Mendelism Part 7

In this way we avoid a difficulty that beset the older view. For on that view no new character could be developed except by the piling up of minute variations through the action of natural selection. Consequently any character found in animals and plants must be supposed to be of some definite use to the individual. Otherwise it could not have developed through the action of natural selection. But there are plenty of characters to which it is exceedingly difficult to ascribe any utility, and the ingenuity of the supporters of this view has often been severely taxed to account for their existence. On the more modern view this difficulty is avoided. The origin of a new variation is independent of natural {143} selection, and provided that it is not directly harmful there is no reason why it should not persist. In this way we are released from the burden of discovering a utilitarian motive behind all the mult.i.tudinous characters of living organisms. For we now recognise that the function of natural selection is selection and not creation. It has nothing to do with the formation of the new variation. It merely decides whether it is to survive or to be eliminated.

One of the arguments made use of by supporters of the older view is that drawn from the study of adaptation. Animals and plants are as a rule remarkably well adapted to living the life which their surroundings impose upon them, and in some cases this adaptation is exceedingly striking.

Especially is this so in the many instances of what is called protective coloration, where the animal comes to resemble its surroundings so closely that it may reasonably be supposed to cheat even the keenest sighted enemy.

Surely, we are told, such perfect adaptation could hardly have arisen through the mere survival of chance sports. Surely there must be some guiding hand moulding the species into the required shape. The argument is an old one. For John Ray that guiding hand was the superior wisdom of the Creator: for the modern Darwinian it is Natural Selection controlling the direction of variation. Mendelism certainly offers no suggestion of any such controlling force. It interprets the {144} variations of living forms in terms of definite physiological factors, and the diversity of animal and plant life is due to the gain or loss of these factors, to the origination of new ones, or to fresh combinations among those already in existence. Nor is there any valid reason against the supposition that even the most remarkable cases of resemblance, such as that of the leaf insect, may have arisen through a process of mutation. Experience with domestic plants and animals shows that the most bizarre forms may arise as sports and perpetuate themselves. Were such forms, arising under natural conditions, to be favoured by natural selection owing to a resemblance to something in their environment we should obtain a striking case of protective adaptation. And here it must not be forgotten that those striking cases to which our attention is generally called are but a very small minority of the existing forms of life.

For that special group of adaptation phenomena cla.s.sed under the head of Mimicry, Mendelism seems to offer an interpretation simpler than that at present in vogue. This perhaps may be more clearly expressed by taking a specific case. There is in Africa a genus of Danaine b.u.t.terflies known as _Amauris_, and there are reasons for considering that the group to which it belongs possesses properties which render it unpalatable to vertebrate enemies such as birds or monkeys. In the same region is also found the genus _Euralia_ belonging to the entirely {145} different family of the Nymphalidae, to which there is no evidence for a.s.signing the disagreeable properties of the Danaines. Now the different species of _Euralia_ show remarkably close resemblances to the species of _Amauris_, which are found flying in the same region, and it is supposed that by "mimicking" the unpalatable forms they impose upon their enemies and thereby acquire immunity from attack. The point at issue is the way in which this seemingly purposeful resemblance has been brought about.

One of the species of _Euralia_ occurs in two very distinct forms (Pl.

VI.), which were previously regarded as separate species under the names _E. wahlbergi_ and _E. mima_. These two forms respectively resemble _Amauris dominica.n.u.s_ and _A. echeria_. For purposes of argument we will a.s.sume _A. echeria_ to be the more recent form of the two. On the modern Darwinian view certain individuals of _A. dominica.n.u.s_ gradually diverged from the _dominica.n.u.s_ type and eventually reached the _echeria_ type, though why this should have happened does not appear to be clear. At the same time those specimens which tended to vary in the direction of _A.

echeria_ in places where this species was more abundant than _A.

dominica.n.u.s_ were encouraged by natural selection, and under its guiding hand the form _mima_ eventually arose from _wahlbergi_.

