J_Jho A*> f'O i si si = ru !-fl ; m |OJ o m o OCT 2 1 MENDEL'S PRINCIPLES OF HEREDITY CAMBRIDGE UNIVERSITY PRESS Hontum: FETTER LANE, E.G. C. F. CLAY, MANAGER ILonUon: H. K. LEWIS, 136, GOWER STREET, W.C. 100, PRINCES STREET Btrlin: A. ASHER AND CO. F. A. BROCKHAUS G. P. PUTNAM'S SONS ani Calcutta: MACMILLAN AND CO., LTD. All rights reserved GREGOR MENDEL, 1866 Enlarged from a group of the brethren of the Konigskloster MENDEL'S PRINCIPLES OF HEREDITY BY W. BATESON, M.A., F.R.S., V.M.H, FELLOW OF ST JOHN'S COLLEGE, PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE Cambridge : at the University Press New York : G. P. Putnam's Sons 1909 (Cambrt&gc : PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS. PREFACE THE object of this book is to give a succinct account of discoveries in regard to Heredity made by the application of Mendel's method of research. Following the clue which his long lost papers provided we have reached a point from which classes of phenomena hitherto proverbial for their seeming irregularity can be recognized as parts of a consistent whole. The study of Heredity thus becomes an organised branch of physiological science, already abun- dant in results, and in promise unsurpassed. A translation of Mendel's two papers, together with a biographical note, is appended. The translation of the first, based on a draft prepared for the Society by Mr C. T. Druery, was printed in the Royal Horticultural Society's Journal, 1901. With modifications I published it separately in 1902, giving a brief summary of Mendelism as then developed, under the title Mendel's Principles of Heredity : A Defence. The object of that publication was to put Mendel's work before the English speaking peoples and to repel the attack which the late Professor Weldon had recently made on Mendelian methods and the conclusions drawn from them. The edition was at once sold out, but I did not reprint the book. As a defence it had served its purpose. Moreover the progress of experiment with the extension of Mendelian conceptions was rapid, and the account of those conceptions there given was in some important respects soon out of date. In particular my vi Preface view of the nature of compound factors was shown to be unnecessarily complex and largely incorrect. Though obviously in a subject fast extending under the influence of many workers there can be no finality, yet at the present time our knowledge of the main facts has reached a definite stage, and a useful and relatively permanent pre- sentation of the phenomena can be attempted. The range and diversity of facts, zoological and botani- cal, from which the material is drawn are so wide that it has been difficult to present them adequately within a moderate compass. Many of the types studied might singly provide the subject of a treatise, and the temptation to annotative excursion has been very great ; but the course which seemed most useful was to admit only such detail as had a clear significance in the exposition of principle, or as a suggestion for further inquiry. The reader therefore will understand that if he turns to the original records specified he will almost always find information, perhaps important, which is omitted here. In the original plan of the book it was intended to discuss somewhat fully the bearing of the new facts on the great problems of Biology, but it is perhaps more fitting that these theoretical considerations should be detached from a presentation of the concrete phenomena. In 1907 I had the honour of delivering the Silliman Lectures in Yale University, and I then took these wider aspects of Genetics as my theme, showing the bearing of the new knowledge on current theory, especially on that of Evolu- tion, and the nature of Variation. The substance of these lectures I propose to publish separately with amplifications, and on the present occasion allusion to these matters has been restricted to the briefest possible indication of the lines of thought which Mendelism inevitably suggests. A chapter dealing with practical applications of Mendelian principles has been introduced. Such applications will Preface vii probably far exceed any limits we can yet perceive. Among them we must foresee not merely advances in the art of breeding animals and plants, but a control over the destiny of our own species. These things are spoken of in their place. To prevent disappointment, however, it must be at once admitted that for fanciers Mendelism can as yet do comparatively little. "Fancying" provides the chief interest in life for thousands of persons in this country. It is an occupation with which the scientific naturalist should have more sympathy than he has commonly evinced. If the scientific world had kept in touch with the operations of the " fancy " much nonsense which has passed into scientific orthodoxy would never have been written. The study of Mendelian phenomena will do something to bring about a fruitful interchange of experience. But for the " fancy " our work can as yet do two things only. First, in the study of the workings of the Mendelian system it will provide a most fascinating pursuit, which if followed with assiduous care may at any moment lead to some consider- able advance in scientific knowledge. Secondly, the prin- ciples already ascertained will be found of practical assistance in the formation of new breeds and may save many mistakes and waste of time. But applied to the business of breeding winners in established breeds they cannot materially help, for almost always the points which tell are too fine to be dealt with in our analysis. In a work of this kind an author must necessarily speak of various subjects on which his knowledge can be super- ficial only, and I trust that if inaccuracies have been intro- duced, readers will be good enough to send me corrections. Much and varied assistance has been given me by many persons. Such help on special points has been acknowledged in the text, but a fuller and more prominent acknowledgment is due to my colleagues. Without their cooperation there would have been, so far as Cambridge is viii Preface concerned, but meagre contributions to record. In the early days of Mendelism, and before, Miss E. R. Saunders collaborated with me. A beautiful series of results, especially relating to the heredity of Stocks (Matthiola], has been the fruit of her labours exclusively. Not only have these results greatly advanced our knowledge of genetic pheno- mena, but I think that at a time when Mendelism was, in England at least, regarded with suspicion, the obvious precision of her work and the persistence of her advocacy did much to convince the scientific world of the reality of our assertions. In 1904 I had the good fortune to gain Mr R. C. Punnett as a partner. Since that date we have worked in close collaboration, and the work that we have thus done has been in every sense a joint product, both as regards design, execution, and interpretation of results. Though for the presentation of the views contained in this book I am solely responsible, those that apply to the subjects of our own work are often his, or have been arrived at in consequence of interchange of ideas with him. On some points of general physiology I have received useful suggestions and criticism from Mr F. F. Blackman, and in this respect I am also especially beholden to Miss F. M. Durham. The Plates of Sweet Peas and Mice are photographic reproductions, on the whole very accurate, of coloured drawings most kindly made for me by Miss Wheldale. The Plate of Primula flowers is taken from an excellent coloured photograph by Mr Waltham*. For Fig. 9 I am obliged to the New Phytologist. * Since the word " magenta," often used in English for the description of a colour, is not understood on the Continent, I may say that it means a purplish or bluish red, as distinguished from a crimson or pink red. On Plate VI, Figs. 8, 9, 14, 15, 20, 21 represent shades of magenta, while Figs. 2, 7, 13, 19 are true reds. Preface ix For several years past I have had an exceptional opportunity of seeing breeding conducted on a large scale through the great kindness of Messrs Sutton of Reading, who have given me the privilege of watching such parts of their work in raising varieties as seemed especially in- structive, with unrestricted access to their pedigree books. From this I have derived much profit, and many hints which have formed the starting point for fuller experiment. My hearty thanks are due to them for this important assistance. W. BATESON. GRANTCHESTER, CAMBRIDGE. February ', 1909. CORRIGENDUM p. 94, line 18. For Plate V read Plate III. CONTENTS PART I. CHAPTER PAGE I. INTRODUCTORY. MENDEL'S DISCOVERY i Introductory — Some pre-Mendelian Writings — Mendel's Discovery — Dominant and Recessive — Segregation. Allelomorphism — Homo- zygote and Heterozygote. Purity of Type. II. THE MATERIAL INVESTIGATED . . . . . .18 List of Structural Characters in Plants and Animals — List of Types in which the inheritance of Colour has been studied — Preliminary Deductions — Dominance and heterozygous characters — Mendel's system distinguished from that of Galton. III. NUMERICAL CONSEQUENCES AND RECOIMBINATIONS . . 57 Representations of the F2 Generation and Novelties due to Re-combina- tion of Factors — Compound Characters — Combs of Fowls — Hetero- stylism — White Flowers from Red x Cream. IV. HEREDITY OF COLOUR 74 Factors determining Colours : the Ratio 9:3: 4 — The " Presence and Absence " Hypothesis. Epistatic and Hypostatic Factors — Colours of Mice — Pied Types — A Dominant Piebald. V. HEREDITY OF COLOUR (continued} ..... 88 Albinos giving Coloured Offspring ; Reversion on Crossing— Various Kinds of Whites — Stocks — Orchids — Pigeons — Fowls — Primula. VI. HEREDITY OF COLOUR (continued} . . . . .107 Eye-Colours. Variations in Colour of the Iris — Deficiency of Eye- Pigments in some Coloured Types. VII. HEREDITY OF COLOUR (continued} . . . . . 115 The Genetics of Yellow Pigments in certain Animals. Yellow Mice not breeding true — The Case of Basset Hounds and the " Law of An- cestral Heredity." Relation of this Principle to Mendelian Rules. VIII. HEREDITY OF COLOUR (continued} 132 Various Specific Phenomena in Colour-Inheritance. Relation of Colour to Hoariness in Stocks. Miscellaneous Cases. Colour of a Special Part controlling that of other Parts — Summary and Discussion — Subtraction-Stages. xii Contents CHAPTER PAGE IX. GAMETIC COUPLING AND SPURIOUS ALLELOMORPHISM . 148 Pollen-Shape and Flower-Colour. Axil-Colour and Sterile Anthers — Hooded Standard and Flower-Colour in Sweet Peas. X. HEREDITY AND SEX 164 Evidence from Breeding Experiments. Bryonia —Sex-limited Heredity. The Horns of Sheep — Colour-Blindness — Sex and Spurious Allelo- morphism. The Currant Moth— The Cinnamon Canary — The Silky Fowl — Aglia tau — Cytological Evidence — Summary. XL DOUBLE FLOWERS . . . . . . . .196 Miscellaneous Cases. Recessive and Dominant Doubling — " Hose-in- Hose " Flowers — The Special Case of Double Stocks. XII. EVIDENCE AS TO MENDELIAN INHERITANCE IN MAN . 205 Normal Characters — -Diseases and Malformations. Dominants — Sex- limited Dominants — Recessives — Notes on collecting Evidence. XIII. INTERMEDIATES BETWEEN VARIETIES AND THE "PURE LINES" OF JOHANNSEN ...... 235 Intermediates as Heterozygous Forms — Subtraction-Stages of Dominants — Interfering Factors — Fluctuational Forms — "Pure Lines." XIV. MISCELLANEOUS EXCEPTIONAL AND UNCONFORMABLE PHE- NOMENA ......... 245 Crosses breeding true without Segregation. Parthenogenetic or Apo- gamic Forms. Hieracium— Sexual Forms — Numerical Aberrations — Irregularities of Dominance — Alternation of Generations — Mater- nal Characters in certain Seeds. XV. BIOLOGICAL CONCEPTIONS IN THE LIGHT OF MENDELIAN DISCOVERIES . . . . . . . . . 266 Nature of Units — Nature of Segregation — Moment of Segregation — Differentiation of Parts compared with Segregation — Reversion and Variation. " Bush " and " Cupid " Sweet Peas — Mendelian Segre- gation and Species — Discontinuity in Variation — Mendelism and Natural Selection. XVI. PRACTICAL APPLICATION OF MENDELIAN PRINCIPLES . 291 Meaning of Pure-bred — Rogueing — Raising Novelties — A Practical Ex- ample— Unfixable Types — Technical Methods — Sociological Appli- cation. PART II. 1. BIOGRAPHICAL NOTICE OF MENDEL ..... 309 2. TRANSLATION OF THE PAPER ON HYBRIDISATION . . . 317 3. TRANSLATION OF THE PAPER ON Hieracium . . . .362 BIBLIOGRAPHY 369 SUPPLEMENTARY LIST . . . 385 INDEX OF SUBJECTS . . . . . . . . . 386 INDEX OF AUTHORS " . . . . 394 LIST OF ILLUSTRATIONS PORTRAITS OF MENDEL. In 1866 About 1862 . About 1880 . Frontispiece to face p. 309 to face p. 317 COLOURED PLATES. Plate I. Lepidoptera . „ II. Mice .... ,, III. Reversion in Sweet Peas IV. Fowls . to face p. 44 between pp. 80-8 1 between pp. 93-94 to face p. 103 V. Spurious Allelomorphism in Sweet Peas between pp. 154-155 VI. Heredity of Colour in Primula Sinensis between pp. 294-295 FIGURES. FIGURE PAGE 1. Tall and " Cupid " dwarf Sweet Peas .... 9 2. Diagram showing consequences of Segregation . . . 12 3. Inheritance of seed-characters in Pea . . . . 15 4. Branched and unbranched forms in Stocks (Matthiold) . 20 5. Hooded and normal Barley . . . . . . 21 6. Heredity in Wheat ........ 23 7. Fern-leaf and palm-leaf in Primula Sinensis ... 24 8. Two-row and six-row Barley . . . . . . 27 9. Starch-grains in Peas ........ 29 10. Round and wrinkled seed in Maize ..... 30 11. Down-colour in Chickens ....... 51 12. Types of combs in Fowls ....... 61 13. Combs of newly-hatched Chickens . . . . . 62 14. Descent of "homostyle" character in Primula ... 69 xiv List of Illustrations FIGURE PAGE 15. Diagram of F2 showing ratio 9:3:4 . . . 77 1 6. Diagram of F2 showing ratio 9:7. . . . . 89 17. Diagram of F2 in Sweet Pea showing ratio 27:9:28 . 91 1 8. Pedigrees of eye-colour in Man ... .108 19. Pollen grains of Sweet Peas . . .150 20. Heredity of horns in Sheep . . . . . . 171 21. Heredity of a peculiar form of curly hair . .207 22. Descent of congenital lock of white hair .... 207 23. Brachydactylous hands ..... 211 24. Skiagram of hands . ... 212 25. Hands of brachydactylous child . 213 26. Pedigree of brachydactylous family ... .214 27. Descent from brachydactylous members . . . 214 28. Drinkwater's pedigree of brachydactyly . . 