Chapter 10 - Inbreeding, Line Breeding And Out Breeding

Among dog breeders, wherever two or three are gathered together, there is likely to arise a discussion of inbreeding, line breeding, and out breeding and of their respective merits and dangers. When the interest is so great and the convictions are so dogmatic (no pun intended!), it is surprising that opinions are not based upon sounder information.

Some breeders outcross their dogs (or think they outcross them) through fear of the degeneracy of type that they believe results from all inbreeding. There are others who line breed (or think they do) in an effort to obtain the good results of inbreeding without assuming its alleged, and sometimes real, hazards. These men are like the Irishman who cuts off a puppy's tail a joint each day so that it will not hurt him so much as to cut it off all at once. There are yet others who, having been told that good dogs are produced by inbreeding, accept inbreeding uncritically as a matter of policy. They inbreed (and know that they inbreed) merely for inbreeding sake. A fourth class inbreed one generation to obtain their type and outcross the two following generations to restore to their stock whatever stamina may have been lost by the inbreeding process. All of these kinds of breeders sometimes produce good dogs, despite that all of the policies and reasons for them are ill founded.

There are, of course, a few breeders who know exactly what they are doing. They inbreed when inbreeding is indicated; they line breed with a purpose; and they outcross to obtain certain characteristics which they desire to annex to their stock.

A sound working knowledge of genetics and of the theory of the genes will enable one to comprehend the reasons why inbreeding, line breeding, and out breeding should produce their respective results. Without such knowledge, a real insight into inbreeding is impossible. The careful consideration of the various chapters on genetics which appear in the earlier part of this book, is essential to the fullest comprehension of what we say here.

It is necessary to keep in mind that the breeder is not mating dogs but is joining together sets of genes enclosed in their chromosome capsules.

We should know exactly what we mean when we speak of inbreeding, line breeding, and out breeding . Few breeders have any clear conception of just where one leaves off and the other begins. In fact, it is impossible to make any hard and fast definition of the three terms; first, because writers have been given to differ somewhat in their application of the words, and, secondly, because a given degree of relationship between two dogs might be greater or less than the same given relationship between two other dogs. Remote cousins may be, although they are not usually, more closely related genetically than are full brother and sister, which latter is considered to be the closest possible kinship.

It is this latter fact which has caused the loose definitions of the words. An understanding of the manner in which the genes and chromosomes carry the family traits, or drop them, from generation to generation, and of genetic probabilities, permits one to see why a brother may be so much more closely related to one full sister than to another or why King George V of England was so much more like Czar Nicholas II of Russia, his first cousin, than like Kaiser Wilhelm II of Germany, also his first cousin.

Thus the mating of cousins may be close inbreeding while the mating of full brother and sister may be only line breeding. This, it is to be understood, is not usually true and is cited only to show why it is impossible to declare exactly where line breeding becomes inbreeding.

Out breeding is, literally speaking, the mating of unrelated animals. As a matter of fact, however, within any purebred variety of dogs, absolute out breeding is all but impossible. This statement will seem a heresy and will, we hope, be challenged by many breeders. It is none the less true.

If one will examine the complete pedigrees of, let us say, the six generations of ancestors behind any two purebred dogs of any given, recognized breed, it may be asserted with confidence that one will find that the two dogs have at least one ancestor in common somewhere in the pedigrees. It is more likely that there will be several common ancestors in the six generations and that the name of one or more of them appears more than once in one or both pedigrees. An examination of their pedigrees, which included the names of the ancestors only to the great grandparental generation, may not reveal the kinship, and the breeder, in mating the purportedly unrelated dogs together, may believe that he is making an outcross.