According to Mendelian views, on the other hand, {146} _A. echeria_ arose suddenly from _A. dominica.n.u.s_ (or _vice versa_), and similarly _mima_ arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was common and _A. dominica.n.u.s_ was rare, its resemblance to the more plentiful distasteful form would give it the advantage over _wahlbergi_ and allow it to establish itself in place of the latter. On the modern Darwinian view natural selection gradually shapes _wahlbergi_ into the _mima_ form owing to the presence of _A. echeria_; on the Mendelian view natural selection merely conserves the _mima_ form when once it has arisen. Now this case of mimicry is one of especial interest, because we have experimental evidence that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one, though at present it is uncertain which is the dominant and which the recessive form. The two have been proved to occur in families bred from the same female without the occurrence of any intermediates, and the fact that the two segregate cleanly is strong evidence in favour of the Mendelian view. On this view the genera _Amauris_ and _Euralia_ contain a similar set of pattern factors, and the conditions, whatever they may be, which bring about mutation in the former lead to the production of a similar mutation in the latter. Of the different forms of _Euralia_ produced in any region that one has the best chance of survival, through the operation of natural selection, which resembles the most plentiful _Amauris_ form. Mimetic resemblance is a true phenomenon, but natural selection plays the part of a conservative, not of a formative agent.

[Ill.u.s.tration: PLATE VI.]

{147}

It is interesting to recall that in earlier years Darwin was inclined to ascribe more importance to "sports" as opposed to continuous minute variation, and to consider that they might play a not inconsiderable part in the formation of new varieties in nature. This view, however, he gave up later, because he thought that the relatively rare sport or mutation would rapidly disappear through the swamping effects of crossing with the more abundant normal form, and so, even though favoured by natural selection, would never succeed in establis.h.i.+ng itself. Mendel's discovery has eliminated this difficulty. For suppose that the sport differed from the normal in the loss of a factor and were recessive. When mated with the normal this character would seem to disappear, though, of course, half of the gametes of its progeny would bear it. By continual crossing with normals a small proportion of heterozygotes would eventually be scattered among the population, and as soon as any two of these mated together the recessive sport would appear in one quarter of their offspring.

A suggestive contribution to this subject was recently made by G. H. Hardy.

Considering the distribution of a single factor in a mixed population consisting of the heterozygous and the two h.o.m.ozygous forms he showed that such a population breeding at random rapidly fell into a {148} stable condition with regard to the proportion of these three forms, whatever may have been the proportion of the three forms to start with. Let us suppose for instance, that the population consists of p h.o.m.ozygotes of one kind, r h.o.m.ozygotes of the other kind, and 2 q heterozygotes. Hardy pointed out that, other things being equal, such a population would be in equilibrium for this particular factor so long as the condition q^2 = pr was fulfilled.

If the condition is fulfilled to start with, the population remains in equilibrium. If the condition is not fulfilled to start with, Hardy showed that a position of equilibrium becomes established after a single generation, and that this position is thereafter maintained. The proportions of the three cla.s.ses which satisfy the equation q^2 = pr are exceedingly numerous, and populations in which they existed in the proportions shown in the appended table would remain in stable equilibrium generation after generation:--

p. 2q. r.

1 2 1 1 4 4 1 6 9 1 8 16 1 20,000 100,000,000 1 2n n^2

This, of course, a.s.sumes that all three cla.s.ses are equally fertile, and that no form of selection is taking place to the {149} benefit of one cla.s.s more than of another. Moreover, it makes no difference whether p represents the h.o.m.ozygous dominants or whether it stands for the recessives. A population containing a very small proportion of dominants and one containing a similar proportion of recessives are equally stable. The term dominant is in some respects apt to be misleading, for a dominant character cannot in virtue of its dominance establish itself at the expense of a recessive one. Brown eyes in man are dominant to blue, but there is no reason to suppose that as years go on the population of these islands will become increasingly brown eyed. Given equality of conditions both are on an equal footing. If, however, either dominant or recessive be favoured by selection the conditions are altered, and it can be shown that even a small advantage possessed by the one will rapidly lead to the elimination of the other. Even with but a 5 per cent selection advantage in its favour it can be shown that a rare sport will oust the normal form in a few hundred generations. In this way we are freed from a difficulty inherent in the older view that varieties arose through a long-continued process involving the acc.u.mulation of very slight variations. On that view the establis.h.i.+ng of a new type was of necessity a very long and tedious business, involving many thousands of generations. For this reason the biologist has been accustomed to demand a very large supply of time, often a great deal more than the physicist is {150} disposed to grant, and this has sometimes led him to expostulate with the latter for cutting off the supply. On the newer views, however, this difficulty need not arise, for we realise that the origin and establis.h.i.+ng of a new form may be a very much more rapid process than has. .h.i.therto been deemed possible.