215 29. Descent of prae- senile cataract . . . . . 216 30. Another cataractous family . . . . . 217 31. Descent of stationary night-blindness . between pp. 220-221 32. Descent of Colour-blindness . . . . . 223 33. Ideal Scheme of descent of Colour-blindness . . 225 PART I CHAPTER I INTRODUCTORY. MENDEL'S DISCOVERY. Introductory — Some pre-Mendelian Writings — Mendel's Discovery — Dominant and Recessive — Segregation. Allelomorphism - - Homozygote and Heterozygote. Purity of Type. AMONG the biological sciences the study of heredity occupies a central position. Whether we be zoologists, botanists, or physiologists, the facts of heredity concern us. Upon this physiological function all the rest in some degree depend. Every advance in knowledge of that central function must affect the course of thought along each several line of biological inquiry. Moreover though, as naturalists, we are not directly concerned with the applications of science, we must perceive that in no region of knowledge is research more likely to increase man's power over nature. The science of sociology, and in many of its developments the science of medicine also, must of necessity form working hypotheses respecting the course of heredity, and we cannot doubt that a percep- tion of the truth in regard to the function of transmission will greatly contribute to the progress of these sciences. Lastly, to the industrial arts of the breeder of plants or animals, the knowledge we are attempting to provide is of such direct importance that upon this consideration no special emphasis is required. In studying heredity, there- fore, we are examining a vital problem of no mean consequence, and those who engage in that work are happy in the thought that they are assisting one of the main advances in natural knowledge. But though we may approach this study of genetics— to use the modern designation — from so many different sides, it is especially in their bearing on the problem of B. H. 2 Introductory [CH. the evolution of species that the facts have hitherto been most profitably investigated. It was in the attempt to ascertain the interrelationships between species that experi- ments in genetics were first made. The words "evolution" and " origin of species " are now so intimately associated with the name of Darw:in that we are apt to forget that the idea of a common descent had been prominent in the minds of naturalists before he wrote, and that, for more than half a century, zealous investigators had been devoting them- selves to the experimental study of that possibility. Promi- nent among this group of experimenters may be mentioned Koelreuter, John Hunter, Herbert, Knight, Gaertner, Jordan, Naudin, Godron, Lecoq, Wichura — men whose names are familiar to every reader of Animals and Plants under Domestication. If we could ask those men to define the object of their experiments, their answer would be that they were seeking to determine the laws of hereditary trans- mission with the purpose of discovering the interrelationships of species. In addition to the observation of the visible structures and habits of plants and animals they attempted by experiment to ascertain those hidden properties of living things which we may speak of as genetic, properties which breeding tests can alone reveal. The vast mass of observation thus accumulated contains much that is of permanent value, hints that if followed might have saved their successors years of wasted effort, and not a few indications which in the light of later discovery will greatly accelerate our own progress. Yet in surveying the work of this school we are conscious of a feeling of disappointment at the outcome. There are signs that the workers themselves shared this disappointment. As we now know, they missed the clue without which the evidence so laboriously collected remained an inscrutable medley of contradictions. While the experimental study of the species problem was in full activity the Darwinian writings appeared. Evolution, from being an unsupported hypothesis, was at length shown to be so plainly deducible from ordinary experience that the reality of the process was no longer doubtful. With the triumph of the evolutionary idea, curiosity as to the significance of specific differences was i] Introductory 3 satisfied. The Origin was published in 1859. During the following decade, while the new views were on trial, the experimental breeders continued their work, but before 1870 the field was practically abandoned. In all that concerns the problem of species the next thirty years are marked by the apathy characteristic of an age of faith. Evolution became the exercising-ground of essayists. The number indeed of naturalists increased ten- fold, but their activities were directed elsewhere. Darwin's achievement so far exceeded anything that was thought possible before, that what should have been hailed as a long-expected beginning was taken for the completed work. I well remember receiving from one of the most earnest of my seniors the friendly warning that it was waste of time to study variation, for " Darwin had swept the field." Parenthetically we may notice that though scientific opinion in general became rapidly converted to the doctrine of pure selection, there was one remarkable exception. Systematists for the most part kept aloof. Everyone was convinced that natural selection operating in a continuously varying population was a sufficient account of the origin of species except the one class of scientific workers whose labours familiarised them with the phenomenon of specific difference. From that time the systematists became, as they still in great measure remain, a class apart. A separation has thus been effected between those who lead theoretical opinion and those who by taste or necessity have retained an acquaintance with the facts. The con- sequences of that separation have been many and grievous. To it are to be traced the extraordinary misapprehensions as to the fundamental phenomena of specific difference which are now prevalent. If species had really arisen by the natural selection for impalpable differences, intermediate forms should abound, and the limits between species should be on the whole indefinite. As this conclusion follows necessarily from the premisses, the selectionists believe and declare that it represents the facts of nature. Differences between species being by axiom indefinite, the differences between varieties must be supposed to be still less definite. Consequently the conclusion that evolution must proceed by insensible 4 Introductory [CH. transformation of masses of individuals has become an established dogma. Systematists, entomologists or botan- ists for example, are daily witnesses to variation occurring as an individual and discontinuous phenomenon, but they stand aside from the debate ; and whoever in a discussion of evolutionary theory appeals to the definiteness of varietal distinctions in colour for instance, or in form, as recognizable by common observation without mechanical aid, must be prepared to meet a charge of want of intelligence or candour. This is no doubt a passing phase and will end so soon as interest in the problems of evolution is combined with some knowledge of variation and heredity. Genetic experiment was first undertaken, as we have seen, in the hope that it would elucidate the problem of species. The time has now come when appeals for the vigorous prosecution of this method should rather be based on other grounds. It is as directly contributing to the advancement of pure physiological science that genetics can present the strongest claim. We have an eye always on the evolution-problem. We know that the facts we are collecting will help in its solution ; but for a period we shall perhaps do well to direct our search more especially to the immediate problems of genetic physiology, the laws of heredity, the nature of variation, the significance of sex and of other manifestations of dimorphism, willing to postpone the application of the results to wider problems as a task more suited to a maturer stage. When the magnitude and definiteness of the advances already made in genetics come to be more generally known, it is to be anticipated that workers in various departments of biology will realise that here at last is common ground. As we now know, the conceptions on which both the systematists and the specula- tive biologists have based their methods need complete revision in the light of the new facts, and till the possibilities of genetic research are more fully explored the task of reconstruction can hardly be begun. In that work of exploration all classes of naturalists will alike find interest. The methods are definite and exact, so we need not fear the alienation of those systematists to whom all theoretical inquiry is repulsive. They are also wide in their scope, and those who would turn from the details of classification i] Pre-Mendelian Huntings 5 as offering matter too trivial for their attention may engage in genetic inquiries with great confidence that every frag- ment of solid evidence thus discovered will quickly take its place in the development of a coordinated structure. Some pre-Mendelian Writings. Of the contributions made during the essayist period three call for notice : Weismann deserves mention for his useful work in asking for the proof that " acquired characters " — or, to speak more precisely, parental ex- perience— can really be transmitted to the offspring. The occurrence of progressive adaptation by transmission of the effects of use had seemed so natural to Darwin and his contemporaries that no proof of the physiological reality of the phenomenon was thought necessary. Weismann's challenge revealed the utter inadequacy of the evidence on which these beliefs were based. There are doubtless isolated observations which may be interpreted as favouring the belief in these transmissions, but such meagre indications as exist are by general consent admitted to be too slight to be of much assistance in the attempt to understand how the more complex adaptatiye mechanisms arose. Nevertheless it was for the purpose of elucidating them that the appeal to inherited experience was made. Weismann's contribution, though negative, has greatly simplified the practical investi- gation of genetic problems. Though it attracted little attention at the time of its appearance, an honourable place in the history of our science must be accorded to the paper published by de Vries (1889) under the title IntracelLulare Pangenesis. This essay is remarkable as a clear foreshadowing of that conception of unit-characters which is destined to play so large a part in the development of genetics. The supreme importance of an exact knowledge of heredity was urged by Galton in various writings published during the period of which I am speaking. He pointed out that the phenomena manifested regularity, and he made the first comprehensive attempt to determine the rules they obey. It was through his work and influence that the existence of some order pervading the facts became generally 6 Pre-Mendelian Writings [CH. recognized. In 1897 ne definitely enunciated his now famous " Law " of heredity, which declared that to the total heritage of the offspring the parents on an average contribute \, the grandparents \, and the great-grandparents -§-, and so on, the total heritage being taken as unity. To this conclusion he had been led by several series of data, but the evidence upon which he especially relied was that of the pedigrees of Basset Hounds furnished him by the late Sir Everett Millais. In that instance the character con- sidered was the presence or absence of black in additipn to yellow and white. The colours were spoken of as tri-colour and non-tri-colour, and the truth of the law was tested by the average numbers of the respective colours which resulted from the various matings of dogs of known ancestral composition. These numbers corresponded so well with the expectations given by the law as to leave no reasonable doubt that the results of calculation were in general har- mony with natural fact. There are features in this important case which need special consideration, and to these I will return. Meanwhile we may note that though there was admittedly a statistical accord between Gallon's theory and some facts of heredity, yet no one familiar with breeding or even with the literature of breeding could possibly accept that theory as a literal or adequate presentation of the facts. Galton himself in pro- mulgating it made some reservations ; but in the practice of breeding, so many classes of unconformable phenomena were already known, that while recognizing the value of his achievement, we could not from the first regard it as more than an adumbration of the truth. As we now know, Galton's method failed for want of analysis. His formula should in all probability be looked upon rather as an occasional consequence of the actual laws of heredity than in any proper sense one of those laws. Of the so-called investigations of heredity pursued by extensions of Galton's non-analytical method and promoted by Professor Pearson and the English Biometrical school it is now scarcely necessary to speak. That such work may ultimately contribute to the development of statistical theory cannot be denied, but as applied to the problems of heredity the effort has resulted only in the concealment of i] Rediscovery of Mendel 7 that order which it was ostensibly undertaken to reveal. A preliminary acquaintance with the natural history of heredity and variation was sufficient to throw doubt on the foundations of these elaborate researches. To those who hereafter may study this episode in the history of biological science it will appear inexplicable that work so unsound in construction should have been respectfully received by the scientific world. With the discovery of segregation it became obvious that methods dispensing with individual analysis of the material are useless. The only alternatives open to the inventors of those methods were either to abandon their delusion or to deny the truth of Mendelian • facts. In choosing the latter course they have certainly succeeded in delaying recognition of the value of Mendelism, but