Fri. Dec 2nd, 2022

Animal Breeding the Law on Principle of Variation

Animal Breeding the Law on Principle of Variation

Animal Breeding the Law on Principle of Variation It has been noticed that in many instances the progeny is not like the parents in every. Sometimes the difference is very slight and confined to but a few particulars.

At other times it is very marked and extends to many features, both of form and characteristics. And since these variations are never entirely absent, it would seem to indicate the existence of a law or principle in heredity that produces results different from the first great law of heredity, that is, the law that like produces like.

The Law of Variation Defined.—The law of variation may be defined as the tendency in animals to produce characters which differ from those of the paternal type. It may not unfitly be termed the law that like does not always produce like.

These two laws, viz., the law that like produces like and the law of variation, would seem to be antagonistic to one another. That two such principles should be found concerned in transmission, however, is not more improbable than that the two opposing principles of good and evil should be found in the one moral nature.

These changes may relate to both form and function. Lack of capacity in the barrel of a female as compared with the same in her dam, illustrates the first, and decreased milk production in the progeny as compared with milk production in the dam, illustrates the second.

In time, these changes may become so pro- THE LAW OR PRINCIPLE OF VARIATION Is announced in certain directions as to become modifications of the systems of animals.

Such are some of the changes that follow a change of environment. Since these variations differ greatly in the time, which is necessary to produce them, and also in their in- tensity, they may be classed as gradual, or general and ordinary, and as sudden or spontaneous and extraordinary.

General Variation Defined.— General variation is that tendency to change from the original type which characterizes in a greater or a less degree all the individuals of a breed. Sometimes it is in the line of improvement and sometimes in that of retrogression.

The general direction which such variation will take will depend upon the causes which lead to it. These are given below under the proper heading. In either case it is an effort of nature to adjust the system to the surrounding conditions. But this does not explain the cause of variations that constantly occur in some degree in animals, when the conditions are as uniform as man can make them.

The causes in these instances are to be looked for from within rather than from without, and they are such as relate to dominance in transmitting properties, or in what may be termed units of transmission.

General Variation Illustrated.— Illustrations of the principle of general variation may be found: First, in the tendency of grain to deteriorate when it has fallen upon an unkindly soil. This variation affects not only the straw but also the grain. There will, likewise, -be variation in the time of maturing and in the ability to withstand disease.

Second, in the quick deterioration of the heavy breeds of sheep when confined to unproductive or rugged pastures. Such sheep cannot find sufficient sustenance on those pastures without expending more strength and energy than the system can spare.

It, therefore, seeks adjustment by reducing the size and weight of the sheep and by lessening the weight of the fleece. And the opposite is true of sheep taken to richer and more level pastures than those which they have previously been ac- customed to.

Third, the tendency to the production of fat developed in the Hereford simultaneously with a diminished production of milk.

A century ago many cows of this breed were abundant producers of milk. Now that early maturity is so much sought in this famous breed of grazing cattle, free milk production in the dams is the exception. And fourth, in the exceeding fineness of fleece developed in the Saxon Merino but at the cost of diminished vigor.

The causes which led to the pro- duction of wool so fine grew out of the management of the flocks and cannot now be further dwelt upon.

The Causes of General Variation.— The causes of general variation in animals are numerous, but chief among them are the following, viz. : Changed conditions of life, as climate, food, and general environment, also habit and dominance in internal powers of transmission. Climate, food, and environment are discussed in Chapters XX. and XXVIII.

Habit is largely the outcome of management. It is happily illustrated in the more or less permanent increase or decrease in milk production in females, based upon the general management of the same through successive generations.

The variations caused by dominance in the internal powers of transmission are less abiding. They are also less well understood, and consequently less perfectly under the control of man.

The Law or Principle of Variation. 3D These influences are so intimately connected that oftentimes it is difficult to determine what is due to each. Doubtless, in some instances, they all act in conjunction. Probably in others some of them are antagonistic ‘forces and made so by natural conditions or by management.

A happy climate and sparse food production would be an instance of the first, and abundant food production accompanied by ill-treatment or neglect an instance of the latter.

General variation will be hastened or retarded very largely in proportion as these causes act in the same direction or otherwise.

Food a Powerful Factor in Variation. —The variations in the improved breeds with reference to the increased production of meat, milk, and wool are largely due to a liberal supply of nutritious food during the period of growth. Without such aid the marked improvement made during the past century could never have been secured, or it can be maintained in the absence of such supplies.

It would be correct to say that up to a certain point, food has been more potent in affecting variation in the line of improvement than any single influence. Tn some in- stances this improvement has been realized in directions where primarily it was not sought. Illustrations are found in the breeding of sheep both in England and America.

With many of the breeds in these respective countries, the dominant aim of the breeder was to effect improvement in the production of mutton. But it was also found that improvement was affected in the production of wool.