One last question with regard to evolution. How far does Mendelism help us in connection with the problem of the origin of species? Among the plants and animals with which we have dealt we have been able to show that distinct differences, often considerable, in colour, size, and structure, may be interpreted in terms of Mendelian factors. It is not unlikely that most of the various characters which the systematist uses to mark off one species from another, the so-called specific characters, are of this nature. They serve as convenient labels, but are not essential to the conception of species. A systematist who defined the wild sweet pea could hardly fail to include in his definition such characters as the proc.u.mbent habit, the tendrils, the form of the pollen, the shape of the flower, and its purple colour. Yet all these and other characters have been proved to depend upon the presence of definite factors which can be removed by appropriate crossing. By this means we can produce a small plant a few inches in height with an erect habit of growth, without tendrils, with round instead of oblong pollen, and with colourless deformed flowers quite different {151} in appearance from those of the wild form. Such a plant would breed perfectly true, and a botanist to whom it was presented, if ignorant of its origin, might easily relegate it to a different genus.

Nevertheless, though so widely divergent in structure, such a plant must yet be regarded as belonging to the species _Lathyrus odoratus_. For it still remains fertile with the many different varieties of sweet pea. It is not visible attributes that const.i.tute the essential difference between one species and another. The essential difference, whatever it may be, is that underlying the phenomenon of sterility. The visible attributes are those made use of by the systematist in cataloguing the different forms of animal and plant life, for he has no other choice. But it must not be forgotten that they are often misleading. Until they were bred together _Euralia wahlbergi_ and _E. mima_ were regarded as perfectly valid species, and there is little doubt that numbers of recognised species will eventually fall to the ground in the same way as soon as we are in a position to apply the test of breeding. Mendelism has helped us to realise that specific characters may be but incidental to a species--that the true criterion of what const.i.tutes a species is sterility, and that particular form of sterility which prevents two healthy gametes on uniting from producing a zygote with normal powers of growth and reproduction. For there are forms of sterility which are purely mechanical. The pollen of _Mirabilis jalapa_ cannot fertilise _M._ {152} _longiflora_, because the pollen tubes of the former are not long enough to penetrate down to the ovules of the latter.

Hybrids can nevertheless be obtained from the reciprocal cross. Nor should we expect offspring from a St. Bernard and a toy terrier without recourse to artificial fertilisation. Or sterility may be due to pathological causes which prevent the gametes from meeting one another in a healthy state. But in most cases it is probable that the sterility is due to some other cause.

It is not inconceivable that definite differences in chemical composition render the protoplasm of one species toxic to the gametes of the other, and if this is so it is not impossible that we may some day be able to express these differences in terms of Mendelian factors. The very nature of the case makes it one of extreme difficulty for experimental investigation. At any rate, we realise more clearly than before that the problem of species is not one that can be resolved by the study of morphology or of systematics. It is a problem in physiology.

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CHAPTER XIV

ECONOMICAL

Since heredity lies at the basis of the breeder's work, it is evident that any contribution to a more exact knowledge of this subject must prove of service to him, and there is no doubt that he will be able to profit by Mendelian knowledge in the conduct of his operations. Indeed, as we shall see later, these ideas have already led to striking results in the raising of new and more profitable varieties. In the first place, heredity is a question of individuals. Ident.i.ty of appearance is no sure guide to reproductive qualities. Two individuals similarly bred and indistinguishable in outward form may nevertheless behave entirely differently when bred from. Take, for instance, the family of sweet peas shown on Plate IV. The F_2 generation here consists of seven distinct types, three sorts of purples, three sorts of reds, and whites. Let us suppose that our object is to obtain a true breeding strain of the pale purple picotee form. Now from the proportions in which they come we know that the dilute colour is due to the absence of the factor which intensifies the colour. Consequently the picotee cannot throw the {154} two deeper shades of red or purple. But it may be heterozygous for the purpling factor when it will throw the dilute red (tinged white), or it may be heterozygous for either or both of the two colour factors (cf. p. 44), in which case it will throw whites. Of the picotees which come in such a family, therefore, some will give picotees, tinged whites, and whites, others will give picotees and tinged whites only, others will give picotees and whites only, while others, again, and these the least numerous, will give nothing but picotees. The new variety is already fixed in a certain definite proportion of the plants; in this particular instance in 1 out of every 27. All that remains to be done is to pick out these plants. Since all the picotees look alike, whatever their breeding capacity, the only way to do this is to save the seed from a number of such plants _individually_, and to raise a further generation. Some of them will be found to breed true. The variety is then established, and may at once be put on the market with full confidence that it will hereafter throw none of the other forms.