with the lapse of time the number of persons who have themselves witnessed the phenomena has increased so much that these denials have lost their dangerous character and may be regarded as merely formal. Rediscovery of Mendel : his Method. With the year 1900 a new era begins. In the spring of that year there appeared, within a few weeks of each other, the three papers of de Vries, Correns, and Tschermak, giving the substance of Mendel's long-forgotten treatise. Each of these three writers was able from his own ex- perience to confirm Mendel's conclusions, and to extend them to other cases. There could therefore, from the first, be no question as to the truth of the facts. To appreciate what Mendel did the reader should refer to the original paper*, which is a model of lucidity and expository skill. His success is due to the clearness with which he thought out the problem. Being familiar with the works of Gaertner and the other experimental breeders he surmised that their failure to reach definite and consistent conclusions was due to a want of precise and continued analysis. In order to obtain a clear result he saw that it was absolutely necessary to start with pure-breeding, homogeneous materials, to consider each character separately, and on no account to confuse the different generations together. Lastly he realised * See Part II. 8 Mendel's Method [CH. that the progeny from distinct individuals must be separately recorded. All these ideas were entirely new in his day. When such precautions had been observed he anticipated that a regular result would be attainable if the experiments were carried out on a sufficient scale. After several preliminary trials he chose the edible Pea (Pisum sativum] for his subject. Varieties in cultivation are distinguished by striking characters recognizable with- out trouble. The plants are habitually self-fertilised, a feature which obviates numerous difficulties. Following his idea that the heredity of each character must be separately investigated, he chose a number of pairs of characters, and made crosses between varieties differing markedly in respect of one pair of characters. The case which illustrates Mendelian methods in the simplest way is that in which heredity in respect of height was studied. Mendel took a pair of varieties of which one was tall, being 6 — 7 feet high, and the other was dwarf, £ to i^ feet. These two were then crossed together. In peas this is an easy operation. The unbroken anthers can be picked out of a bud with a pair of fine forceps and the pollen of the plant chosen for the father may be at once applied to the stigma of the emasculated flower. The cross-bred seeds thus produced grew into plants which were always tall, having a height not sensibly different from that of the pure tall variety. In our modern terminology such a cross-bred, the first filial generation, is called Fr From the fact that the character, tallness, appears in the cross-bred to the exclusion of the opposite character, Mendel called it a dominant character ; dwarfness, which disappears in the Fl plant, he called recessive. The tall cross-bred, so produced, in its turn bore seeds by self-fertilisation. These are the next generation, /%. When grown up they prove to be mixed, many being tall, some being short, like the tall and the short grand- parents respectively. Fig. i shows such an /% family in the Sweet Pea. Upon counting the members of this Fz generation it was discovered that the proportion of tails to shorts exhibited a certain constancy, averaging about three tails to one short, or in other words, 75 per cent, dominants to 25 per cent, recessives. i] MendeFs Method 9 These /% plants were again allowed to fertilise them- selves and the offspring of each plant was separately sown. It was then found that the offspring, F.A, of the recessives '- rt "3 I *j < c >> a-!- C rt bO consisted entirely of recessives. Further generations bred from these recessives again produced recessives only, and therefore the recessives which appeared in F^ are seen to io Segregation [CH. be pure to the recessive character, namely, in the case we are considering, to dwarfness. But the tall /% dominants when tested by a study of their offspring (-/%), instead of being all alike (as the dwarfs or recessives were), proved to be of two kinds, viz. (a) Plants which gave a mixed F3 consisting of both tails and dwarfs, the proportion showing again an average of three tails to one dwarf. (b] Plants which gave tails only and are thus pure to tallness. The ratio of the impure (a) plants to the pure (b) plants was as 2 to i. The whole Fz generation therefore, formed by self- fertilisation of the original hybrid consists of three kinds of plants : o / o / o / 25 /o. 50 /0 25 /, pure dominants impure dominants pure recessives or 3 dominants : i recessive. Segregation. Allelomorphism. The conclusion which Mendel drew from these observa- tions is one which will suggest itself to any one who reflects on the facts. The result is exactly what would be expected if both male and female germ-cells of the cross-bred F^ were in equal numbers bearersHof either the dominant (D] or recessive (R) character, but not both. If this were so, and if the union of the male and female germ-cells occurs at random, the result would be an Fz family made up of But, as th,e first cross showed, when D meets R in fertilisation the resulting individual is in appearance D ; therefore F^ appears as $D : iR. The results of the Fz i] and its Consequences 1 1 generation are in exact agreement with this suggestion : for the R plants give R only ; and of the D plants one- third give D only, while two-thirds give the same mixture, (£13, 3/? : i^, which was produced by F^ (Fig. 2. I). The descent may be represented diagrammatically thus : Parents Tall ( TT ) x Short Tall TT pure tall Tall Tt Tall tT Short tt pure short F3 TT Tt tT tt TT Tt tT tt Now since the fertilised ovum or zygote, formed by the original cross, was made by the union of two germ-cells or gametes bearing respectively tallness and dwarfness, both these elements entered into the composition of the original F^ zygote; but if the germ-cells which that zygote eventually forms are bearers of either tallness or dwarfness, there must at some stage in the process of germ-formation be a separation of the two characters, or rather of the ultimate factors which cause those characters to be developed in the plants. This phenomenon, the dissociation of characters from each other in the course of the formation of the germs, we speak of as segregation, and the characters which segregate from each other are described as allelomorphic, i.e. alternative to each other in the constitution of the gametes (Fig. 2). That this is the true account was proved by further . experiments which Mendel made by crossing the f*\ with fa pure dominants and with pure recessives. For DR x DD gave an offspring all dominant in appearance, though in reality consisting of both DR plants and DD plants, on an average in equal numbers. On the other hand DR x RR gives an equal number of dominants and recessives, of which the dominants are all DR plants, and the recessives are all pure recessives. These various experiments illus- trate the composition of the four simple types of Mendelian families, which may be set out thus : x vr- 12 Segregation [CH. DD x RR gives DRxRR gives DR^DD gives x DR gives v\\DR iRR : iRR appearing as 3 dominants : i recessive. RC=!> .ff x 7? III. The mating DR x Z>Z>. The way in which these ratios are produced may be easily represented by means of a number of draught-men. Pairs of draughts then represent zygotes ; single draughts represent germ-cells. That there is a propriety in repre- senting zygotic or somatic cells as double structures and germ-cells as single structures will be evident to biologists ; for we know that each somatic nucleus in plants and animals is a double structure, containing twice the number of chromosomes present in each mature germ-cell. Two black draughts may then be taken to represent a pure black individual, two white draughts a white individual. When they are crossed together F^ is represented by a black i] and its Consequences 13 draught and a white one (Fig. 2. I). Supposing the black to be a dominant the fact may be represented by putting it on the top. When segregation of the allelomorphs, black- ness and whiteness, takes place in gameto-genesis, the germ-cells of the cross-bred are again bearers of blackness or of whiteness, and it may readily be shown experimentally that the results of their various random combinations give rise to the ratios stated above. The fact of segregation was the essential discovery which Mendel made. As we now know, such segregation is one of the normal phenomena of nature. It is segregation which determines the regularity perceptible in the here- ditary transmission of differences, and the definiteness or discontinuity so often conspicuous in the variation of animals and plants is a consequence of the same phenomenon. Segregation thus defines the units concerned in the consti- tution of organisms and provides the clue by which an analysis of the complex heterogeneity of living forms may be begun. There are doubtless limits beyond which such analysis cannot be pursued, but a vast field of research must be explored before they are reached or determined. It is likely also that in certain cases the units are so small that no sensible segregation can be proved to exist. As yet, how- ever, no such example has been adequately investigated ; nor, until the properties and laws of interaction of the segregable units have been much more thoroughly examined, can this class of negative observations be considered with great prospect of success. The dominance of certain characters is often an impor- tant but qever an essential feature of Mendelian heredity. Those who first treated of Mendel's work most unfortu- nately fell into the error of enunciating a " Law of Domi- nance " as a proposition comparable with the discovery of segregation. Mendel himself enunciates no such law. Dominance of course frequently exists. The consequences of its occurrence and the complications it introduces must be understood as a preliminary to the practical investigation of the phenomena of heredity, but it is only a subordinate incident of special cases, and Mendel's principles of inherit- ance apply equally to cases where there is no dominance^ r 14 Segregation [CH. •HflU and the heterozygous type is intermediate in character between the two pure types. To the detection of the genetic system of any given case it is however necessary that the results of combinations should be sensibly regular. When, as occasionally happens, a character may sometimes behave as a dominant and sometimes not, we have as yet no satisfactory means of further analysis. These irregularities in dominance may confidently be attributed to the disturbing effects of other factors or of conditions, but the detection of such unknown factors must be a long and perhaps impossible task. Mendel applied his method to the following seven distinct pairs of characters in peas, and found that in each the inheritance was similar. The dominant character is put first. 1. Height : whether tall or short. 2. Distribution of flowers on the stem : whether arranged along the axis of the plant, or bunched together at the top so as to form a false umbel *. 3. Colour of unripe pod : whether a shade of green or bright yellow. 4. Shape of pod : whether simply inflated, or deeply constricted between the seeds, i.e. as in "sugar- peas " or " Pois sans parchemin." 5. Colour of seed-skin : whether various shades of grey or brown, with or without violet spotting, or white. The "grey" skins are always asso- ciated with coloured flowers and almost always with a purple or red mark in the axils. 6. Colour of cotyledons : whether yellow or green. 7. Shape of seeds : whether rounded or wrinkled. It will be observed that the first five are plant-characters. In order to see the result of crossing, the seeds must be sown and allowed to grow into plants. The last two characters belong to the seeds themselves. The seeds of course are members of a generation later than that of the plant which bears them. Thus when a cross is made the * This is a fasciated and semi-monstrous form. and its Consequences resultant seeds are F^ showing the dominant character yellowness or roundness, but the seed-skins are maternal tissue. Such F^ seeds grow into Fl plants and bear F» seeds which show the typical mixture of dominants and recessives in the pods (Fig. 3). In each case Mendel's x ) YR rt I i i D I u § c|R Fig. 3. Inheritance of seed-characters in Pea. The seed of a green round variety fertilised by pollen of a yellow wrinkled variety are yellow and round (J\). The reciprocal cross would give the same result. Two pods of F^ seed borne by the Fl plant are shown. There were 6 yellow round, 3 green round, 3 yellow wrinkled, i green wrinkled. observations have been substantially confirmed by later observers, and the operation of similar processes has now been recognized in a long series of most diverse characters in both animals and plants. Consequences of Segregation : Homozygote and Heterozygote. Before considering the various extensions of Men- delian research, it may be well to indicate in general terms the chief significance of the facts. The first conception to which we are led is that of unit-characters, units because they may be treated as such in the cell-divisions of gametogenesis. It is evidently upon some process of 1 6 Purity of Type [CH. qualitative segregation occurring in one or more of these cell-divisions that allelomorphism depends. The opposite members of each pair of characters being allelomorphic to each other, every zygote*, or individual produced in ferti- lisation, must, in respect of any such pair, be either a homozygote, that is to say, a zygote formed by the union of two gametes each bearing the same allelomorph, as AA and aa, or a keterozygote formed by the union of two germs bearing different allelomorphs, as Aa. Therefore in respect of any pair of allelomorphic characters, the individuals composing the whole population are of three kinds only : 1. Homozygotes of the form A A, 2. Homozygotes of the