A pedigree of twenty generations shows (with duplications) some two million ancestors; a pedigree of thirty generations shows (with duplications) more than two billion ancestors. A clerk working every day, including Sundays, and recording ten thousand names per day, would require more than five hundred years to compile such a pedigree. It is possible to produce a canine generation each year, although the average canine generation is perhaps some two to three years. Since the population of purebred dogs of all breeds of the entire world has never at any one time been a billion or even half that enormous number, it is easy to see that duplications of names would be necessary in such a pedigree. The registrations in the stud book of The American Kennel Club for all breeds of dogs have recently passed the two million mark. It is quite unnecessary to go to the pair of dogs (or was it a pair of each breed?) with which Noah herded the other animals on the Ark, to find common ancestors for any two domestic dogs.

Within every generation of every breed there are fashionable or propontent sires which are much used and which influence the breed's destiny. Not only is it unnecessary to consider thirty generation pedigrees, each with its two billion names, but a six generation pedigree of any two contemporary dogs of the same breed will" reveal the name of one or more dogs as the common ancestor of both.

Genetically, the perfect outcross would be one between two animals of the same breed, one of which was homozygous dominant for all traits and the other homozygous recessive for all traits. This kind of genetic purity does not exist in modern breeds.

It is easy, therefore, to perceive how impossible it is to make what is known as a perfect outcross within a given breed of dogs.

Elsewhere we have shown that kinship is not always genetically what it seems to be, that it is theoretically possible that even full brothers may be actually unrelated. These facts do, indeed, make out crossing theoretically possible from a genetic point of view. However, if the dog breeders who utter their glib opinions and prejudices about the comparative merits of inbreeding and out breeding have ever accepted or considered those facts, this writer is not aware of it. Out crossing as generally discussed is the mating together of animals whose pedigrees list no ancestors in common.

Line breeding is the breeding together of animals whose kinship one to another is shown by their pedigrees to be more or less remote. Inbreeding is the breeding together of closely related mates.

Both terms are loosely used and what one breeder may consider to be rather close inbreeding, another breeder may think of only as line breeding. Mating such as brother to full sister or to half sister, parents to progeny, full uncle or aunt to niece or nephew respectively, are almost universally accepted as inbreeding. The mating together of cousins (animals with one or two grandparents in common) and of animals of more remote kinship one to another, is usually considered as line breeding. What shall we say of the mating of an animal to a half-sister or half-brother of a parent, of two animals having all four grandparents in common but neither parent in common (double first cousins), of grandparent and grand-progeny.

A consideration of genetics and the genes leads to the conclusion not only that it is undesirable to draw too sharp a line of demarcation between line breeding and inbreeding, but also that such demarcation is impossible. The same relationship which may amount to mere line breeding between one pair of dogs may be intensive inbreeding between some other pair of dogs.

It is perhaps neither necessary nor desirable to abandon the terms inbreeding and out breeding ; but they do demand to be clarified and if they are to be used at all they should be used with a full awareness of their meaning.

That meaning is much deeper than anything that can be conveyed by a mere statement of the formerly recognized kinships, such as brother and sister, cousins, uncle and niece, half this and half that. In the light of biology, such kinships may mean little or may mean much.

The true relationship between animals is determined by the likeness of their chromosomes and the genes within them. When like alleles from a common ancestor come together from each gamete to form the zygote, inbreeding occurs, although the parents of that zygote may be but tenth cousins. When unlike alleles unite to form the zygote, even if the parents are full brother and sister, out breeding occurs.

Except in identical twins, which are always of the same sex and cannot be mated together, it is for practical purposes impossible to find two dogs with all genes of identical origin; and even if such a pair were found, the reduction of the basic sex cells would not, in all likelihood, bring together in the zygote the haploid sets of chromosomes to make a diploid set (except as to the sex chromosomes) exactly like that of the parents. Such a mating, if it were possible, would be the most intensive of inbreeding and would result in progeny almost exactly like the parents. Since it is not possible, the breeder who chooses to inbreed his stock intensively can approximate such mating only so far as his perception in choosing mates genetically alike, and his good fortune in the reduction of the basic sex cells will permit.

Line breeding is effective as such only insofar as it brings together in the zygote haploid sets of like genes. Inbreeding tends to bring together such like sets of genes even more than does line breeding. In its very essence, that is what inbreeding is. Line breeding, then, is only modified inbreeding, and when it serves the purpose intended it is indeed inbreeding.