The all-important thing is to save and sow the seed of separate individuals separately. However alike they look, the seed from different individuals must on no account be mixed. Provided that due care is taken in this respect no long and tedious process of selection is required for the fixation of any given variety. Every possible variety arising from a cross appears in the F_2 generation if only a sufficient {155} number is raised, and of all these different varieties a certain proportion of each is already fixed. Heredity is a question of individuals, and the recognition of this will save the breeder much labour, and enable him to fix his varieties in the shortest possible time.

Such cases as these of the sweet pea throw a fresh light upon another of the breeder's conceptions, that of purity of type. Hitherto the criterion of a "pure-bred" thing, whether plant or animal, has been its pedigree, and the individual was regarded as more or less pure bred for a given quality according as it could show a longer or shorter list of ancestors possessing this quality. To-day we realise that this is not essential. The pure-bred picotee appears in our F_2 family though its parent was a purple bicolor, and its remoter ancestors whites for generations. So also from the cross between pure strains of black and albino rabbits we may obtain in the F_2 generation animals of the wild agouti colour which breed as true to type as the pure wild rabbit of irreproachable pedigree. The true test of the pure breeding thing lies not in its ancestry but in the nature of the gametes which have gone to its making. Whenever two similarly const.i.tuted gametes unite, whatever the nature of the parents from which they arose, the resulting individual is h.o.m.ozygous in all respects and must consequently breed true. In deciding questions of purity it is to the gamete, and not to ancestry, that our appeal must henceforth be made. {156}

Improvement is after all the keynote to the breeder's operations. He is aiming at the production of a strain which shall combine the greatest number of desirable properties with the least number of undesirable ones.

This good quality he must take from one strain, that from another, and that again from a third, while at the same time avoiding all the poor qualities that these different strains possess. It is evident that the Mendelian conception of characters based upon definite factors which are transmitted on a definite scheme must prove of the greatest service to him. For once these factors have been determined, their distribution is brought under control, and they can be a.s.sociated together or dissociated at the breeder's will. The chief labour involved is that necessary for the determination of the factors upon which the various characters depend. For it often happens that what appears to be a simple character turns out when a.n.a.lysed to depend upon the simultaneous presence of several distinct factors. Thus the Malay fowl breeds true to the walnut comb, as does also the Leghorn to the single comb, and when pure strains are crossed all the offspring have walnut combs. At first sight it would be not unnatural to regard the difference as dependent upon the presence or absence of a single factor. Yet, as we have already seen, two other types of comb, the pea and the rose, make their appearance in the F_2 generation. a.n.a.lysis shows that the difference between the walnut {157} and the single is a difference of two factors, and it is not until this has been determined that we can proceed with certainty to transfer the walnut character to a single-combed breed. Moreover, in his process of a.n.a.lysis the breeder must be prepared to encounter the various phenomena that we have described under the headings of interaction of factors, coupling and repulsion, and the recognition of these phenomena will naturally influence his procedure. Or again, his experiments may show him that one of the characters he wants, like the blue of the Andalusian fowl, is dependent upon the heterozygous nature of the individual which exhibits it, and if such is the case he will be wise to refrain from any futile attempt at fixing it. If it is essential it must be built up again in each generation, and he will recognise that the most economical way of doing this is to cross the two pure strains so that all the offspring may possess the desired character. The labour of a.n.a.lysis is often an intricate and tedious business. But once done it is done once for all. As soon as the various factors are determined, upon which the various characters of the individual depend, as soon as the material to be made use of has been properly a.n.a.lysed, the production and fixation of the required combinations becomes a matter of simple detail.