form aa, 3. Heterozygotes of the form Aa. The gametes are of two kinds only, A and a. Each kind of homozygote is pure to the character of the gametes which compose it. Purity of Type. Purity of type thus acquires a precise meaning. It is dependent on gametic segregation, and has nothing to do with a prolonged course of selection, natural or artificial. All this is of course consonant with the visible facts that have been discovered by the cytologists, in so far as the nucleus of each somatic cell is a double structure, while the nucleus of each gametic cell is a single structure. It is, in my judgment, impossible as yet to form definite views as to the relations of the various parts of the cell to the function of heredity. The details of cytology and their interpretation are beyond our present province, but this much is certain : that when in these discussions we idealize the characters as borne by the gamete in an unpaired state and by the zygote in a paired state, we make no assumption which is not in full accord with histological appearances. From the fact that the development of characters in animals or plants depends on the presence of definite units * In botany the term zygote is usually restricted to the single cell which results from the process of fertilisation, but by a natural extension the word may be used for the individual which develops by somatic divisions from that cell. i] Purity of Type 17 or factors in their germ-cells, the paradox at once follows that an organism may be pure-bred in respect of a given character though its parents were cross-bred in the same respect. Purity depends on the meeting of two gametes bearing similar factors, and when two similarly-constituted gametes do thus meet in fertilisation, the product of their union is pure. The belief, so long prevalent, that purity of type depends essentially on continued selection is thus shown to have no physiological foundation. Similarly it is evident that an individual may be pure in respect of one character and cross-bred or impure in respect of others. As a consequence of the application of Mendel's prin- ciples, that vast medley of seemingly capricious facts which have been recorded as to heredity and variation is rapidly being shaped into an orderly and consistent whole. A new world of intricate order previously undreamt of is disclosed. We are thus endowed with an instrument of peculiar range and precision, and we reach to certainty in problems of physiology which we might have supposed destined to continue for ages inscrutable. After such a discovery it is obvious that old ideas must be revised. Systematists debating the limits of " specific rank " or the range of variability, morphologists seeking to reconstruct phylogenetic history, physiologists unravelling the interaction of bodily functions, cytologists attempting to interpret the processes of cell-division — each of these classes of naturalists must now examine the current con- ceptions of his study in the light of the new knowledge. The practical breeder of animals or plants, basing his methods on a determination of the Mendelian units and their properties, will in many of his operations be able to proceed with confidence and rapidity. Lastly, those who as evolutionists or sociologists are striving for wider views of the past or of the future of living things may by the use of Mendelian analysis attain to a new and as yet limitless horizon. B. CHAPTER II THE MATERIAL INVESTIGATED. List of Structural Characters in Plants and Animals — List of Types in which the inheritance of Colour has been studied — Preliminary Deductions — Dominance and heterozygous characters — Mendel's system distin- guished from that of Gallon. HEREDITY following the general rules described in the last chapter has been witnessed in a great diversity of animals and plants. The characters already proved to follow such rules show an equal diversity. The following is a list of some of them. Adequately to represent the facts respecting- each of these cases lengthy description would be needed. In regard to several of them occurrences which do not readily fall into line have been recorded. Of these some are probably due to errors of observation or mistakes of various kinds, but a few will doubtless prove to be genuine exceptions to rule and may constitute points of departure for fresh and more extended research. In the outline of the phenomena, which is all that this book can profess to offer, it seemed best to restrict as far as possible the enumeration of these details, which can only be thoroughly appreciated by reference to the original papers ; but such annotations as appeared necessary either in elucida- tion of the phenomena or by way of incentive to further work are briefly given with references to the original sources. These annotations will be better understood after the later chapters have been read. In the following list when one character is conspicuously dominant it is put first, but in several cases the dominance is imperfect. Plants. i. Tallness and dwarfness. Peas (Pisum) and Sweet Peas (Lat/iyrus odoratus]. Runner and French Beans (Pkaseolus}. As regards Peas the facts have been recorded by Mendel (195), Tschermak (269, 270, &c.), R.E.C. * (20). When varieties differing * R.E.C. stands throughout for Reports to the Evolution Committee of the Royal Society, giving an account of the experiments of W. Bateson, E. R. Saunders and R. C. Punnett. Other contributors to these Reports are mentioned by name. CH. n] Structural Characters: Plants 19 greatly in height are used, dominance is complete, and the two parent forms are represented as three to one in F^. No clear exception has yet been observed. Peas (Pisum) exist in a vast number of distinct horticultural varieties which can roughly be classified as tall (about 5 — 6 ft), half-dwarfs (about 4 ft.), dwarfs (about 9 ins. to 3 ft.). The genetic relations of the half-dwarfs to the others are not fully explored, and further investigation will probably lead to the discovery of important facts. The cross half- dwarf x tall giving tall as dominant has produced some extreme dwarfs in F«, doubtless by recombination (q-v.), R.E.C. 20, p. 69. The cross half-dwarf x dwarf has given intermediates in F^ (ibid.). The cross between tall and dwarf " Cupid " Sweet Peas gives complete dominance of tallness and simple segregation in F2, " Cupids " indis- tinguishable from the original "Cupid" parent reappearing (Fig. i). Phaseolus has been investigated especially by Tschermak (278) who records some apparently anomalous results, de Vries (298, n. p. 76) states that he found that extracted F2 dwarf Antirrhinum did not breed true, but threw plants of various heights. The experiment should be repeated. 2. Branching habit and the unbranched habit. Sun- flower (Helianthus, Shull, 241) and Cotton (Balls, 6). The branched form of Stock (Matthiola incana] is dominant to the unbranched Brompton type. In F^ the unbranched type reappears, but the ratio has not been determined (Fig. 4). E. R. Saunders (unpublished). 3. The straggling habit of both the tall and dwarf " Cupid " Sweet Peas, and the much-branched erect habit of the " Bush" Sweet Peas (R.E.C. 22). The relation of these two types to each other is not altogether simple. As described (q.v.} F^ from Cupid x Bush is a reversionary form exactly like the normal tall variety. Neither the tall varieties nor the Cupids show the profuse branching of the Bush Sweet Peas which gives them their peculiar appearance. This is evidently recessive to the unbranched condition, and the fact thus stands out in contrast to those observed in the case of Sunflower and Cotton. But in the Sweet Pea we have the additional complication that the factor which represses the excessive branching by its presence gives increase of height. The tall and the Bush differ from each other in respect of this factor only. It is present in the tall but absent from the Bush. In the cross between Bush and Cupid two pairs of factors are concerned as explained in the passage referred to. 4. Hairiness and glabrousness. Lychnis. Matthiola (Stocks). Wheat. The case of Lychnis has been studied by de Vries (288) and R.E.C. (19). In crosses between fully hairy and glabrous strains the discon- tinuity is complete. Various forms intermediate in hairiness may nevertheless be found wild and are by no means rare. Silene inflata 2O Structtiral Characters: Plants [CH. often exists in two forms, hairy and glabrous, growing side by side, and doubtless their genetic relations are the same as those found for the corresponding varieties of Lychnis. In this species a third form is found with hairs on the edges only (12). The case of Matthiola is important and presents features of special interest, R.E.C. (19, 20, 21, see also Correns, 61). Between thoroughly hoary and glabrous strains the discontinuity is absolute, and the glabrous Fig. 4. Matthiola. Branched and unbranched forms in F». A photo- graph of Miss Saunders' plants, the leaves removed. (Supplied by Miss Killby.) are entirely destitute of hairs. The dominance is complete and homo- zygotes cannot be distinguished from heterozygotes. A third, or "half hoary " form exists, which is glabrous or nearly so on the upper surface only. Its behaviour has not been fully investigated (19, p. 33.) The genetics of hairiness in wheat have been studied by Spillman (247), Tschermak (270), Biffen (27). The heterozygotes are sometimes inter- mediate in hairiness. The Peach and the Nectarine are probably related to each other as hairy dominant and glabrous recessive. Peculiar results are recorded in Cotton (Balls, 6). II] Structural Characters: Plants 21 Datura. 5. Prickliness and smoothness of fruits. (R.E.C. 19, 20.) Ranuncuhis arvensis (20). The case of Datura is interesting from the fact that it sometimes has mosaic fruits, one quarter or one half being prickly and the rest smooth. This is perhaps to be regarded as indicative of segregation occurring among zygotic cells (see Chap. xv.). Ranunculus arvensis has three types, spiny, tuberculated, and smooth. The first is a simple dominant. Tuberculated x smooth gave Fl partially spiny (2 1, p. 55). 6. Absence of glands (Matthiola incana] on leaves was dominant to presence of glands (M. sinuatd] (R.E.C. 20, p. 40). Fig. 5. Cross between a normally awned Barley and a variety with " hooded " awns. P, P, the parents. Fl shows partial dominance of hoods. The increase in length of ear is noticeable. The case also illustrates the result of crossing a 2-row type with a 6-row type, showing dominance of the former. (From Professor Biffen's specimens. ) 22 Structtiral Characters: Plants [CH. 7. Rough and smooth foliage. Wheat. Biffen (27). 8. Keeled glumes and rounded glumes. Wheat. Ibid. 9. Beardless and bearded ears. Wheat. Ibid. Also Spillman (247) and Tschermak (270). Most, if not all, of the "beardless" varieties exhibit a slight and variable amount of awn especially on the uppermost spikelets (Fig. 4). 10. The "hoods" or " Kapuzen " characteristic of certain Barleys show a partial dominance over the normal type. These hoods, Professor Biffen states, are, structurally, aborted florets (Fig. 5). Tschermak (270), Biffen (30). 11. Hollow and solid straw. Wheat. Biffen (27). This is a structural character of an interesting kind, and one upon which the commercial value of straw very largely depends. It was shown that many factors were concerned in the production of the stem-characters ; and in F^ by the recombination of these factors a great variety of straws appeared. 12. Blunt and pointed pods. Pisum. Tschermak (271), R.E.C. (20). Phaseolus. Tschermak (272). The dominance in this case is complete. Some varieties exist in both a blunt and a pointed type (e.g. Button's Continuity). The nature of these cases is discussed later. 13. Lax and dense ears of Wheat and Barley give different results according to the varieties used. Sometimes F1 is lax, sometimes it is intermediate (Spillman, 247; Biffen, 27, 28). See Fig. 6. In Barley an increase in ear- length has been observed (Fig. 5). 14. Development of fibrous parchment-like lining to pods, and the absence of the same which constitutes the "sugar peas." Pisiim. In Phaseolus (kidney-beans), where similar types occur, the evidence is that the dominance is reversed (Emerson, 120, 121). This is one of the features originally investigated by Mendel. He regarded the parchmented type as a dominant. In our experiments F1 has always had some parchment but the quantity is so much reduced as to cause the heterozygote to have a very distinct appearance (R.E.C. 20). 15. Much serrated and little serrated edges of leaves. Urtica (cp. Phyteuma, Correns, 70, p. 197). This cross n] Structural Characters: Plants was described by Correns (77) who gives a striking diagram representing his results. The cross was made between two forms known as Dodartii and -bilulifera, which were ^« P C X .^ -§'s« •or5 a ^H g "H ^ S i 8 -o ™ P-1 rt 'o ^ T3 -C TD I! g '"H, rt — ' CX " C 3 U * -^ g (•* O *-• C3 <: « M- OJ U3 K — , o Bi ** .H rt tf> ^J ID OJ bb £ regarded by Linnaeus as distinct species. The almost entire-leaved Dodartii has been treated by later authors as a variety QipHulifera. 24 Stmctural Characters: Plants [CH. 1 6. Palmatifid or "palm-leaf" and pinnatind or " fern -leaf." Primula Sinensis (Fig. 7). The fern-leaved form arose in English horticulture about 1860 as a variation from the normal type. I have had opportunities of seeing its genetic behaviour on a large scale at Messrs Sutton's, and many experiments have been made with it by Mr R. P. Gregory in conjunction with me. Dominance is usually complete, but at Messrs Sutton's I have seen on two occasions strains containing plants of intermediate leaf-shape, which were presumably hetero- zygous, for the two types occurred on sister-plants. The leaf-shape is entirely independent of the colours and other features of the plant, and can be transferred bodily from one colour-type to another. Messrs Sutton's varieties " Mont Blanc" and "Sirdar," for example, are sold both in the palm-leaved and in the fern -leaved forms. Fern-leaf (R) Palm-leaf (Z>) Fig. 7. The two types of leaf found in Primula Sinensis. 