Thus, the distinction between line breeding and inbreeding is a false one. If line breeding succeeds in intensifying and purifying the attributes of the line-bred strain, it is because it functions as inbreeding, and its hazards, if it accomplishes its purpose, are as great as inbreeding. If it does not succeed in intensifying and purifying the strain, and if the hazards of inbreeding are avoided, it not only is not inbreeding but, in a genetic sense, is not even line breeding.

The line breeder is like the urchin who goes to the creek the first sunshiny days of spring and is reluctant to plunge into the cold water. He splashes a foot into the stream tentatively while he stands shivering in goose-flesh on the bank. He knows that he wants to swim, yet is afraid of the coldness of the water. The in breeder taunts that the last guy in is a sissy, and dives in. He may get cold, but he has had his swim.

In breeding, as in anything else, one cannot make an omelet without breaking some eggs.

A clear comprehension of the behavior of the chromosomes and their genes enables one to realize just how and why the results of inbreeding are what they are. Such realization is impossible without a knowledge of the gene theory. The reader who has mastered that theory can and will make his own deductions from it as to the policy of inbreeding.

Long before the genes were known, before Mendel published his experiments and made public the laws he had discovered, inbreeding of plants and animals was practiced. Many of the plants are self-fertilized and even in the wild state are inbred from generation to generation. It is also fully recognized that animals in their breeding habits are not constrained by incest "taboos."

It may be argued that in the wild state natural selection eliminates the weak and the unfit, to which replication is to be made that in domestic strains of livestock, artificial selection, the will of the breeder, does or at least should do the same service to the species.

When, as a child, the senior author of this book read in Dr. Mill's The Dog in Health and Disease that statement that "he who cannot drown should not breed," it appeared to him as heartless cruelty. Now, the statement to him assumes the humane meaning which its writer intended it to convey, and to it must be added that especially ought not such a one to inbreed.

Every purebred variety of domestic livestock that we have, including all of our breeds of dogs, was produced, maintained, and developed through inbreeding. The earlier breeders who brought those varieties into being employed inbreeding empirically. It worked. They knew not why it worked. Often it failed, and they knew not why it failed.

They could make no progress without inbreeding and yet the sorry results which inbreeding often enough produced for them frightened them. They had not the theory in the light of which to analyze those results, good or bad. Our fears are all of things we do not understand. They were like children afraid of the dark.

Seeking the benefits of inbreeding, while abjuring its injuries, these early breeders invented line breeding. They, of course, did not know that line breeding, insofar as it succeeds in its purpose, is, in fact, inbreeding. They only knew that less degeneracy resulted from it, despite that improvement was likely to be less rapid. They were content to crawl ahead with line breeding rather than run forward with inbreeding, since the danger of stumbling was not so great. They made haste slowly. But they were not avoiding the pitfalls as they believed they were. They were only covering up recessive faults with hybrid-dominance. The faults remained in the stock.

Degeneracy is purgation of the strain. Inbreeding brings it to the surface where it may be skimmed off and got rid of. In the long run, degeneracy’s are a blessing to the breeder. It is by their expression in the phenotype that they can be eliminated from the stock and its purity fixed.

When absolute outcrosses do succeed in producing typical stock, the explanation lies in the phenomenon of heterosis. Heterosis, the hybridization of unrelated strains or varieties, produces great vigor and stamina for a single generation in the burying of undesirable recessive factors in each line under favorable dominants from the other line. The recessives are, however, only buried, not eliminated, and may crop out alive and kicking in subsequent generations. We cannot cheat nature as easily as we think. Mules are useful animals and their stamina is proverbial. It is just as well that they are not fertile, however, because their stamina of hybridism would be lost in their progeny as it is in the progeny of such hybrids as are able to produce offspring.