An excellent example of the practical application of Mendelian principles is afforded by the experiments which Professor Biffen has recently carried out in Cambridge. {158} Taken as a whole English wheats compare favourably with foreign ones in respect of their cropping power. On the other hand, they have two serious defects. They are liable to suffer from the attacks of the fungus which causes rust, and they do not bake into a good loaf.

This last property depends upon the amount of gluten present, and it is the greater proportion of this which gives to the "hard" foreign wheat its quality of causing the loaf to rise well when baked. For some time it was held that "hard" wheat with a high glutinous content could not be grown in the English climate, and undoubtedly most of the hard varieties imported for trial deteriorated greatly in a very short time. Professor Biffen managed to obtain a hard wheat which kept its qualities when grown in England. But in spite of the superior quality of its grain from the baker's point of view its cropping capacity was too low for it to be grown profitably in compet.i.tion with English wheats. Like the latter, it was also subject to rust. Among the many varieties which Professor Biffen collected and grew for observation he managed to find one which was completely immune to the attacks of the rust fungus, though in other respects it had no desirable quality to recommend it. Now as the result of an elaborate series of investigations he was able to show that the qualities of heavy cropping capacity, "hardness" of grain, and immunity to rust can all be expressed in terms of Mendelian factors. Having once a.n.a.lysed his material {159} the rest was comparatively simple, and in a few years he has been able to build up a strain of wheat which combines the cropping capacity of the best English varieties with the hardness of the foreign kinds, and at the same time is completely immune to rust. This wheat has already been shown to keep its qualities unchanged for several years, and there is little doubt that when it comes to be grown in quant.i.ty it will exert an appreciable influence on wheat-growing in Great Britain.

[Ill.u.s.tration: FIG. 30.

Curves to ill.u.s.trate the influence of selection.]

It may be objected that it is often with small differences rather than with the larger and more striking ones that the breeder is mainly concerned. It does not matter much to him whether the colour of a pea flower is purple or pink or white. But it does matter whether the plant bears rather larger seeds than usual, or rather more of them. Even a small difference when multiplied by the {160} size of the crop will effect a considerable difference in the profit. It is the general experience of seedsmen and others that differences of this nature are often capable of being developed up to a certain point by a process of careful selection each generation. At first sight this appears to be something very like the gradual acc.u.mulation of minute variations through the continuous application of a selective process. Some recent experiments by Professor Johannsen of Copenhagen set the matter in a different light. One of his investigations deals with the inheritance of the weight of beans, but as an account of these experiments would involve us in the consideration of a large amount of detail we may take a simple imaginary case to ill.u.s.trate the nature of the conclusions at which he arrived. If we weigh a number of seeds collected from a patch of plants such as Johannsen's beans we should find that they varied considerably in size. The majority would probably not diverge very greatly from the general average, and as we approached the high or low extreme we should find a constantly decreasing number of individuals with these weights. Let us suppose that the weight of our seed varied between 4 and 20 grains, that the greatest number of seeds were of the mean weight, viz. 12 grains, and that as we pa.s.sed to either extreme at 4 and 20 the number became regularly less. The weight relation of such a collection of seeds can be expressed by the accompanying curve (Fig. 30). Now if we select for {161} sowing only that seed which weighs over 12 grains, we shall find that in the next generation the average weight of the seed is raised and the curve becomes somewhat s.h.i.+fted to the right as in the dotted line of Fig.

30. By continually selecting we can s.h.i.+ft our curve a little more to the right, _i.e._ we can increase the average weight of the seeds until at last we come to a limit beyond which further selection has no effect. This phenomenon has been long known, and it was customary to regard these variations as of a continuous nature, _i.e._ as all chance fluctuations in a h.o.m.ogeneous ma.s.s, and the effect of selection was supposed to afford evidence that small continuous variations could be increased by this process. But Johannsen's results point to another interpretation. Instead of our material being h.o.m.ogeneous it is probably a mixture of several strains each with its own average weight about {162} which the varying conditions of the environment cause it to fluctuate. Each of these strains is termed a PURE LINE. If we imagine that there are three such pure lines in our imaginary case, with average weights 10, 12, 14 grains respectively, and if the range of fluctuation of each of these pure lines is 12 grains, then our curve must be represented as made up of the three components

A fluctuating between 4 and 16 with a mean of 10 B " " 6 " 18 " " 12 C " " 8 " 20 " " 14

[Ill.u.s.tration: FIG. 31.

