concern is to ascertain what we can
of the law of similarity.
The lesson which this law teaches might be stated in five words, to
wit: _Breed only from the best_--but the teaching may be more
impressive, and will more likely be heeded, if we understand the
extent and scope of the law.
Facts in abundance show the hereditary tendency of physical, mental
and moral qualities in men, and very few would hesitate to admit that
the external form and general characteristics of parents descend to
children in both the human and brute races; but not all are aware that
this law reaches to such minute particulars as facts show to be the
case.
We see hereditary transmission of a peculiar type upon an extensive
scale, in some of the distinct races, the Jews, and the Gypsies, for
example. Although exposed for centuries to the modifying influences of
diverse climates, to association with peoples of widely differing
customs and habits, they never merge their peculiarities in those of
any people with whom they dwell, but continue distinct. They retain the
same features, the same figures, the same manners, customs and habits.
The Jew in Poland, in Austria, in London, or in New York, is the same;
and the money-changers of the Temple at Jerusalem in the time of our
Lord may be seen to-day on change in any of the larger marts of trade.
How is this? Just because the Jew is a "thorough-bred." There is with
him no intermarriage with the Gentile--no crossing, no mingling of his
organization with that of another. When this ensues "permanence of
race" will cease and give place to variations of any or of all sorts.
Some families are remarkable during long periods for tall and handsome
figures and striking regularity of features, while in others a less
perfect form, or some peculiar deformity reappears with equal
constancy. A family in Yorkshire is known for several generations to
have been furnished with six fingers and toes. A family possessing the
same peculiarity resides in the valley of the Kennebec, and the same
has reappeared in one or more other families connected with it by
marriage.
The thick upper lip of the imperial house of Austria, introduced by
the marriage of the Emperor Maximillian with Mary of Burgundy, has
been a marked feature in that family for hundreds of years, and is,
visible in their descendants to this day. Equally noticeable is the
"Bourbon nose" in the former reigning family of France. All the Barons
de Vessins had a peculiar mark between their shoulders, and it is said
that by means of it a posthumous son of a late Baron de Vessins was
discovered in a London shoemaker's apprentice. Haller cites the case
of a family where an external tumor was transmitted from father to son
which always swelled when the atmosphere was moist.
A remarkable example of a singular organic peculiarity and of its
transmission to descendants, is furnished in the case of the English
family of "Porcupine men," so called from having all the body except
the head and face, and the soles and palms, covered with hard
dark-colored excrescences of a horny nature. The first of these was
Edward Lambert, born in Suffolk in 1718, and exhibited before the
Royal Society when fourteen years of age. The other children of his
parents were naturally formed; and Edward, aside from this
peculiarity, was good looking and enjoyed good health. He afterward
had six children, all of whom inherited the same formation, as did
also several grand-children.
Numerous instances are on record tending to show that even accidents
do sometimes, although not usually, become hereditary. Blumenbach
mentions the case of a man whose little finger was crushed and twisted
by an accident to his right hand. His sons inherited right hands with
the little finger distorted. A bitch had her hinder parts paralyzed
for some days by a blow. Six of her seven pups were deformed, or so
weak in their hinder parts that they were drowned as useless. A
pregnant cat got her tail injured; in each of her five kittens the
tail was distorted, and had an enlargement or knob near the end of
each. Horses marked during successive generations with red-hot irons
in the same place, transmit visible traces of such marks to their
colts.
Very curious are the facts which go to show that acquired habits
sometimes become hereditary. Pritchard, in his "Natural History of
Man," says that the horses bred on the table lands of the Cordilleras
"are carefully taught a peculiar pace which is a sort of running
amble;" that after a few generations this pace becomes a natural one;
young untrained horses adopting it without compulsion. But a still
more curious fact is, that if these domesticated stallions breed with
mares of the wild herd, which abound in the surrounding plains, they
"become the sires of a race in which the ambling pace is natural and
requires no teaching."
