Animal Breeding the Law of Atavism
Animal Breeding the Law of Atavism It has been shown that by the first great law of breeding, viz., the law that like produces like, improvement may be secured in a definite line through judicious breeding.
It has also been shown that through the second law of breeding, viz., the law of variation, higher improvement may be secured when the proper steps are taken to perpetuate desirable variations and to eliminate those that are undesirable.
There is yet another feature of breeding frequently spoken of as atavism, that cannot, properly speaking, be said to come within the realm of either of the above laws. The evidence of it is so frequently apparent as to justify the conclusion that, notwithstanding the erratic character of its action, it is under the direction of law. It will, therefore, be denominated as the third law of breeding.
Definition of Atavism. —By atavism means that innate tendency in animals to revert to the original type. It is frequently spoken of as reversion, at other times as throwing back and yet again as breeding back. It differs from the law that produces like in the production of resemblances to an ancestry more or less remote rather than to the parents or to a near ancestry.
How far back these resemblances may be traced is not certainly known, but they have been traced to several, even to many generations.
It differs from the law of variation in producing resemblances to an ancestry more remote than the parents or the near ancestry, whereas the latter produces dissimilarity to the ancestry whether near or remote.
Illustrations of Atavic Transmission. — Illustrations of atavic transmission may be found:
1, In the occasional appearance of scurs or horns in the polled breeds of cattle bred pure for many generations.
2, In the Shorthorn herd books, many instances of atavic inheritance are found which appertain to color.
3, In the occasional appearance of tan-colored spots on the ears and face of the American Merino. And
4, in the occasional out-cropping of physical defects and peculiarities in the human family after the interval of generations. Scurs or miniature horns appear with more or less frequency in the polled breeds, notwithstanding the efforts of the breeders to remove them entirely.
In some instances, these efforts have been persistent for more than a century. The same is true of the white color in Shorthorns, notwithstanding the deep-rooted prejudice against this color during the past decades.
The breeders of American Merinos have sought for more than half a century to remove the tan-colored spots which at one time more or less characterized the face and ears of many individuals of the old Spanish Merino breed, and yet they appear occasionally.
Physical defects, such as, for instance, a deficiency in the proper number of fingers, have frequently appeared in descendants re- moved several generations from ancestors thus affect- ed.
Forms of Atavic Transmission.— Atavic transmission may relate to form, color, habits, mental traits, predisposition to disease, and, indeed, to any feature of the organization. The comparative frequency of the light thigh in the Hereford, notwithstanding the efforts of breeders to remove it during recent decades, illustrates atavic transmission relating to form.
The occasional appearance of a belted Galloway furnishes an illustration of atavic transmission relating to color in addition to that given above. The occasional production of a superior milking cow in breeds long bred almost wholly for beef production is an instance of atavic transmission pertaining to habit, and to function.
The love of the descendants of the Indian for a comparatively idle life and also for a roaming life after the lapse of generations and after the effort of generations to teach him habits of industry, furnishes an illustration of the atavic transmission of mental traits.
The appearance of certain tuberculous diseases after the lapse of several generations not only illustrates the atavic transmission of those diseases, but of the tendency to be the same through the intervening generations.
These illustrations could be multiplied indefinitely. Not only is it true that these peculiarities may appear in the off- spring without having appeared for many generations previously, but it is also true that they have been transmitted without the possibility of detecting even a trace of their presence, and yet it is doubtless true that the tendency to produce them was present all the while, though in what may be termed the latent form.
Other influences were doubtless present which kept those tendencies quiescent for the time being.
Atavic Transmission not Well Understood. — The laws which control atavic transmission are very imperfectly understood. Much less is known with reference to the influences which control them than with reference to those which control ordinary variation. From certain lines of breeding, as, for instance, crossbreeding, variation may be expected.
The same is true of animals of the same breed widely dissimilar in form when bred together. It could not be otherwise, since elements positively alien are blended in the first instance and elements of dissimilarity in form are fused in the second.
The attempt, at least, is made to blend in one instance and to fuse in the other. But it cannot be known beforehand just when atavic transmission will appear any more than the extent of the same may be known when it does appear.
The conclusion is legitimate then, that the laws which control atavic transmission are apparently uncertain and variable in their action. But this uncertainty and variability is doubtless only apparent. What appears to be erratic results is doubtless the outcome of influences, some of which are so subtle as to be beyond the realm of human scrutiny.
In some instances, it is not easy to distinguish between what appertains to atavic transmission from that which is the outcome of spontaneous variation. It may be that in some instances the two principles act in con- junction.
Two Classes of Atavic Transmissionism.—The observed instances of atavic transmission have been divided into two classes. To the first class is referred the reappearance of lost characters in purebreds after the interval of a number of generations. To this class would belong the reappearance of the undesired dark muzzle in Shorthorns, the reappearance of scurs in the dark faced breeds of sheep, of bristles in the improved breeds of swine, and of the red color in the Aberdeen Angus Polls.