Curves to ill.u.s.trate the conception of pure lines in a population.]

as is shown in Fig. 31. A seed that weighs 12 grains may belong to any of these three strains. It may be an average seed of B, or a rather large seed of A, or a rather small seed of C. If it belongs to B its offspring will average 12 grains, if to A they will average 10 grains, and if to C they will average 14 grains. Seeds of similar weight may give a different result because they happen to be fluctuations of different pure lines. But within the pure line any seed, large or small, produces the average result for that line. Thus a seed of line C which weighs 20 grains will give practically the same result as one that weighs 10 grains.

On this view we can understand why selection of the largest seed raises the average weight in the next generation. We are picking out more of C and less of A and B, and as this process is repeated the proportion of C gradually increases and we get the appearance of selection {163} acting on a continuously varying h.o.m.ogeneous material and producing a permanent effect. This is because the interval between the average weight of the different pure lines is small compared with the environmental fluctuations.

None the less it is there, and the secret of separating and fixing any of these pure lines is again to breed from the individual separately. As soon as the pure line is separated further selection becomes superfluous.

Since the publication of Darwin's famous work upon the effects of cross and self fertilisation, it has been generally accepted that the effect of a cross is commonly, though not always, to introduce fresh vigour into the offspring, though why this should be so we are quite at a loss to explain.

Continued close inbreeding, on the contrary, eventually leads to deterioration, though, as in many self-fertilised plants, a considerable number of generations may elapse before it shows itself in any marked degree. The fine quality of many of the seedsman's choice varieties of vegetables probably depends upon the fact that they had resulted from a cross but a few generations back, and it is possible that they often oust the older kinds not because they started as something intrinsically better, but because the latter had gradually deteriorated through continuous self-fertilisation. Most breeders are fully alive to the beneficial results of a cross so far as vigour is concerned, but they often hesitate to embark upon it owing to what was held {164} to be the inevitably lengthy and laborious business of recovering the original variety and refixing it, even if in the process it was not altogether lost. That danger Mendelism has removed, and we now know that by working on these lines it is possible in three or four generations to recover the original variety in a fixed state with all the superadded vigour that follows from a cross.

Nor is the problem one that concerns self-fertilised plants only. Plants that are reproduced as.e.xually often appear to deteriorate after a few generations unless a s.e.xual generation is introduced. New varieties of potato, for example, are frequently put upon the market, and their excellent qualities give them a considerable vogue. Much is expected of them, but time after time they deteriorate in a disappointing way and are lost to sight. It is not improbable that we are here concerned with a case in which the plants lose their vigour after a few as.e.xual generations of reproduction from tubers, and can only recover it with the stimulus that results from the interpolation of a s.e.xual generation. Unfortunately this generally means that the variety is lost, for owing to the haphazard way in which new kinds of potatoes are reproduced it is probable that most cultivated varieties are complex heterozygotes. Were the potato plant subjected to careful a.n.a.lysis and the various factors determined upon which its variations depend, we should be in a position to remake continually any good potato without {165} running the risk of losing it altogether, as is now so often the case.

The application of Mendelian principles is likely to prove of more immediate service for plants than animals, for owing to the large numbers which can be rapidly raised from a single individual and the prevalence of self-fertilisation, the process of a.n.a.lysis is greatly simplified. Even apart from the circ.u.mstance that the two s.e.xes may sometimes differ in their powers of transmission, the mere fact of their separation renders the a.n.a.lysis of their properties more difficult. And as the const.i.tution of the individual is determined by the nature and quality of its offspring, it is not easy to obtain this knowledge where the offspring, as in most animals, are relatively few. Still, as has been abundantly shown, the same principles hold good here also, and there is no reason why the process of a.n.a.lysis, though more troublesome, should not be effectively carried out.