The crossing of two distinct and unrelated breeds of dogs may produce mongrels of great stamina in the first generation, mongrels whose average size at maturity will be greater than the mean size of the two parents. Such mongrelism will also bury many of the traits of each breed which have been selected and preserved through many generations. In the breeding of such mongrels, either among themselves or to purebred mates of the same breed of either parent, the stamina derived from heterosis is lost and at least some of the attributes of the purebred lines will reappear. The stamina is for the hybrid generation only, not for the strain. Any purebred attributes which are recessive are only buried and not lost, although it may require many generations of selective breeding to eliminate from the progeny of such mongrels all of the traits of the outcross parent.

The hybridizing of strains within a breed brings results which are in kind, even if not in degree, analogous to those which derive from the hybridization of breeds. This will be obvious if the reader will consider that a strain is simply a variety within a variety. Dogs are dogs; Foxhounds are Foxhounds, and are more or less distinct from other breeds of dogs; each of the old and long established packs of English Foxhounds constitutes a strain, the attributes of which distinguish its members from the Hounds of every other pack.

Foxhounds are cited as an example to show that strains are breeds within breeds, because the strains of Foxhounds are kept more distinct and pure than are the various strains of most other breeds of dogs. In most breeds there is a frequent crossing of strains, each of which is rendered less distinct by the addition of the germ plasma (we refrain from the temptation to say "blood" instead of "germ plasma ") from another strain.

The dogs of the various packs of Foxhounds could be crossed and the progeny would be Foxhounds, but would not be distinct and pure members of the strain of either of the parent packs, would derive some attributes from each, and might, through a combination of dominant factors, manifest traits which neither parent strain was known to possess. Just as Foxhound parents of distinct strains would produce Foxhounds, so Pekingese crossed to Great Danes would produce dogs, but those dogs would be neither Pekingese nor Great Danes.

The breeds have been established through inbreeding and continuous selection toward a definite end, all of the recognized and distinct varieties having been derived from the various wild varieties in combination. So the various strains have been derived from inbreeding within varieties and the continued selection for wanted attributes.

Continued hybridization of varieties tends to bring about reversion to the mean of the race, and persistent intercrossing of distinct and unrelated breeds results in a race which approximate in its type the wild races from which the domestic varieties have been derived. The Dingo, once believed to be a wild dog indigenous to Australia, is now recognized to be a feral domestic dog which has developed a uniformity of type through the crossing and intercrossing of domestic dogs reverted to the wild, and the influence of natural selection.

As the crossing of varieties tends to cause the progeny to revert to the mean of the races, so the crossing of strains tends to produce a reversion to the stock from which the separate strains have developed and to result in mediocrity.

This is not intended to imply that strains should never be crossed. Indeed, it is not intended to imply that breeds should never be crossed. It is only to warn that the re-establishment of true breeding purity after an outcross of either a strain or a breed is a tedious and difficult business.

The crossing of breeds should be undertaken only with the end in view of establishing a new breed which will fulfill some purpose or answer some need for which there is no already recognized variety. The developing and fixing of type for such a new breed requires many generations and is not a task to be lightly assumed.

The manufacture of the Airedale Terrier and of the Dober-man Pinscher breeds by the crossing, re-crossing, and intercrossing of breeds already recognized and fixed were noteworthy achievements in breed hybridization. So, from the intercrossing of other breeds, was the production of the Irish Wolfhound breed which, it is believed, had become extinct as a breed. If any valuable breeds of dogs we now possess should be wiped out, it would be possible to reproduce its type from the crossing and re-crossing of other breeds; but it would be a long and arduous process. Unless his middle name be Persistence, a breeder is well advised not to make such an effort.

So with the crossing of strains within a breed. Strains may be crossed, re-crossed, and intercrossed to produce another strain which may be better than either of the parent strains. This involves not only the crossing of strains but the deliberate, rigorous and unremitting selection of type to a definite end.