17. Leaves and petals normal or laciniated. Cheli- donium majus. de Vries (290) and (298), i. p. 134. This case is interesting in comparison with No. 15. .In the Nettle, serration is a dominant, while here laciniation is a recessive. A careful study of the physiological distinction between the two processes would probably lead to important results (cp. Leake, 170, on leaves of Cotton). n] Structural Characters: Plants 25 1 8. Certain leaf characters in Capsella bursa-pastoris. Shepherd's Purse. (Unpublished work of Shull ; about to appear as a publication of the Carnegie Institution.) 19. Various characters in the seed of Cotton. Balls (6). Many of these are of great commercial importance. Balls (6) gives the following list : Dominant. Recessive. Long staple. Short staple. Regular distribution. Irregular distribution. Coloured lint. White lint. Silky lint. Harsh lint. More fuzz. Less fuzz. He says that all the desirable characters are dominant, and that hence the chances of picking out a stable form by common selection are very small. Individual selection must be adopted. 20. Biennial habit and annual habit. Hyoscyamus. Correns (73). More research on the relations of annuals to biennials is greatly to be desired. Points of the highest physiological interest are involved. In connection with root crops also some questions of commercial importance are raised. In R.E.C. (19, p. 135) I ventured to suggest that the persist- ence of " runners " which go to seed in such plants as Beet and Mangel may be due to want of individual selection of pure dominants, and in view of Correns' observation the probability of this suggestion is increased. 21. Normal stem and fasciated stem associated with peculiar distribution of inflorescences. Peas (Pisum). Mendel (195), R.E.C. (20). In our experience various intermediates occur in /%. 22. Susceptibility to rust-disease (Puccinia glumaruni) and resistance to the same. Wheat. Biffen (27, 29). This is perhaps one of the most important instances to which Mendelian method has yet been applied. Using a variety very susceptible to rust and another practically immune to its attacks Biffen found that F^ was not perceptibly less attacked than the rusty type. Fz showed ordinary segregation, and the green, resistant plants, standing among the yellow rusty ones, formed a very striking spectacle. The recessives bred true and their progeny has remained rust-proof. It has not yet been shown to what the resistance is due. Working with Professor Biffen, Miss Marryat (193) found that the rust-hyphae are checked after entering the stomata of the resistant plants. If, as may be suspected, the resistance is due to the presence of some anti-toxin, the dominance of " susceptibility " must be taken to indicate that the formation of the anti-toxin is prevented by the presence of a factor in the dominant forms, a conclusion which may lead to definite progress in the physiology of disease-resistance. 26 Structural Characters: Plants [CH. 23. Flat standard and hooded standard in the flower of Sweet Pea. R.E.C. (22). See Plate V. Some very curious phenomena have been observed in this case, which are described in Chap. ix. The type known as "Snapdragon," perhaps an extreme form of hood, is also a recessive to the flat type. R.E.C. (20, p. 83). 24. Imbricated petals and stellate or "star "-type. Primula Sinensis. Observations made at Messrs Sutton's and experiments of R. P. Gregory with W. Bateson. (See Fig. 14.) 25. The monstrous condition of the calyx in which it resembles the corolla, seen in " hose-in-hose " Campanula is an imperfect dominant to the normal. Associated with this homoeotic variation, the female organs are more or less completely sterile in certain strains. Correns (76). This subject is discussed in Chap. xi. 26. Abortion of the female organs in the lateral florets of Barley, as found in the 2 -rowed types, and the complete or hermaphrodite development of the florets, as in the 6-rowed types (Fig. 8). Tschermak (270) and (275), p. 1 1. Biffen (30). This case is somewhat complex. There are three types, (i) Six-row, in which 3 perfect hermaphrodite florets are developed in each spikelet. All set seed and the result is that the ear has 6 rows of seeds. (2) Types in which the lateral florets have anthers but no female organs. (3) The " Abyssinian " type in which the lateral florets contain neither male nor female organs. The types (2) and (3), being able to make seeds only in the central florets of each spikelet, alike develop two rows of seeds. When (i) is crossed with (3), Fv is like (2); and (2) crossed with (3) gives FI also like (2). Some further complexities have been observed, but in general it appears that the dominant factor has the power of partially preventing the formation of the reproductive organs in the lateral florets. The facts may perhaps be interpreted as bearing on the phenomenon of Sex. Tschermak (275) describes crosses between a 2-row and a "4-row" type. From Professor Biffen I understand that the latter is in reality a lax-eared 6-row type. F^ is 2-row, and in Fz the ratio is 12 2-row : 3 "4-row" : I 6-row. This is a special case of the ratio 9:3:3:1, lax- ear and 2-row being dominants. Tschermak and Shull (242) regard it as an illustration of the effects of a latent factor. n] Structural Characters: Plants 27 27. The two-celled type of fruit is dominant to the many-celled type in Tomato. Price and Drinkard (221). The case is one of the few in which the genetic behaviour of a meristic or divisional feature has been investigated apart from any complexity introduced by differentiation. Fig. 8. Cross between Abyssinian 2-row Barley and a club-headed 6-row type. The middle figure shows F^. The length of ear is increased. (From Professor Biffen's specimens.) 28. Style short, associated with large pollen grains, constituting the "thrum" type, and style long, associated with small pollen grains constituting the "pin-eyed" type. Primula Sinensis and acaulis. Bateson and Gregory (17). The short styled type has been found in the homozygous condition in P. Sinensis but not yet in /*. acaulis. For the relations of these types to the " homostyled " form, see p. 68. Dominance is complete. 28 Structural Characters: Plants [CH. 29. Long style and short style in Oenothera. (This difference is probably quite distinct in nature from ordinary heterostylism as seen in Primula, &c.) de Vries (290). The same fact has been observed by Balls (6) in Cotton. 30. Normal long pollen grains with three pores, and rounded pollen grains usually with two pores. Sweet Pea (Lathyrus odoratus]. R.E.C. (20, 21, 22). See Fig. 18. 31. Normal anthers and sterile anthers. Sweet Pea. R.E.C. (20, 21, 22). With regard to these two last features numerous complications occur, which are described in later chapters. 32. Roundness of seed connected with the presence of starch in large elongated simple grains, and wrinkledness of seed connected with the presence of peculiar compound starch-grains. Pisum. This is one of the most familiar of Mendel's original examples (see Fig. 3). It has been re-investigated by many observers. Correns (60) ; Tschermak (269, &c.) ; R.E.C. (20); Hurst (155); Lock (172, 173). The F^ seeds made by fertilising an emasculated flower of a wrinkled variety with pollen from a round variety, or vice versa, are generally ordinary round seeds, and F \ shows the common ratio 3 round : i wrinkled, the two types being mixed in the pods of the Fl plants. Among the multitude of varieties of peas now cultivated there is a great diversity both of rounds and of wrinkleds. The interrelations of these several types, even as regards seed-shape, have as yet been imperfectly explored. The degree to which the wrinkles are formed is fairly uniform for any one type, but the various types show different degrees of wrinkling. The differences obviously depend chiefly on the chemical and physical properties of the reserve- materials in the cotyledons, and an analysis of these peculiarities might lead to further discoveries. Gregory (134) found that the starch in round peas occurs chiefly as large elongated simple grains, whereas in wrinkled peas it is in the form of small grains of irregular shape which are often compounded together (Fig. 9). Darbishire (94) added the interesting fact that in F± the grains are intermediate, many being large and simple, but round instead of elongated, with an admixture of compound grains. He confirmed also Denaiffe's observation* that wrinkled take up more water than round, but he found that F-L is intermediate in this respect, and he suggests that the size, the * Denaiffe, Les Pois potagers, p. 9. II] Structural Characters: Plants 29 shape of the grains, and their simple or compound nature, may be governed by distinct factors. He regards the absorptive power as again separable from these features. Round and Indent. Wrinkled. Fig. 9. Outlines of starch grains in the different types of peas. The wrinkled contains many compound grains. (From Gregory.) Magni- fication the same in both figures. A third type of pea, of which the purple sugar-peas (sans parchemiri) are a good instance, may be described as "indent." These also have large, simple starch grains. Such seeds are of irregular flattened shape and may be confounded with true wrinkled peas. Their properties are entirely different, and the two sorts must be carefully distinguished. One of their special properties will be discussed in a later chapter, but here it must suffice to say that their genetic properties are essentially those of round peas. Much confusion has been introduced by want of care in distinguish- ing these types. Intermediates, which on casual sorting, cannot be classed either as round or wrinkled, sometimes occur. Some of the round types (e.g. Victoria Marrow) contain a large proportion of such seeds. Their peculiarity is almost certainly due to environmental influence, though obviously the liability to this affection may be transmitted. When such seeds are found in F» from a cross between thoroughly round and wrinkled varieties, the pitting, when it exists, generally affects all the round seeds of the pods in which it occurs. With experience such pitting can immediately be recognized as distinct from the true genetic wrinkling, and in our experiments the results of a further sowing have repeatedly confirmed the judgment made by inspection of the seeds. A complete account of all the phenomena would run to great length. The interrelations of round and wrinkled seeds are to be recommended as offering perhaps the most favourable example for an investigation of the chemical nature of a genetic factor. The wrinkling is evidently the consequence of a particular method of drying, and this must depend on the nature of the reserve-materials. A first step would be to determine the relative amounts of sugar and starch in the two chief types. It is natural 30 Striictural Characters: Plants [CH. to suppose that the wrinkled peas are those in which the transformation of sugar into starch has gone less far than in the round peas ; but, as much starch is formed in the wrinkleds, one ferment having this transformative power must be present in them. Hence we are led to suppose that in the round pea a second ferment is present which can carry the process further. As offering an attractive problem in physiological chemistry the phenomena are recommended to those who have the requisite skill to investigate them. 33. Starchy endosperm giving a full, rounded seed, and sugary endosperm giving a shrivelled and wrinkled seed. Maize. Fig. 10. de Vries (290); Correns(63); Lock (i?2, I74)- Fig. 10. A cob of Maize (Zea mays] borne by an F^ plant from the cross round x wrinkled, fertilised with its own pollen, showing the mixture of round (dominant) and wrinkled (recessive) seeds. (From a specimen given by Dr Webber.) Of the various Mendelian experiments this is one of the most demon- strative. Dominance is perfect so far as external observation goes. Correns records a remarkable excess of round seeds as recurring with great constancy in certain families when F^ is self-fertilised (see later). It often happens that pollen from one variety of maize is blown by the wind to the stigmas of another variety. If this pollen possesses a dominant factor capable of affecting the seed, seeds exhibiting it are formed. If for instance pollen from a round maize is blown on to a wrinkled or sugar-corn, round seeds will be formed among the normally wrinkled seeds. When formerly it was supposed that the endosperm, which contains the reserve- materials, was a maternal structure, the change in the seed was regarded as an influence exerted by the embryo on the maternal tissues. The effects of such influences were called by Focke " Xenia" There are a few examples of such influence which may with probability be regarded as genuine*; but since the discovery of the fact that the endosperm of maize results from a double fertilisation effected by the second nucleus of the * The phenomena are discussed by Darwin, An. and Pits., ed. n. 1885, i. pp. 428-433. It seems likely that in some of these instances the factor introduced by the pollen-grain can influence or infect tissues in contact with the embryo. n] Structural Characters: Plants 31 pollen-tube, cases like that of maize are not strictly to be classed as Xenia (see Correns, 58). 34. Glutenous and starchy endosperms. Wheat. Biffen Professor Biffen's researches respecting these important features are not yet completed. The glutenous, translucent, hard type has definite dominance over the opaque, soft, starchy type. 35. Single flowers usually behave as dominants to doubles, as in Stocks, Primula, &c. In Carnations the doubleness dominates. The most extensive researches on the genetics of double- ness are those of Miss Saunders in the case of Stocks (Matthiola)) R.E.C. (20-23). The peculiar phenomena discovered are discussed in a separate chapter (q.v.}. 