The weight of the fleece was not only increased, but the strength of the fiber was also improved. It has also been found that when improvement has been carried beyond a certain limit in one direction, in some instances it has been followed bv retrogression in other directions.

Improvement in the beef and mutton form of cattle and sheep, respectively, has frequently been carried out so far that it has resulted in a decrease in milk pro- duction and also to a lessened power to breed well. Increased compactness of form in some of the breeds of swine has also been followed by decreased fecundity.

Spontaneous Variation Defined.— Spontaneous variation may be defined as that tendency sometimes found in animals to produce progeny more or less unlike either the parents or the ancestry of these. It differs from general variation in its violence and in- tensity, that is to say, it differs in degree, and it differs also in the greater tendency to individualization.

The changes in ordinary variation are gradual. They only become marked, when, by increase or decrease through repetition, there is, as it were, accumulation in variation. When the changes are sudden and extreme, they may be said to be violent, and when the tendency is strong in these sudden variations to reproduce and perpetuate themselves, they may be said to be intense.

Moreover, in ordinary variation the tendency to change in certain directions may affect many of the animals in a herd or even in a hived, whereas, in spontaneous variation they relate to but one animal. As with general variation, the change is sometimes in the direction of improvement and at other times in the direction of retrogression. It is also less under the control of the individual than ordinary variation.

The latter is in many of its phases measurably controlled by the breeder, but not the former since it cannot be known beforehand when it will appear nor in what form.

Spontaneous Variation not Well Understood. — But little is known definitely regarding the action of this law or the principles that control it. It is thought in some instances that spontaneous variations arise from a sudden shock given to the pregnant mother, and in other instances from mental impressions at the time of conception or during the initial stages of pregnancy. Thus, it is probable that monstrosities are sometimes produced.

These influences, as already intimated, would seem to be much more potent during the earlier stages of development in the fetus. Whether this will apply to psychical as to physical development has not been positively ascertained. But these influences do not explain nearly all the instances of spontaneous variation which occur.

They do not occur regularly or in any fixed order and since man cannot anticipate them, he is entirely helpless to prevent them.

And yet it may be true that they are as much under the control of law as transmission in the direction of likeness.

Variation. —Illustrations of the principle of spontaneous variation are found, first, in the occasional production of monstrosities. These are products of conception, sometimes alive, but more frequently not living at birth, and so malformed as to shock the sense of the fitness of things. Sometimes they are greatly defective in certain physical features of their being, and in other instances they have these in excess.

A rabbit with but one ear would furnish what may be termed a mild instance of the former, and a calf with two heads or six legs an instance of the latter. The cause of malformations is discussed in Chapter XV.

They are found, second, in the production of progeny very unlike the parents or the ancestry in color, form, and other characteristics.

A black sheep appearing in a Hock in which no animals of that color had ever appeared before would furnish an instance of the first ; a dairy bred calf possessed in a considerable degree of the essentials of beef production furnishes an instance of the second ; and a child of unusual timidity, the offspring of courageous parents and descended from a courageous ancestry, furnishes an instance of the third.

A further illustration is found in the case of a woman on exhibition in Minneapolis in 1895, who was more than eight feet high, although neither of her parents were of more than average size. They are found, third, in the various hornless breeds of cattle.

It is now considered certain that these are all descended from races which at one time were horned. This conclusion is sustained by the absence of hornless specimens in the more ancient of the geological formations in which the skeletons of cattle are found, and also by what is known regarding the origin of at least some of the hornless breeds.

Spontaneous Variations Cannot Perpetuate Themselves, —That spontaneous variations cannot perpetuate themselves unaided by man is owing largely to the infrequency with which they occur. Even under circumstances that are deemed most favorable to their production their occurrence is infrequent. So infrequent are they in some well-bred herds, that the owners may not be able to cite a single instance of such variation at all pronounced in a lifelong experience in breeding.

It is certainly fortunate that it is so, for in a large majority of instances they are disturbing factors in breeding. So infrequent are they that not with- standing the marked power which they often must reproduce themselves in they are soon obliterated through the overwhelming preponderance of blood flowing in normal channels.

Because of this it would probably be impossible for any instance of spontaneous variation to perpetuate itself so as to become a peculiarity of the breed, without the aid of man. But when man comes to the rescue^ as he has done in forming the hornless breeds, he can, through judicious selection, make the new characteristic a characteristic of all the animals of the breed. The fact, however, must not be overlooked that some forms of ordinary variation are secured and perpetuated through changed conditions.

Take, for instance, Shetland ponies to a milder climate and surroundings of improved food production, and they will increase in size even though running wild.