The more distinct and more nearly unrelated the strains that are crossed, the less the uniformity that is to be expected in the progeny. Outcrosses, if undertaken at all, should be with a definite purpose and it should not be expected to fulfill that purpose in a single generation. Any virtues which may be added to a strain through out crossing to another strain cannot be looked upon as inherent in the first strain until they have been purified and fixed within that strain through inbreeding. Out crossing is only to be employed as a means to an end and as a preliminary to the fixation of its good results, if any, through inbreeding.

As a matter of practical fact, the various so-called strains within the various recognized breeds of dogs are seldom distinct and uncorrupted with the germ plasma of other so-called strains. The various strains within a breed are usually all derived from some one or more mutual ancestors who are to be found not very far back in the pedigrees of all of the dogs of the breed. If these common ancestors who occur in the various strains were great dogs (and they would not have been employed as the foundation blood for strains had they not been for their own generation), there is little hazard in the so-called outcross. But the common ancestors preclude its being truly an outcross and it is only because it is not an outcross that it is not hazardous.

Bad results from out crossing can be eliminated through persistent inbreeding and selection, and the good results so derived may be retained and perpetuated by the same means. Out crossing is for the breeder who is willing to devote generations to the addition to his strain of a virtue which it lacks. If possible, that virtue should be brought into the strain through an only partial outcross, since out crossing is quite as likely to destroy virtues already possessed as to add others that are wanting and wanted.

The crossing of strains for the experienced and pertinacious breeder who knows what he wants and undertakes it for a specific end may bring to his strain the added virtues which he asks of it. It is a risky expedient for the breeder who wants immediate, first generation results or for one who does not know exactly what he is about.

If such a one succeeds in getting what he wants in the first generation from what he believes to be an outcross, it is in all likelihood because what appears to him to be an outcross is in very fact not an outcross at all, and the two so-called strains are so closely related that the good results may be attributed to line breeding rather than to out breeding .

And it is not the experienced and informed breeders who are forever experimenting with out crossing of strains. It is rather the novice and inexperienced who hope in one fell swoop to garner in one generation all of the virtues of both out crossed strains and to eliminate their faults.

Inbreeding is accepted, by those who do not know, as synonymous with degeneracy. The fear of it and the ignorant prejudices against it are widespread. Like most folk beliefs, there must be some justification for the attitude of the uninformed public and of a large part of the breeders toward the practice. In fact, there is some such justification.

The perils of inbreeding are, however, much more apparent than real. It is at once the surest means of establishing in our stock the breeding purity of desirable attributes and the means of bringing to light undesirable recessive factors which may lurk in the germ plasma of a strain. If such of those animals as show the defective traits are eliminated from the breeding program and are not permitted to reproduce themselves, generation by generation those undesirable qualities can be cast out of the strain, their appearance being less and less frequent, until the strain is finally purged of them.

Meanwhile, the desirable dominant factors are, through inbreeding, changed from hybrid-dominance to pure dominance, and desirable recessives are captured and retained. Thus is developed the prepotence of the members of the strain. For that, indeed, is what prepotence amounts to—the possession of like genes in the two haploid sets of chromosomes that form the zygote.

An animal's greatness as a breeding force depends upon that prepotence. Formerly prepotence was more or less fortuitous. We knew that a few dogs had it; most dogs lacked it—even most good dogs. Now we know what it is, how it can be built up and how it can be conserved from generation to generation.

Some of the great proponent dogs have derived from pedigrees which seem to show no close inbreeding; but the inbreeding is there—like genes have met. And when like genes meet in the zygote, inbreeding results.

It is only because closely related mates are prone to be similar in their genetic makeup, and therefore like genes are likely to meet, that inbreeding is effective. It may occur without apparent inbreeding, but it is by deliberate inbreeding of selected individuals that we are able to bring it about with comparative frequency.

It is necessary to divorce our concept of inbreeding as it applies to dogs and as it applies to our own species. In man, under our present state of civilization, inbreeding would doubtless be disastrous, despite that some of the world's finest minds have resulted from cousin marriages.