36. In Phaseolus hypo-geal cotyledons are dominant to epi-geal. Various intermediates in /%. Tschermak (278, p- 54)- This list and that which follows make no pretension to completeness. Those features are enumerated which either seem of special interest, or have been studied with some thoroughness. Indications respecting many more are to be found in the original papers (see especially for Peas and Phaseolus the writings of Tschermak and Lock ; for Cotton, Balls; for Oenothera, &c., de Vries, and Macdougal (186); for Wheat and Barley, Biffen, and Tschermak; for Maize, Correns, and Lock; for various plants, Correns, and de Vries). In the genus Brassica numerous crosses have been studied by Sutton (262). In his experiments it was found, among other important results, that the bulbing of the Swede, Turnip, and Kohl Rabi disappeared completely in crosses with non-bulbing Kales, and that in Fz imperfect bulbing reappeared. Professor Biffen, who is continuing work on the same lines, tells me .that in regard to these and similar characters cultural conditions play a great part, and lead to curious and conflicting results. 32 Structural Characters: Animals [CH. Animals. Structural Characters. MAN. A considerable number of diseases and malformations have been shown to behave usually as dominants. A few conditions may be said, more doubtfully, to behave as recessives. The subject of human inheritance is discussed in Chap. xn. Of normal characteristics, eye-colour is the only one yet studied (Hurst, 161)^ sufficiently to justify a positive statement as to the existence of a Mendelian system of descent. CATTLE. 37. Absence of horns in polled breeds of Cattle is dominant to the presence of horns (R.E.C. 19; Spillman, 246). In sheep the inheritance of horns is sex-limited (g.v.\ and from evidence given me by Mr E. P. Boys-Smith I suspect that this is true in the case of Goats also. HORSE. 38. There is little doubt that the gait known as "pacing" is recessive to the ordinary trotting gait in the American trotters. Trotters bred together may produce pacers, but hitherto I have found no authentic instance of genuine natural pacers, when mated together, producing trotters. Correspondents have sent me word of several apparent exceptions to this rule, but all on inquiry have proved to be erroneous. In the pacing gait the two legs of the same side of the body are moved together or nearly so, while in trotting the foreleg of one side moves almost with the hind leg of the other. Horses may be trained with more or less success to adopt either gait, but the distinction between natural pacers and natural trotters is a fairly sharp one (16). The physiological nature of the difference is quite obscure, but presumably it is of nervous origin. MOUSE. 39. From time to time mice are found hairless, with the skin thrown up into corrugated folds. Experimenting with such mice Mr Archibald Campbell found the condition to * See also Davenport (107). ii] Structural Characters: Animals 33 be a recessive, the presence of normal fur being a dominant. The fur grows at first normally and falls off as maturity is reached. Of 12 Fz mice 3 lost their hair. I am indebted to Mr Campbell for information respecting this interesting case, and for living specimens. The attempt to breed the recessives together failed, but in Gaskoin's case* naked parents produced young like themselves. From his account it appears that the young which he observed never grew their hair, but the fact is not absolutely certain from the description. 40. The normal condition and the ''waltzing" habit in Japanese mice. The waltzers exhibit a peculiar vertiginous movement of the head when they come out into the light, and spin often with extreme rapidity, running after their tails till apparently exhausted. Our knowledge of this case is derived from Von Guaita (135) and Darbishire (90). The dominance of the normal type is complete, and in F.2 waltzers reappear. The F ., numbers obtained by Darbishire were 458 normals, 97 waltzers, where the expectation is 386 normals, 139 waltzers. The deficiency may perhaps indicate a complication, but more probably it is due to the greater delicacy of the abnormal mice, which was so great that all attempts to breed them together were unsuccessful. RABBIT, GUINEA-PIG. 41. Normal short hair and the long "Angora" hair Rabbit, Guinea-pig, and doubtless Cat (see Hurst, 157 ; Castle, 45 and 48 ; Sollas, unpublished ; Castle and Forbes, 55). Castle (48), p. 64, gives important details as to the physiological nature of the distinction between the normal and "Angora" hair, which he regards as resulting from a special method of growth. 42. The rough or resetted condition of the coat in the Guinea-pig dominates over the normally smooth condition (Castle, 48 ; Sollas, unpublished). Castle found occasionally that animals partially resetted occurred in Fz. * For references see Bateson, Materials for Study of Variation, 1894, p. 56. A good figure is given by Gaskoin, Proc. Zool. Soc. 1856. B. H. * 34 Structural Characters: Animals [CH. 43. Polydactylism occurred in a Guinea-pig, the off- spring of normal parents, and ran an irregular course in its subsequent descent (Castle, 49). CAT. 44. The abbreviated tail of the Manx Cat is a dominant (more or less imperfect) to the normal tail (see Anthony, 2 ; Hind, 151; Davenport, 98 ; Kennel, 166,0). 45. Polydactylism is almost certainly dominant; but, as in other types, irregularities doubtless occur. FOWLS. For the study of heredity Fowls are especially well suited. In addition to their many colour-characteristics the various breeds present a great range and variety of struc- tural features. Among the long series of offspring which hens of the more fertile breeds produce, the descent of these charac- teristics can be watched in families of ample length. The chief papers dealing with Fowls are R.E.C. (19-22); Hurst (156); Davenport (101). The following is a list of the principal facts already elicited as to the behaviour of these structural features but much remains to be done. 46. Various shapes of comb, for example the rose comb and the pea comb, are both dominant to the single comb. The double or longitudinally split condition is also dominant to the unsplit. See pp. 61-7. Many of the finer details in regard to the heredity of comb-shapes are not yet clear. The classification of the comb-types in the newly-hatched chickens is generally very easy, but in occasional strains forms intermediate between the pea and the single occur, which may probably be due to subtraction-stages of the pea factor (q.v.\ Some of the singles extracted in f\ from various crosses have lateral " sprigs " — as fanciers say. It is not impossible that these irregular processes are due to additional minor factors, but they are subject to so much fluctuation that their descent would be very difficult to trace. The comb of the Silky fowl is a rose, + a trifid element which causes its posterior end to be divided into three irregular points. In F^ from Silky x Single, regular rose combs are produced in those individuals which have the rose factor without this trifid element. Attention may be called to the dominance of the median splitting of the comb found in certain breeds, for the facts may have a bearing on the genetics of meristic characters. Splitting of the comb may occur in one of n] Structural Characters: Animals 35 several distinct ways. It may affect mainly the anterior portion, or the posterior. The split combs of established breeds have possessed ordinary dominance; but a form of posterior splitting somewhat like that of the F^ from Breda x Single occurred apparently as a mutation among extracted singles, and exhibited a curious genetic behaviour suggesting irregularity of dominance (20, pp. 108 and 113). 47. The normally webbed feathers are dominant to the peculiar feathers of the Silky fowl. 48. Extra toe is usually dominant to the normal four- toed condition, but exceptions occur. This irregularity of dominance is exhibited by all cases of polydactylism yet studied in birds or mammals. It seems to be a property of certain strains. Some families run a perfectly regular Mendelian course, others contain members with only the normal four toes, which are yet capable of transmitting the extra toe. The numbers in such families are not favour- able to the suggestion that the irregularity is caused by a definite disturbing factor. 49. Crest is dominant to no crest. Fz may contain individuals with crests far larger than those of the parent crested breed, a fact which suggests that in breeds with small crests (e.g. Silky) the full development of the crest is kept in check by some other factor. 50. Feathered leg partially dominates over clean leg. Both Hurst (156) and Davenport found dominance very irregular. F^ is intermediate, and traces of leg-feathering are occasionally seen in the offspring of clean-legged birds. 51. "Frizzling," or turning back of the feathers, is dominant to the plain straight feathers of the normal. 52. Normal size of feathers on the hocks, or tibio-tarsal region, is dominant to elongation of these feathers to form quills — the " Vulture-hock " of fanciers. 53. Muff, or tuft of feathers at sides of the bill and throat, as in Faverolles, is dominant to no muff, as in ordinary breeds. 54. Imperfect development of coccyx and tail-feathers with absence of tail, as in " Rumpless " fowls, is dominant to the normal development of those parts. This interesting case was investigated by Davenport (101). It is exactly comparable with that of the Manx Cat. 3—2 36 Structural Characters: Animals [CH. 55. Certain breeds (Houdan, Polish, Breda) have an extraordinary development of the nostril, which is patulous, with alae horizontal instead of curving downwards. This peculiarity is recessive to the normal (Davenport, 101). Hurst has observed the same thing and Mr Punnett and I have similar evidence from the Breda. Davenport states that in his experience the "high" nostril is never combined with a fully developed comb. 56. The tendency to go broody and sit on eggs dominates over the absence of this instinct, characteristic of several Mediterranean breeds. There is probably segre- gation in regard to these two dispositions, but this cannot yet be asserted positively. In regard to fertility as measured by egg-production there is as yet no clear evidence. 57. The loud and penetrating shrieks which the cocks (and to a less degree the hens) of an Egyptian breed give out when caught, were reproduced almost exactly by the J\ generation from a cross with a non-shrieking breed. Though numerical data in regard to such a character are scarcely attainable, there is little doubt of the segregation as evidenced by /%. PIGEONS. 58. The normal foot is dominant to the webbed con- dition of the toes which sometimes occurs as an abnormality (Staples-Browne, 254). Mr J. L. Bonhote tells me that in his experiments normal birds have produced webbed offspring. He is making further experiments with this family. 59. The "shell," or turning-back of the head-feathers of the Nun is dominant to the normal plain head (ibid.\ 60. Birds with normal, i2-feathered tails crossed with the many-feathered Fantail give intermediate numbers in F ^ In /% i2-feathered tails reappear, but, so far, no real Fan has come from the cross-breds. Mr Staples- Browne, to whom I am indebted for this information, will publish a complete account of his evidence. He tells me that the extracted 12 -feathered birds do not breed true, but may throw birds with 13 or 14 feathers. n] Structural Characters : Animals 37 CANARIES. 61. Crest is dominant to plain-head, as the non-crested condition is called by fanciers (R.E.C. 19, p. 131 ; Daven- port, 105). The type of crest which fanciers admire consists of feathers neatly laid down over the head. To produce such birds crested individuals are bred with plain-heads, and it is clear that the exhibition type of crest is a heterozygous form. When crested birds are bred together it is said that an ugly, standing crest frequently is produced, and presumably this is the homozygous type of crest. The mating of two crested parents is by several authors said to give rise to some bald birds. Other writers (e.g. Blakston) have ridiculed this statement, and formerly I was inclined to regard it as a mere exaggeration, but Davenport in his recent paper mentions bald heads as sometimes occurring among his crested birds. He has kindly supplemented his published account with the statement that the bald patch is an area " on the back of the head varying from four to six millimetres in diameter practically without feathers and remaining featherless throughout life. The crest, however, on top of the skull remains perfectly evident, and often baldness can only be detected by blowing the feathers." In no case was such a bald patch found in a plain-head. The bald patch on the occiput is recognized by Blakston (Cage Birds, p. 104) as a property of crested birds, and presumably the "balds" alleged to come from the mating of two crests are birds homozygous for crest- factor, in which the crest stands up and allows the bald patch to be seen. Davenport had a crested bird without any bare patch, and he found that the feathering in this region was due to a separate dominant factor. Animals and Plants in which Colour-Characters have been shown to have a Mendelian Inheritance. The phenomena of colour-inheritance are complicated in several ways. Some of these complications which are of great importance and interest will be considered in subsequent chapters. It is, however, convenient to enu- merate the genera in which Mendelian heredity has been observed in order to illustrate the scope of the principle. The following- list of genera contains the chief of those in which heredity according to a Mendelian system has been shown to occur. In some of them as the result of extensive research many Mendelian features of colour have been discovered, and the existence of numerous colour- factors is demonstrated. In others only one such factor for colour has been detected. 