It would not be quite correct to say that these are in no degree under the control of man, since it has been observed that the tendencies to reversion decrease with increase in the duration of the breeding in a certain line. To the second class belong those instances in crossbreeding where a peculiarity of the animal used to affect the cross appears which had not formerly occurred in the cross-bred descendants, or which had been early lost on the return to the use of a single strain upon the descendants of the cross.
For instance, when a Galloway sire is used on a pure-bred of one of the horned breeds, the horns are certainly absent in the first cross because of the greater prepotency of the Galloway blood, based, doubtless, in a considerable degree on the longer period of pure breeding in the Galloways.
The absence of horns will certainly characterize successive generations of the descendants when the use of pure Galloway blood is continued. But there might be instances when horns or scurs would again crop out. If so, they would almost certainly arise because of the crossbreeding, that is to say, because of what may be termed latent potency in the pure breed first crossed upon by the Galloway.
Such a conclusion finds countenance in the fact that atavic transmission is much less rare in the progeny of pure bred animals than in the progeny of cross breeds. Observation has also shown that atavic transmission is much less frequent in the progeny of animals of mixed breeding that are unexpectedly crossed upon by pure breeds, than if both breeds were pure when first mated.
Influences that tend to Produce Atavic Transmission. —The tendencies to atavism would seem to be strong in proportion: 1, To the want of duration.
In the purity of breeding.
2, To the lack of purity in the blood when alien pure breeds are bred together.
3, To the lack of purity in the blood when animals of the so-called pure breeds are mated, that is animals of the same breed. And
4, to the lack of prepotency on the part of the parents. Scurs appear more frequently in the Aberdeen Angus Polls than in the Galloways. This is doubtless owing to the less prolonged period during which the former was bred pure than the latter.
Pure Yorkshire swine is very rarely “off” in their color markings. The same cannot be said of pure Berkshires. The former has been bred pure for a long period. Reversion is much less common in the Merinos than in the Oxford Downs, since the former is the more ancient breed, and thus it is with various other pure breeds that may be thus contrasted. These facts point to the conclusion that sometime in the breeding of pure breeds a place will be reached where the influence of atavic transmission would practically cease as far as it relates to the original characters of the breed.
That want of purity in breeding and duration in the same increases the tendency to reversion has already been touched upon in the preceding section. The more nearly balanced then, that breeds are, in antiquity of breeding, the more numerous will be the instances of atavic transmission when they are crossed, for then the resistance to fusion would be stronger than under different conditions.
Under such circumstances it has been noticed that there is a tendency to reversion to the original traits of one breed or the other rather than to blending or fusion. The reasons cannot be satisfactorily given. With animals of a so-called pure breed of recent formation, the tendencies to atavism will be strong in proportion to the recentness of the formation of the breed.
This, at least, will be true of such breeds as are of composite blood, for then alien blood is present, and it has been shown that the blood of crossbreeds tends to increase the inclination to atavism.
That the tendencies to atavism as to variation would increase as prepotency is weak, would seem to be reasonable, since prepotency from its very nature tends to produce resemblance to the parents rather than to their ancestors, near or remote.
Alternations to Atavic Transmission. — In some forms of atavic transmission there is a tendency to alternations of generation in the inheritance and more especially with reference to certain forms of disease. Such transmission may be more or less regular in its appearance, as, for instance, in every second or third generation, or it may be irregular, owing, in part, at least, to the presence or absence of exciting causes acting upon the rudiments of yet future diseases that have been transmitted.
In many of the lower animals the alternation of generations is a fixed law of transmission. According to Miles, in a certain order of plant lice, the aphides, nine or ten generations of individuals are produced in succession before those having sexual organs that are capable of producing eggs make their appearance.
But alternations of this class vary in the number of generations which they cover in different orders.
Reversion surrounded with Difficulties. — Because of the alternations just referred to and also the apparent irregularities in transmission, the theory of reversion is surrounded with many difficulties. This irregularity may arise, in part, from the inheritance of two or more than two antagonistic characters, either one of which ma}’ become dominant in the offspring.
They may dominate in a regular or irregular alternation. That one or the other of these characters should dominate in what may be termed irregular alternation may be accounted for through the influence of natural causes, as, for instance, changed conditions, and to some extent they may be and doubtless are influenced by the character of the breeding.
And yet these influences are so subtle in their action that, heretofore, they have baffled all attempts to explain them. This, at least, is true of many of them. But the explanation just given does not sufficiently account for transmission in an alternation of generations whether the alternation be regular or irregular.