No animal except man offers any objection to incest, nor, indeed, is believed to recognize it as such. That, even in man, the taboo against it is not instinctive is proved by the frequency with which even now the laws prohibiting it are known to be violated, and by the incestuous marriage-ma tings which were obligatory among the Ptolemies, the Peruvian Incas, and the ancient Hawaiian royal lines. The early Greeks, a people not notably intellectually or physically deficient, permitted the marriage of half-brother and half-sister.

Of the numerous hypotheses thus far advanced to explain the incest taboo in the human family, by far the most satisfactory is that of Bronislaw Malinowski. In his article, "Culture," in the Encyclopedia of the Social Sciences, Malinowski posits that the violent emotions arising from sexual affection, if not limited to the father and mother, would tear apart the family. "A house divided against itself cannot stand; it will become all one thing, or all the other." The incest taboo therefore serves to preserve, not only the basic family unit, but also the intimate kin group as it is denned by the given society.

In any event, there is now no known human society, primitive or civilized, in which the taboo does not function. It was a part of the Mosaic law, and from the Hebrew religion was annexed into the Christian doctrine which has dominated the culture of Western Europe and America.

It is well that it should do so. So long as we harbor and protect the unfit, especially so long as we permit the unfit to reproduce themselves, and so long as monogamy enables the mediocre to reproduce at the same rate as does the fittest of the race, incestuous mating could not be tolerated in the human race. With scientifically supervised human marriages (or at least mating ) and a Spartan law which would render possible the destruction of the unfit, the human race could, through judicious inbreeding, mold its own betterment with the same certitude with which it molds the destinies of its domestic animals. It could breed to order savants and scientists, pugilists and pole vaulters. In a few generations of supervised inbreeding, idiocy, various insanities, predispositions to various infectious diseases, color blindness, haemophilia, and a host of ills that human flesh is heir to could be eliminated or at least greatly reduced in the general population.

This is no plea that such a regeneration of the human race should be undertaken. This book is no Huxleyan Brave New World. The mention of the possible application of inbreeding to our own Homo sapiens is made only to argue that the justified taboo of the practice, as pertains to the human species in a culture like our own, becomes a stupid prejudice when we apply it to our dogs and other domestic animals. In them, we can supervise and determine the mating ; we can breed only from the best; we can mate the few superior males to a vast number of females; and we can eliminate the even moderately unfit from our breeding programs. On no count does any objection to inbreeding in mankind apply to inbreeding of dogs.

There can be no doubt that the taboo against human incest has functioned to deter and often prevent breeders from the employment of inbreeding for the improvement of their livestock. It was only because reason prevailed over prejudice that inbreeding was employed for the very establishment of the various breeds and that it has been utilized consciously for their development.

Are there then no hazards in the inbreeding of dogs? Is inbreeding the infallible prescription for the attainment of perfection?

It is by no means so simple. There are hazards involved, but not so great as is sometimes believed. And ill-considered inbreeding is the worst possible policy in any effort to produce fine dogs. Inbreeding is a loaded gun which must be pointed in the right direction to accomplish the purpose for which it exists.

It can perpetuate and intensify the faults of a strain or of an individual dog in the same manner in which it can perpetuate and intensify the desirable attributes. And it can manifest in the phenotype faults which exist in the genotype but which cannot be located there with our present techniques.

It is generally believed that inbreeding produces general constitutional weakness and loss of stamina. The fact is that it intensifies the existing genetic traits. Inbreed to strength, and strength in even greater measure will result; inbreed to weakness, and the result is further degeneracy of type.

Whether one outcross or whether one inbreed, selection is necessary for improvement of the line. Outcrossed stock will so tend to mediocrity and the burying of unwanted recessive factors of one line under dominant factors of the other line that the rigorous selection which is necessary for the preservation of the stamina of inbred stock is impossible in outcrossed stock. But it is to be remembered that such unwanted recessives are only buried; they are not lost. They are liable to recur in subsequent generations. And the virtues which are presumed to be derived from out crossing are, insofar as the outcross is real and not merely apparent, only the hybrid dominance of certain genes, subject to disappear in another generation.