At the same time, it affords the breeder a rational basis for some familiar but puzzling phenomena. The fact, for instance, that certain characters often "skip a generation" is simply the effect of dominance in F_1 and the reappearance of the recessive character in the following generation.

"Reversion" and "atavism," again, are phenomena which are no longer mysterious, but can be simply expressed in Mendelian terms as we have already suggested in Chap. VI. The occasional appearance of a sport in a supposedly pure strain is {166} often due to the reappearance of a recessive character. Thus even in the most highly pedigreed strains of polled cattle such as the Aberdeen Angus, occasional individuals with horns appear. The polled character is dominant to the horned, and the occasional reappearance of the horned animal is due to the fact that some of the polled herd are heterozygous in this character. When two such individuals are mated, the chances are 1 in 4 that the offspring will be horned. Though the heterozygous individuals may be indistinguishable in appearance from the pure dominant, they can be readily separated by the breeding test. For when crossed by the recessive, in this case horned animals, the pure dominant gives only polled beasts, while the heterozygous individual gives equal numbers of polled and horned ones. In this particular instance it would probably be impracticable to test all the cows by crossing with a horned bull. For in each case it would be necessary to have several polled calves from each before they could with reasonable certainty be regarded as pure dominants. But to ensure that no horned calves should come, it is enough to use a bull which is pure for that character. This can easily be tested by crossing him with a dozen or so horned cows. If he gets no horned calves out of these he may be regarded as a pure dominant and thenceforward put to his own cows, whether horned or polled, with the certainty that all his calves will be polled. {167}

Or, again, suppose that a breeder has a chestnut mare and wishes to make certain of a bay foal from her. We know that bay is dominant to chestnut, and that if a h.o.m.ozygous bay stallion is used a bay foal must result. In his choice of a sire, therefore, the breeder must be guided by the previous record of the animal, and select one that has never given anything but bays when put to either bay or chestnut mares. In this way he will a.s.sure himself of a bay foal from his chestnut mare, whereas if the record of the sire shows that he has given chestnuts he will be heterozygous, and the chances of his getting a bay or a chestnut out of a chestnut mare are equal.

It is not impossible that the breeder may be unwilling to test his animals by crossing them with a different breed through fear that their purity may be thereby impaired, and that the influence of the previous cross may show itself in succeeding generations. He might hesitate, for instance, to test his polled cows by crossing them with a horned bull for fear of getting horned calves when the cows were afterwards put to a polled bull of their own breed. The belief in the power of a sire to influence subsequent generations, or telegony as it is sometimes called, is not uncommon even to-day. Nevertheless, carefully conducted experiments by more than one competent observer have failed to elicit a single shred of unequivocal evidence in favour of the view. Until we have evidence based upon experiments which are capable of {168} repet.i.tion, we may safely ignore telegony as a factor in heredity.

Heterozygous forms play a greater part in the breeding of animals than of plants, for many of the qualities sought after by the breeder are of this nature. Such is the blue of the Andalusian fowl, and, according to Professor Wilson, the roan of the Shorthorn is similar, being the heterozygous form produced by mating red with white. The characters of certain breeds of canaries and pigeons again appear to depend upon their heterozygous nature. Such forms cannot, of course, ever be bred true, and where several factors are concerned they may when bred together produce but a small proportion of offspring like themselves. As soon, however, as their const.i.tution has been a.n.a.lysed and expressed in terms of Mendelian factors, pure strains can be built up which when crossed will give nothing but offspring of the desired heterozygous form.

The points with which the breeder is concerned are often fine ones, not very evident except to the practised eye. Between an ordinary Dutch rabbit and a winner, or between the comb of a Hamburgh that is fit to show and one that is not, the differences are not very apparent to the uninitiated.

Whether Mendelism will a.s.sist the breeder in the production of these finer points is at present doubtful. It may be that these small differences are heritable, such as those that form the basis of Johannsen's pure lines. In this case the breeder's outlook is {169} hopeful. But it may be that the variations which he seeks to perpetuate are of the nature of fluctuations, dependent upon the earlier life conditions of the individual, and not upon the const.i.tution of the gametes by which it was formed. If such is the case, he will get no help from the science of heredity, for we know of no evidence which might lead us to suppose that variations of this sort can ever become fixed and heritable.

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CHAPTER XV

MAN