External influences of an even character should tend to produce uniformity in trans- mission. When the external evidence, therefore, are of this character, and yet there are alternations in transmission, such transmission certainly points to the conclusion that these belong as strictly to the organization through inheritance as any other part of the system.
Reversion not Spontaneous Variation.— Individual instances of reversion cannot be referred to spontaneous variation. The difference between these has already been touched upon when defining atavism. It was there explained that atavism produced re- semblances to an ancestry more or less remote, whereas spontaneous variation produced dissimilarity to the ancestry whether near or remote.
Spontaneous variations are extremely irregular, and they are not only dissimilar to the ancestry, but also to one another. In individual instances of atavic transmission, there is not only resemblance to the ancestry, but resemblances to one another are frequent in the in- stances of such transmission.
There can be no effect without a cause. The more constant the effect, the more apparent the cause is. The constancy, therefore, with which some pre-existing characters are transmitted proves the existence of definite physiological laws governing atavic transmission.
The mistake, however, must not be made that because spontaneous variations are so extremely irregular, they are not under the domain of law. But it must be acknowledged that extremely little is known in the meantime of the laws that control atavic transmission, whatever may be known of these in the future.
Theorizing on Valuation. — Some have accounted for the phenomena of all variation, including reversion, on the hypothesis that the union of two different natures in reproduction may give a result different from either. If this were true, there could be no assurance of constancy in the transmission of ancestral forms.
The first great law of breeding could then be no longer looked upon as a law. It would be of no value to the breeder because of the extreme uncertainty of the character of the trans- mission. This hypothesis is not true even in instances of spontaneous variation that are quite pronounced in character. In such instances the variation belongs only to certain features of the animal. Take, for instance, Polled Durham cattle.
One branch of the breed is purely Shorthorn. Through spontaneous variation foundation animals were secured that were hornless. Notwithstanding such variation, the Polled Durham’s have all the characteristics of Shorthorns with the exception that the horns are absent.
Such a hypothesis would imply the correctness of the theory that the elements of the organization may be again resolved into their original constituents. This would scarcely seem possible, for if it were so, accumulation in dominant properties could not be secured. The tendencies to reversion might then be as strong as those in the direction of transmitting like properties.
Dominant and Latent Characters. —In the discussion of the various forms of heredity it is necessary to distinguish between dominant and latent characters in heredity. The dominant characters include those that are prominent and obvious.
For instance, the power to transmit fine and dense wool in the Merino sheep is a dominant character. It is known to be so, since it has been observed that they always transmit wool fine and dense.
A century ago, the Spanish Merino was narrow in body and flat in the rib. American breeders have sought to widen the body and round out the rib, and with considerable success, and yet frequently individuals appear with those characteristics as they were originally. The characters which reproduce them are latent. The presence of the dominant characters are known by the constant character of the transmission.
The latent characters can only be shown to have existed by occasional transmission in the offspring. It may be true that all characters are transmitted as physiological units, some of which are dominant and others latent. This theory, if correct, would throw some light on the subject of heredity, but it would not throw any light on the causes of dominance in the various units of transmission. On much that relates to transmission the word mystery can be written, and it is the only word that will fitly apply at present, whatever the future may bring forth.
An Assemblage of Characters not Inherited. — It is not probable that the offspring inherits an assemblage of peculiarities representing the aggregate of parental characters. That it is so is shown in the inheritance of certain diseases. The morbid characteristics of one or the other parent are frequently either completely repeated or are altogether absent, and yet other prominent characteristics will be inherited from the parent whose morbid characteristics were not transmitted.
It is doubtless true, therefore, that dominant features in the offspring may be inherited from both parents, while other features in each may not be inherited to any marked degree except in what may be termed the latent form. Hence it is, that the offspring may inherit the defects of one parent and the good qualities of the other.
It also happens that a defect disappears for a number of generations and then suddenly appears. Theoretically it has been bred out, but the fact of its reappearance sIioavs that it has been transmitted all the while but has in the interval been quiescent. Atavism not necessarily
Antagonistic to Improvement.—Atavism is usually looked upon as antagonistic to improvement in breeding, but good may come from it in some instances on the principle that good may result from what is in itself an evil. So-called improvement in breeding has not always been unmixed improvement. Poland China swine have been improved as compared with their ancestors in form and feeding qualities, but in very many instances at the sacrifice of stamina. When, therefore, atavic transmission relating to inheritance of this character appears it is. distinct gain.
But it is only when improvement in some features has been accompanied by retrogression in others that any direct benefit can be gained from atavic transmission. When, however, breeders are led, through the fear of atavic transmission, to discard the use of grade sires on their farms and to avoid crossbreeding in an aimless and uncertain way, great good may result from the fact of its existence. It then becomes a rod, as it were, to whip the breeder into line who might otherwise be careless in his methods.
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