Variations more readily Produced in Domestic Animals.— Variations in both forms and the susceptibility to them are more readily produced in domesticated than in wild animals. This is owing to the greater changes in the conditions that surround the former. The conditions that surround wild animals are much the same from generation to generation.

Those to which domesticated animals are subjected are frequently changed, and in some instances the changes are marked. All changes in surroundings and management tend to produce variation, as previously shown, and this tendency is markedly strengthened by the admixing of alien blood elements.

That variations would multiply, therefore, as changed conditions and mixed breeding increase, is what is to be expected, and that the violence of such changes would increase with the intensity of changed conditions is a natural sequence. It is also owing to the greater resistance to variation offered by wild animals through fixity of type of long duration.

It has been shown that purity of breeding long continued is a dominant factor in producing certainty in transmission. (See page 32.) It follows, therefore, that the opposite of this will also prove true, that is to say, that the decrease in the duration of the. period of pure breeding will lead to an increase in variation. But it would not be quite safe to say that duration in breeding in any line, however long continued, would so intensify heredity that variations even spontaneous in character would never occur.

Perpetuating Variations. — In a preceding paragraph it has been shown that variations cannot perpetuate themselves unaided by man, save through changed conditions, and that even with changed conditions, spontaneous variations cannot perpetuate themselves without such aid. It is true, nevertheless, that when variations do occur, there is in them frequently an inherent tendency to reproduce and perpetuate themselves.

Particularly is this true of some forms of spontaneous variation. For instance, when, in a pure horned breed a hornless male appears, and is mated with females of the same breed, it is almost certain that a majority of the progeny will be hornless. Because of this, improvement has been made possible in breeds by selecting the desirable variants and breeding from them so as to effect further improvement.

On this principle, also, new breeds have been formed possessed of distinct peculiarities, as, for instance, one branch of the Polled Durham’s. ^Nearly all the breeds of sheep and swine that have been improved chiefly by using materials within the breed itself, have been so improved by taking ad- vantage of distinctive variations that existed at the time when the improvement began and that subsequently appeared and breeding the animals with a view to render these permanent.

JS Totably was this true of the old Dishley breed of sheep which the genius of Bakewell transformed into what has since been known as the Leicester breed. It is true nevertheless, that in a larger number of instances trans- forming power has been brought in through cross- breeding, and for the reason that the desirable variations were more readily secured in animals of another breed. But when the latter method is chosen the tendency to revert to the original type is much stronger, hence permanent improvement is slower.

This tendency to reversion is very marked in hybrid plants. For many generations do they show a tendency to reversion. Variations in type within a breed that have assumed what may be termed fixity of type, have also been used to effect improvement by fusing or intercrossing them, if the term may be thus used. It was thus that the Cruikshank Shorthorns evolved through the blending of Bates and Booth blood, accompanied by selecting a type different from either.

Variation Consonant with Highest Development. —The repeated and systematic exercise of any organ or set of organs is necessary to secure and maintain variation consonant with the highest development, as witnessed in the training of athletes. Again and again, it is necessary for them to repeat the same acts until performing them requires but little effort, unless the performance of these acts is very extreme. But even when thus secured in the individual, several generations of such training would be necessary before what may be termed a family of athletes could be produced.

Such a necessity is also shown in the development of the milk-giving function in cows. Milking qualities of the highest type could not be secured by transmission alone by selecting from cows noted for milk production and by breeding only from them.

It is also further necessary to milk them by hand so that all the milk may be taken from them, to breed them young that the energies of the system may be early turned to milk production, and to milk them for a long period that persistence in milk production may be secured.

In other words, it is necessary to strengthen the milk giving function through what may be termed repetition in milk-giving. The same is true of the strengthening of intellectual powers.

By repetition in effort the mental powers of the individual are strengthened. What is thus grained is secured in part by transmission, and through repetition in effort in the same direction, a higher level is reached. Thus, it is that nations become possessed of individual characteristics, and thus it is that they are lifted to higher levels of attainment.

Power of Transmission in Some Families. — Some families of a breed have a much greater power in transmitting their peculiarities than others, and for the reason that these have been intensified by a certain line of breeding. Illustrations may be found in the Webb Southdown’s’, in what is sometimes termed the Dishley Leicester’s and Longhorns, and in the Ben Tompkins sort of Herefords. In these respective instances families had been evolved within the breed.

They had been so evolved through the aid of in-and-in breeding, a process which speedily intensified properties. Males of correct form and qualities that have been thus bred are able to secure desirable variations in the progeny, and to render them permanent. It is this persistence of transmission which makes variation possible in improving breeds in a certain direction already established and in producing new ones. Were it otherwise, what would be gained in one generation would be lost in the next.

Regards, Coyalita

Copyright © 2021-2022 https://1-stop-sporting-goods.com All Rights Reserved

Leave a Reply

Your email address will not be published. Required fields are marked *