Inbreeding is not an end in itself but is only a means to an end. It is a method of holding fast to that which is good and of casting out that which is bad. It establishes homozygous dominant purity and brings into manifestation homozygous recessive alleles, good or bad, by the union of like pairs of genes.

Purebred varieties, such as our various races of dogs, have been selected for certain attributes since the origin of the breeds, and in them, animals lacking in those desirable attributes have been cast out of the breed and have not been permitted to reproduce their kind. Through constant selection, such breeds have been purged of the factors which were considered as undesirable, and as a consequence undesirable recessives are less likely to appear in purebred races, when they are inbred, than in such stocks as those of the human race, which have not been inbred and the undesired qualities eliminated.

There is a point beyond which degeneracy cannot go and the organism survive. The continuous elimination of degenerate factors from the various strains of purebred livestock has purged the races of their degenerative recessives until we may now inbreed those races without the fear of degenerative consequences. The longer the races exist, the more intensely they are bred in and in, and the more rigorously selection is maintained, the less the dangers from inbreeding. The breeders who developed the various breeds in the only manner that development was possible, which was by means of intensive inbreeding, were beset much more by the dangers attendant in that process than are the modern breeders with their established races already purged for them of those degeneracy’s they fear. Once the purgation is complete, and it is now in purebred dogs so nearly complete that the hazards are minimized, no further degeneracy from inbreeding is possible.

The degeneracy’s, which were buried in the germ plasma s and brought to the surface through inbreeding, and with which earlier breeders had to contend, inculcated in them a healthy fear of the practice and they have handed that fear on to us along with purebred and purged races in which the hazards do not for practical purposes exist at all.

The vast number of self-fertilizing plants (that is, inbred in every generation since the origin of the species) have been purged of their degeneracy’s through natural selection which destroyed the organisms which were unable to cope with their environments. The artificial selection of the breeder prevents from reproduction domestic plants and animals which manifest attributes which fail to please him or serve his ends.

An experiment in the prolonged inbreeding of white rats has been carried on by Miss King, a worker at the Wistar Institute. For twenty generations she bred together full brothers and sisters, which is recognized as the most intensive form of inbreeding, selecting her breeding stock constantly for its vigor and stamina. At the end of the twenty generations of intensive inbreeding, Miss King had a race of rats of greater than average size, greater fecundity, and greater longevity than the stock with which the experiment was begun. Inbreeding plus selection resulted in the improvement of the strain. Without selection, this would, of course, have been impossible.

What, then, are our conclusions about inbreeding, line breeding, and out breeding as policies for the dog breeder? First, we find that the terms, and therefore the policies, within a purebred variety are not so different as has sometimes been supposed. Outcrosses are not absolute since all typical animals of any breed are more or less closely related. In inbreeding it is only theoretically possible to bring together identical haploid sets of chromosomes, no matter how close the relationship of the mated animals. Line breeding is but a modification of inbreeding, and so far as it achieves its purpose is in very fact inbreeding.

Secondly, outcrosses, as nearly as it is possible to make them, should be made only for the purpose of annexing some desirable attribute or attributes to a strain, and they must be subsequently fixed within that strain through inbreeding.

Thirdly, inbreeding of purebred stock is not so hazardous as has been supposed, and with adequate selection for vigor, stamina, and fertility those qualities can be intensified through persistent inbreeding.

Fourthly, inbreeding and selection make for the genetic purity which results in prepotence, which can be deliberately achieved and preserved in a strain.

Fifth, and most important of all, we find that the inbreeding results in the union of like genes in the zygote. A comprehension of the theory of the chromosomes and genes, as the conveyors of its inheritance to the organism, enables us to understand why inbreeding plus selection should purify the race, produce prepotency, and enable us to produce the kind of dogs we want. All is in the genes and the chromosomes which carry them. Not to understand their behavior is to miss the point of this entire argument.

In subsequent Chapters XV and XVI, entitled respectively, "Choosing a Brood Bitch," and "Choosing a Stud Dog," we shall make further use of the facts of this chapter.

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