Mr. T.A. Knight, in a paper read before the Royal Society, says, "the
hereditary propensities of the offspring of Norwegian ponies, whether
full or half-bred, are very singular. Their ancestors have been in the
habit of _obeying the voice_ of their riders and _not the bridle_; and
horse-breakers complain that it is impossible to produce this last
habit in the young colts. They are, however, exceedingly docile and
obedient when they understand the commands of their masters."
A late writer in one of the foreign journals, says that he had a "pup
taken from its mother at six weeks old, who although never taught to
'beg' (an accomplishment his mother had been taught) spontaneously
took to begging for every thing he wanted when about seven or eight
months old; he would beg for food, beg to be let out of the room, and
one day was found opposite a rabbit hutch apparently begging the
rabbits to come and play."
If even in such minute particulars as these, hereditary transmission
may be distinctly seen, it becomes the breeder to look closely to the
"like" which he wishes to see reproduced. Judicious selection is
indispensable to success in breeding, and this should have regard to
_every particular_--general appearance, length of limb, shape of
carcass, development of chest; if in cattle, the size, shape and
position of udder, thickness of skin, "touch," length and texture of
hair, docility, &c., &c.; if in horses, their adaptation to any
special excellence depending on form, or temperament, or nervous
energy.
Not only should care be taken to avoid _structural defects_, but
especially to secure freedom from _hereditary diseases_, as both
defects and diseases appear to be more easily transmissible than
desirable qualities. There is often no obvious peculiarity of
structure, or appearance, indicating the possession of diseases or
defects which are transmissible, and so, special care and continued
acquaintance are necessary in order to be assured of their absence in
breeding animals; but such a tendency although invisible or
inappreciable to cursory observation, must still, judging from its
effects, have as real and certain an existence, as any peculiarity of
form or color.
Every one who believes that a disease may be hereditary at all, must
admit that certain individuals possess certain tendencies which render
them especially liable to certain diseases, as consumption or
scrofula; yet it is not easy to say precisely in what this
predisposition consists. It seems probable, however, that it may be
due either to some want of harmony between different organs, some
faulty formation or combination of parts, or to some peculiar physical
or chemical condition of the blood or tissues; and that this altered
state, constituting the inherent congenital tendency to the disease,
is duly transmitted from parent to offspring like any other quality
more readily apparent to observation.
Hereditary diseases exhibit certain eminently characteristic
phenomena, which a late writer[2] enumerates as follows:
1. "They are transmitted by the male as well as by the female
parent, and are doubly severe in the offspring of parents both
of which are affected by them.
2. They develop themselves not only in the immediate progeny of
one affected by them, but also in many subsequent generations.
3. They do not, however, always appear in each generation in the
same form; one disease is sometimes substituted for another,
analogous to it, and this again after some generations becomes
changed into that to which the breed was originally liable--as
phthisis (consumption) and dysentery. Thus, a stock of cattle
previously subject to phthisis, sometimes become affected for
several generations with dysentery to the exclusion of phthisis,
but by and by, dysentery disappears to give place to phthisis.
4. Hereditary diseases occur to a certain extent independently
of external circumstances; appearing under all sorts of
management, and being little affected by changes of locality,
separation from diseased stock, or such causes as modify the
production of non-hereditary diseases.
5. They are, however, most certainly and speedily developed in
circumstances inimical to general good health, and often occur
at certain, so called, critical periods of life, when unusual
demands on the vital powers take place.
6. They show a striking tendency to modify and absorb into
themselves all extraneous diseases; for example, in an animal of
consumptive constitution, pneumonia seldom runs its ordinary
course, and when arrested, often passes into consumption.
7. Hereditary diseases are less effectually treated by ordinary
remedies than other diseases. Thus, although an attack of
phthisis, rheumatism or opthalmia may be subdued, and the
patient put out of pain and danger, the tendency to the disease
will still remain and be greatly aggravated by each attack.
In horses and neat cattle, hereditary diseases do not usually
show themselves at birth, and sometimes the tendency remains
latent for many years, perhaps through one or two generations
and afterwards breaks out with all its former severity."