38 Colours of Plants [CH. Plants. Antirrhinum (Snapdragon). Orchids (several genera). Atropa. Papaver. Brassica (Turnips and Swedes). Phaseolus. Clarkia. Phyteuma. Coreopsis. Pisum. Datura. Polemonium. Gossypium (Cotton). Primula. Helianthus (Sunflower). Salvia. Hordeum (Barley). Solanum (Tomato). Hyoscyamus (Henbane). Triticum (Wheat). Lathyrus (Sweet Pea). Verbascum (Mullein). Lychnis. Viola. Matthiola (Stocks). Zea (Maize, Indian Corn). Mirabilis. Animals. Man. Fowls. Mice. Pigeons. Rats. Canaries. Rabbits. Axolotl. Guinea-pigs. Lepidoptera, various (Silkworm ; A- Horse. braxas grossulariata ; Angerona Pigs. prunaria, &c.). Sheep. Coleoptera (Lina ; Leptinotarsa • Cattle. Crioceris). Cats. Helix. For the convenience of readers acquainted with the phenomena in outline and desirous of pursuing the subject further the following brief annotations are placed here. Until the chemistry of pigmentation is better understood, a comparison between the behaviour and properties of the several types cannot be instituted with much confidence. Antirrhinum. Wheldale (303) has shown that the lowest or hypostatic factor dominant to albino gives yellow in the " lips " of the flower ; the addition of various other factors produces anthocyan reds which superposed on the yellow give deep crimson red colour. A second series of reds, more purplish or magenta in tint (colour of wild A. maj'us), results from addition of a factor which in absence of anthocyans gives an ivory colour. This ivory is epistatic to yellow. It is remarkable that the lowest anthocyan factor gives red in the tube with a tinge in the lips, while the addition of the next above it gives the self-coloured flower. There is also a white-tubed type of each colour-combination ("Delila" of de Vries, 298). All the factors except those for yellow and ivory can be carried by the albino. Among the reds several heterozygous combinations can be recog- nized. The heredity of striping is still under investigation. Atropa Belladonna. The normal dark-fruited type is dominant to yellow-fruited (de Vries, 290; Saunders, 19). n] Colours of Plants 39 Brassica. White chromoplasts dominant to yellow in Swedes and Turnips (Sutton, 262). Clarkia elegans. Common magenta-red dominant to salmon pink (Bateson and Punnett). Coreopsis tinctoria. Ordinary yellow type dominant to var. brunnea with brown flowers (de Vries, 290). The brown flowers like those of Cheiranthus (Wall-flower) are no doubt due to presence of much dark anthocyan, and the case is probably one in which the development of little anthocyan dominates over the development of much anthocyan (cp. Lathy rus, Primula, &c.). Datura. Purple in flower or stem dominant to white flower and green stem (de Vries, 290; Saunders, 19). Gossypium (Cotton). Dominance of many colour-characters in plant, flower, and seed (Balls, 6). F% details not yet published. Helianthus. Purple disk dominant to yellow disk (Shull, 241). Hordeum (Barley). Black pigment in paleae dominant to its absence (Tschermak, 270; Biffen, 30). Hyoscyamus niger annuus x H. niger pallidus were found by Correns (69) to give F-i flowers of intermediate tint. Lathyrus (Sweet Pea). Anthocyan colours dominant. Purples dominant to reds. Colour depends on two complementary factors. Yellow chromo- plasts recessive to colourless. Facts fully described in later chapters. Plants with coloured flowers have dark seed-coats. Whites have seed-coats colourless. Lychnis. F^ between L. diurna and L. vespertina has flowers of inter- mediate tint ranging through many grades (see de Vries, 290 ; Correns, 69; R.E.C. 19). Segregation imperfectly studied. Matthiola (Stocks). Colours as in Sweet Pea (R.E.C. 19-21; Tschermak, 278; Correns, 61). For colours of seeds see R.E.C. 19. Mirabilis. Colours consist of a complex series of reds and yellows, the interrelations of which are not yet clear (see Correns, 67, 74, 77). Miss Marryat's experiments (unpublished) prove the existence of a number of heterozygous forms. Orchids. Dominance of anthocyan colour in Cypripedium is clear. In that genus it results from union of two complementary factors (Hurst, Gard. Chron. 1908, i. p. 173). As regards distribution of colour the facts are complex, but several indications of Mendelian distribution have been recognized (Hurst, 153, 160). See p. 96. Papaver. Presence of dark purple spot at base of petals dominant to the absence of such colour (de Vries, 290). Phaseolus. The elaborate researches of Tschermak (271-3, 275, 278) have demonstrated the existence of numerous factors controlling the colour of the flowers and seed-coats in P. vulgaris, P. multiflorus and their hybrids. The flower-colours are purples, reds, and white, with a bicolour form of the red ("Painted Lady"). Colour has not yet been produced by union of 40 Colours of Plants [CH. two whites, -but on the analogy of the Sweet Pea such a result may be attainable. Otherwise the same rules apply generally. At least two sets of pigments take part in coloration of seed-coats : (i) brown, (2) purple. White- flowered plants have seed-coats unpigmented, and the bicolour flowers go with parti-coloured half-white seeds. The development of purple in the coats and the pattern in which it is deposited depend on various factors which can be carried by the albino. Various complications were met with (see originals). The cross between the two species showed some degree of sterility. Similar results were obtained by Emerson (120, 121). The seed-coats of heterozygous plants in some cases were distinguishable. He also found green pods dominant to yellow pods (cp. Pisum), Further facts, with a scheme elucidating some of the curious ratios which the seed-colours may exhibit (e.g. 18 : 18 : 6 : 6 : 16) in F2, are given by Shull (242). Phyteuma Halleri (dark violet) x P. spicatum (white) gave two types in FI, 5 plants being bright blue with violet tinge, 4 violet (Correns, 70). Pisum (Edible Peas). Flower-colours of three types, (i) Purple, Standard a pale purplish white ; wings deep chocolate purple. (2) Pink, Standards pinkish white ; wings a fine salmon-pink. (3) White. F% containing all three types is the usual 9 : 3 : 4 in order named. Mark in axils of leaves, if present, is purple in (i), red in (2), absent in whites. Tschermak experimented with a purple strain without the axil-mark, and found that, as in Sweet Peas, the factor for that character can be carried by the albino. Seed-coats colourless or greenish in white-flowered plants. In plants with coloured flowers one or more of three distinct kinds of pigments always present: (i) a purple, occurring in spots, (2) a brown, distributed either generally over the surface, or in bands (as in Maple peas), (3) an insoluble greenish grey, distributed over the whole testa. Neither (i) nor (2) can be developed in the absence of (3), but traces of (2) may sometimes be seen in white-flowered plants. There are separate factors for (i), (2), and (3), of which (i) and (2) may be carried by the whites (Lock, 176). Cotyledon-colours are yellow, and green. Yellow is a dominant, and heterozygotes are indistinguishable from homozygous dominants. In rare cases green has been seen as exceptions in Flt but these are probably due to abnormal conditions. Many modern varieties have cotyledons patched with green and yellow. Genetically these are greens which show a special liability to bleaching. If protected while ripening they remain green. Colours of Pisum have been chiefly studied by Mendel (195) ; Tscher- mak (269, 271-3); Correns (60); R.E.C. (20); Lock (172, 173, 175-6); Hurst (155). Polemonium. Correns (70) found the white var. of P. coeruleum dominant to the yellow of P. flavum ; and the blue type of coeruleum * flavum gave Fl blue. It may be inferred that the yellow of flarum is a chromo- plast colour, and that the blue anthocyan dominated as usual. Hybrids sterile. Primula, P. Sinensis exists in a long series of colour-types the relations of which are still being investigated by R. P. Gregory in con- n] Colours of Plants 4 1 junction with me. Some of the more striking facts are referred to in later chapters. White flowers with green stem constitute an albino, recessive to all colours. The magenta shades have a factor epistatic to crimson and pink. Blue is hypostatic to all the red shades. The whites which have red or reddish stems are dominant whites, showing only a pale shade or tinge of colour in f1. Deep colours cannot appear on stems that are not red except in the white-edged "Sirdar" (q.v.). Salvia Horminum. Purple, red, white, related as in Pisum, &c. Saunders, R.E.C. (20). Triticum (Wheat). Red chaff is dominant to white chaff. Grey chaff is epistatic to red and dominant to white. Tschermak (270) ; Biffen (27); Spillman (247). Verbascum blattaria. Yellow (a sap-colour) dominant to white. Shull (241). Viola. White is recessive to colour (de Vries, 290) in V. cornuta. The brown seed-colour of V. papilionacea is dominant to buff of V. hirsutula, and the purple of the capsule of hirsutula to the absence of purple in papilionacea (Brainerd, 41). Zea (Maize). Yellow endosperm dominant to white. Blue in aleurone layer an irregular dominant to absence of blue. (Definite exceptions are frequent.) Red pericarp, a plant-character, dominant to absence of red. The relations of the striped types have not been clearly determined. Correns (63); Lock (172, 174). Colours of Animals. Man. Eye-colour ( naturalist would hesitate to regard them as four distinct specific characters ; and even as the special properties of domesticated birds I suppose they would by many be regarded as evidence of long-continued selection. Nevertheless, as will be seen, these four forms stand to each other in a simple genetic relation and the fact suggests wide possibilities in regard to many hitherto unexceptionable differences " of specific value " recognized among animals and plants. The interpretation of the facts was at first by no means easy, and I am sorry to have been responsible for the promulgation of a quite erroneous suggestion regarding them. Further knowledge of kindred phenomena has, however, made the elucidation of this case now perfectly clear and simple. To return to the experimental results. Having found that rose x pea gives walnut, the next thing to be done was to test the genetic properties of birds thus produced. This was done in two ways (i) by breeding the walnuts together, (2) by breeding them with singles. In what follows, the names may be abbreviated thus : R, rose ; P, pea ; RP, walnut ; and S, single. Experiment showed that RP x RP gave an 7% family RP, R, P, and S. The appearance of S, which was not known to have been put in, is not at first sight intelligible. Repeated trials proved that the ratio in which these combs appeared was 9RP : 3R : 3P : iS. It was further proved by experiment that the R birds were either pure R or contained the recessive S, but gave no more P or RP ; that the P birds similarly could only give P, or P and .S ; while the 6" were all pure to that character. Ill] The Comb Consistent with this result were the offspring obtained from first-cross RP x S, for this mating gave the ratio iRP : iR : \P : iS. As it was already established that R and P were each dominant to S, the inference was certain that the gametes produced by the Fl (RP) birds were RP, R, P, and 5 in equal numbers. The whole series of phenomena may be represented as due to the combinations of two pairs of allelomorphic characters or factors, namely 1. Rose (domt) R, absence of rose (rec.) r. 2. Pea (domt) P, absence of pea (rec.)/. RP RP RP RP RP Rp rP rp walnut walnut walnut walnut pure giving rose giving pea giving all 4 RP RP Rp Rp Rp rP Rp rp walnut rose walnut rose giving rose pure giving all 4 giving single rP rP rP rP RP Rp rP rp walnut giving pea walnut giving all 4 pea pure pea giving single rp RP rp Rp rp rP rp rp walnut giving all 4 rose giving single pea giving single single pure Reference to the diagram of combinations shows that the two " absences " of rose and pea respectively will meet once on an average in 16 times, and to such a meeting without doubt the appearance of the single combs as a novelty, in 7%, is to be ascribed. In the early years of this experiment, knowing that R B. H. 66 The Breda Comb [CH. and P were allelomorphic to S, I came to regard them as also allelomorphic to each other. This idea led to con- fusion, but we know now that no case justifies such an application of the principle of allelomorphism. A rose comb is not due to an elemental factor which can segregate from the pea comb factor. The two factors belong to distinct allelomorphic pairs and each in the gametogenesis of the heterozygote segregates from its own allelomorph, which is simply the absence of the factor in question. The single comb contains neither R nor P. The rose comb is a single comb modified by the presence of R, while the pea comb is produced by the presence of P. We may therefore describe the rose as R no P, and the pea as P no R. It is convenient to use capital letters for dominants and small letters for recessives, the rose being thus written Rp, and the Pea, rP. The walnut comb is the RP, while rp gives the single. The allelomorphism of the elements which go to the constitution of the shapes of combs in fowls may without doubt be carried very much further. For example there are indications that the size of the comb depends to some extent at least on other pairs of factors. Another curious set of phenomena, perhaps worth investigating further, -may be studied in a cross between a single comb breed and the 41 Breda" fowl. The Breda is usually said to have "no comb." As a matter of fact it has two very minute tubercles which represent the comb. When this breed is crossed with the single comb, /^ has what may be called a "double" single comb. It consists of two large lobes or leaves diverging outwards from a common base*. Such a comb is evidently due to the introduction by the Breda of a factor which may be called " bifidity." This factor acts on the large comb brought in by the single-combed parent and the result of the combination is a large, double comb. /% from this cross has not yet been raised, but there can be no doubt that it will contain members having the " no comb " of the Breda and the "absence of bifidity" of the single- combed breed. Such birds probably have only a minute tubercle at the posterior end of the comb region. * The two lobes sometimes unite anteriorly to a greater or less extent. In] The Breda Comb 67 Mr Hurst suggested to us that an excellent confirmation of the truth of the analytical method by which the composi- tion of the rose and pea combs has been represented could be obtained by a cross between the rose comb and the Breda, which, as has been stated, has the bifidity-factor but practically no comb at all. The elements involved are Rose, R. No rose, r. Comb present, C. Comb absent, c. Bifidity, B. No bifidity, b. The Fl generation has the composition