The diseases which are found to be hereditary in horses are scrofula,
rheumatism, rickets, chronic cough, roaring, ophthalmia or
inflammation of the eye,--grease or scratches, bone spavin, curb, &c.
Indeed, Youatt says, "there is scarcely a malady to which the horse is
subject, that is not hereditary. Contracted feet, curb, spavin,
roaring, thick wind, blindness, notoriously descend from the sire or
dam to the foal."
The diseases which are found hereditary in neat cattle are scrofula,
consumption, dysentery, diarrhea, rheumatism and malignant tumors.
Neat cattle being less exposed to the exciting causes of disease, and
less liable to be overtasked or exposed to violent changes of
temperature, or otherwise put in jeopardy, their diseases are not so
numerous, and what they have are less violent than in the horse, and
generally of a chronic character.
Scrofula is not uncommon among sheep, and it presents itself in
various forms. Sometimes it is connected with consumption; sometimes
it affects the viscera of the abdomen, and particularly the mesenteric
glands in a manner similar to consumption in the lungs. The scrofulous
taint has been known to be so strong as to affect the foetus, and
lambs have occasionally been born with it, but much oftener they show
it at an early age, and any affected in this way are liable to fall an
easy prey to any ordinary or prevailing disease which develops in such
with unusual severity. Sheep are also liable to several diseases of
the brain and of the respiratory and digestive organs. Epilepsy, or
"fits," and rheumatism sometimes occur.
Swine are subject to nearly the same hereditary diseases as sheep.
Epilepsy is more common with them than with the latter, and they are
more liable to scrofula than any other domestic animals.
When properly and carefully managed, swine are not ordinarily very
liable to disease, but when, as too often kept in small, damp, filthy
styes, and obliged constantly to inhale noxious effluvia, and to eat
unsuitable food, we cannot wonder either that they become victims of
disease or transmit to their progeny a weak and sickly organization.
Swine are not naturally the dirty beasts which many suppose.
"Wallowing in the mire," so proverbial of them, is rather from a wish
for protection from insects and for coolness, than from any inherent
love of filth, and if well cared for they will be comparatively
cleanly.
The practice of close breeding, which is probably carried to greater
extent with swine than with any other domestic animal, undoubtedly
contributes to their liability to hereditary diseases, and when those
possessing any such diseases are coupled, the ruin of the stock is
easily and quickly effected, for as already stated, they are
propagated by either parent, and always most certainly and in most
aggravated form, when occurring in both.
With regard to hereditary diseases, it is eminently true that "an
ounce of prevention is worth a pound of cure." As a general and almost
invariable rule, animals possessing either defects or a tendency to
disease should not be employed for breeding. If, however, for special
reasons it seems desirable to breed from one which has some slight
defect of symmetry, or a faint tendency to disease, although for the
latter it is doubtful if the possession of any good qualities can
fully compensate, it should be mated with one which excels in every
respect in which the other is deficient, and on no account with one
which is near of kin to it.
Notwithstanding the importance due to the subject of hereditary
diseases, it is also true that few diseases invariably owe their
development to hereditary causes. Even such as are usually hereditary
are sometimes produced accidentally, (as of course there must be a
beginning to everything,) and in such case, they may, or may not be,
transmitted to their progeny. As before shown, it is certain that they
sometimes are, which is sufficient reason to avoid such for breeding
purposes. It is also well known that, in the horse, for instance,
certain forms of limbs predispose to certain diseases, as bone spavin
is most commonly seen where there is a disproportion in the size of
the limb above and below the hock, and others might be named of
similar character; in all such cases the disease may be caused by an
agency which would be wholly inadequate in one of more perfect form,
but once existing, it is liable to be reproduced in the offspring--all
tending to show the great importance of _giving due heed_ in selecting
breeding animals _to all qualities, both external and internal_, so
long as "like produces like."
FOOTNOTES:
[2] Finlay Dun, V.S., in Journal of the Royal Agricultural Society.