Rr, Cc, Bb, namely a double rose comb. Fn generation contains a great variety of forms, of which those having B and C, but no R, have the high bifid comb like that of the /*\ raised in the cross Breda x Single, and those which contain only r and b in combination with C have single combs of the ordinary high type (Fig. 12). Both these kinds occurred in F2, and their appearance is entirely confirmatory of the scheme of representation adopted. Since neither the rose nor the Breda have outwardly any suggestion of single comb in their appearance, were it not for a knowledge of Mendelian analysis such a result must have seemed utterly unaccount- able. When therefore we look at such an organ as the comb of a fowl and attempt to conceive its genetic properties, we have to remember that the structure as a whole may be composite in origin, and that the visible appearances and properties may result from the interaction of a number of distinct elements each transmitted independently in gameto- genesis. Everything however points to the conclusion that the number of these elements is finite, and that their properties are not beyond the reach of orderly analysis. In the example of the walnut combs the interaction of two dominant factors is such that the 9 members of the Fz series of 16 have a feature, the walnut comb, distinct from those of the original parents, while the i member of the series which contains neither dominant factor, also has a feature, the single comb, distinct from anything visibly 5—2 68 Heterostylism [CH. introduced. The next case also illustrates the appearance of a novelty in F,,, but, as will be seen, it is one of the groups of 3 members which manifests it. Heterostylism in Primula. The dimorphic, heterostyled condition of Primula plants is well known and need not be described in detail. The two types are distinguished thus : A. Thrum, or Short-styled type, 1. Style short, the stigma standing at the level of constriction of the tube. 2. Anthers at the mouth of the tube. 3. Pollen grains large. B. Pin, or Long-styled type. 1. Style long, the stigma usually standing in the mouth of the tube. 2. Anthers at the level of constriction. 3. Pollen grains small. Experiments made by Mr R. P. Gregory in conjunction with me showed that the inheritance of these- two types is Mendelian*, the short-styled or thrum behaving as dominant. The long-styled, being recessive, always breed true to that type on self-fertilisation or when bred inter se. In connection with this fact it is interesting to observe * I know no authentic case of the presence of both long- and short- styled flowers on the same plant. Such occurrences are frequently announced, but so far as I can discover the records are based on mistakes. In occasional flowers on long-styled plants, especially in the beginning of the flowering period, the style does not attain its proper length. Such flowers are through carelessness sometimes misdescribed as short- styled. The anthers however are at the lower level, and the pollen-grains are small, so the essentially long-styled nature of these plants is quite clear. The statement is also sometimes made that pin plants have produced thrum-eyed offspring without the intervention of a cross. This mistake is due to the appearance of a type with "exsert" anthers. Such anthers project from the mouth of the tube and give a thrum-like look to the flower. But careful examination shows that the anther-filaments are inserted at the lower level, and the pollen-grains are small. Such plants are therefore long-styled. ml Heterosiylism 69 that Primula Sinensis, which is preferred by fanciers in the pin form, was easily bred true to that type, and is always so maintained in good strains. For our experiments it was with considerable difficulty that we procured any thrum plants. On the other hand it was decided long ago that the Auricula and the Polyanthus for exhibition purposes must always be thrums, but though thrums have thus been largely selected for breeding, pin-eyed plants are continu- ally reproduced, as must be expected, for the thrum is a dominant, and therefore liable to contain the recessive type. The facts about to be described relate to an experiment made with a peculiar race of P. Sinensis grown by Messrs Fig. 14. Some of the types of flowers in F^ from the cross short-style (thrum) ; small eye x homostyle ; large eye. A. The long-styled flower : with small eye. B. The homostyled : large eye. C. The short-styled : small eye. D. The short- styled : large eye. In the two upper flowers the corolla is of the " star " type. D is the ordinary, imbricated type of Sinensis. C is more or less inter- mediate in corolla-shape. This shape is the usual heterozygote formed between star type and Sinensis type. The corolla -shapes are of course quite independent of the style and " eye " characters. 70 Heterostylisni [CH. Sutton under the name " Primrose Queen." To the casual observer this race differs from an ordinary Primula in the fact that the yellow " eye," instead of forming a small and well-defined pentagon, is continued as a yellow flush ex- tending far over the limb of the corolla. The anthers of these flowers stand at the lower level and the pollen-grains are small ; but the style instead of projecting into the mouth of the tube stops at the anther-level, being thus practically the same length as in the short-styled type. Such a form of flower was called by Darwin homostyled. Crosses were made between this homostyle race with the yellow flush and an ordinary thrum with the pentagonal or small eye. /\ is thrum with the small eye, showing that the yellow flush is recessive. F* gave the following series : 9 thrum or short-style with small eye, 3 „ ,, with the large eye, 3 ordinary pin or long-style with small eye, i homostyle with the large eye like Primrose Queen. The long-styled or pin type, which apparently was not put in, is evidently due to the re-combination of the charac- ters. The two pairs of characters are Thrum type (domt). Pin type (rec.). Small eye (domt). Large eye (rec.). The homostyle is the form which the pin type assumes when the large eye is developed ; but when in F^ the pin type meets the small eye, the ordinary pin or long-styled form is produced. Fa gave a simple and confirmatory result ; for of the JF2 long-styled plants, some proved pure to the long-styled character, while others threw the recessive homostyle. White Flowers in F., from Red 'x Cream. Exactly comparable with the foregoing case is the paradoxical appearance of WwV^-flowered individuals in the F^ from the cross of a sap-coloured variety with a variety having cream-coloured flowers. For example, in Sweet Peas or Stocks (Matthiold} when a red-flowered type is crossed with a cream, Fl is red, without any cream-colour. In] Novelties by Re-combination 71 Fz consists of 9 reds without cream, 3 reds with cream, 3 whites, i cream. When the allelomorphs are correctly distinguished the significance of this series is obvious. The operations may be shown in a tabular form, thus : Parents ...' Red variety x Cream variety / Red sap (Z>) Allelomorphs \r , ^ ~> * (Colourless corps. (D) ( Red sap [Colourless corpuscles Colourless sap (R) Yellow corpuscles (J?) 7 J Red sap 2 (Colourless corps. Ippearance 9 Red Red. sap Yellow corps. 3 Red Cream Colourless sap Colourless corps. 3 White Colourless sap Yellow corps. i Cream These cases of novelties resulting through a re-combi- nation of the factors brought in by the original pure types are striking because it is not at first sight evident how the novelty has been produced. Generally speaking, however, the re-combinations form in Fz a series of types many of which are obviously new combinations of features which could be recognized on inspection as present in the pure parents. Thus the cross between a bearded, rough chaff, red wheat, and a beardless, smooth chaff, white wheat give in Fl a beardless, rough chaff, red. But in /% all the different possible combinations occur, such as bearded, smooth, red ; beardless, rough, white ; bearded, rough, white, &c., each in their appropriate numerical proportions. In the Guinea-pig, starting from albino, smooth coat, long hair, and crossing it with coloured, rough coat, short hair, Fl is coloured, rough coat, short hair. But Fz contains the various re-combinations of these three pairs of characters, such as albino, rough coat, short hair ; coloured, smooth coat, long hair, &c. Thus by selecting any desired type in F.2 any of these new combinations can be fixed and perpetuated. Basing his procedure on a knowledge of the dominance or recessiveness of each character the breeder may thus guide his operations with certainty. That this has been the mode by which most of the new breeds of domesticated plants and animals have been created is obvious. The traces of it remain in many cases. For 72 Practical Examples [CH. instance, Sutton's " Nonpareil," one of the marrow peas, consists in about equal numbers of yellow-cotyledon seeds and green-cotyledon seeds. Like all the new peas it must have arisen at some definite moment by the selection of an individual yellow-seeded plant which was true for the various good qualities of Nonpareil — being homozygous for them in other words — but in cotyledon-colour it was heterozygous. As the diversity of colour was not thought objectionable it persists. If any one wishes to make an exclusively green-seeded Nonpareil, all he has to do is to take green seeds from a sack of that variety and sow them, saving the seeds they bear. If he desires the yellow type pure, he may similarly sow yellow seeds from the same sack. Most of these by now will presumably be pure to yellow, but some may not. By keeping the seeds of any plant which gives only yellow seeds a pure yellow Nonpareil will be constituted. Similarly Sutton's " Continuity " is a pea which is allowed to be either pointed in pod or blunt. The variety is true in other respects and it is clear that its original progenitor was a plant homozygous for the peculiarities of Continuity but heterozygous in respect of the pod-shape. The pointed-pod plants would be found true for that character, since it is recessive, while the blunt-pod plants might or might not be true for it. So in the Chinese Primrose, several varieties, e.g. Sutton's Mont Blanc, Sirdar, &c., distinguished by peculi- arities of colour have been fixed both in a palm-leaved and in a fern-leaved form, these having of course been saved in Fn or later generations from a heterozygote in which the palm and fern-leaved characters were combined. In Sweet Peas, after the original dwarf " Cupid " was found — accord- ing to tradition a chance seedling among tall plants — it was easy to transfer the characteristic colour of the various tall types on to a Cupid foundation, and now any colour almost can be had either as a tall or as a Cupid. Possible Limits to Re-combination. These illustrations might be extended indefinitely. It will probably occur to many that there are limits to these possibilities of transference, and so undoubtedly there are. m] Limits to Interchangeability 73 The detection of these limits is one of the more important tasks still awaiting us. Though on this head little can yet be asserted with confidence it is likely that such limitations are constituted in two distinct ways : First, from all we know of the capacities of animals and plants we must anticipate that some characters are incompatible in the same individual. For example in cattle the highest milk- production is not to be found in the breeds which make the best beef. Meat-production and milk-production are to some extent alternative and can only be combined by compro- mising one quality or both. That such an alternative distribution is merely a result of allelomorphism seems on the whole unlikely, though certainly not impossible. Then again we must surely expect that these transferable characters are attached to or implanted upon some basal organisation, and the attributes or powers which collectively form that residue may perhaps be distinguishable from the transferable qualities. The detection of the limits thus set upon the interchangeability of characters would be a dis- covery of high importance and would have a most direct bearing on the problem of the ultimate nature of Species. CHAPTER IV HEREDITY OF COLOUR. Factors determining Colours : the Ratio 9:3: 4. — The "Presence and Absence" Hypothesis. Epistatic and Hypo static Factors — Colours of Mice — Pied Types— A Dominant Piebald. WITH regard to the application of the Mendelian system to problems of colour inheritance the evidence is now considerable. The fact that in both animals and plants albinism behaved as a recessive to colours was soon dis- covered. Several examples among plants are mentioned by de Vries in his first paper on this subject, and shortly after similar facts were recorded in regard to animals. Since in the course of a large range of experiments with many species of animals and plants no case to the contrary has been met with, it may perhaps be asserted as a general truth that pigmentation is always dominant to total absence of pig- ment*. When however we proceed to the investigation of the genetic properties of varieties which are so far deficient in pigment as to be called sometimes partial albinos, we find that various specific rules are followed and no uni- formity of behaviour has yet been discovered. White fowls for instance are thus commonly spoken of as partial albinos, but the pigmentation of their eyes sharply distinguishes them from albinos which are destitute of pigment, and many of their genetic properties are found experimentally to be quite distinct from those of real albinos. The same is true of certain varieties of plants, which though varying from the specific type by possessing white flowers have yet some red or purple sap in the stem or elsewhere. With- * The Axolotl is perhaps an exception. See p. 43. The fact that in plants colourless chromoplasts are dominant to yellow chromoplasts scarcely constitutes an exception, for the yellow of the chromoplasts is not pigment in the usual acceptation of that term. CH. iv] The Ratio 9:3:4 75 out experiment no prediction can be made with confidence as to the behaviour of such types in their crosses. Albinism being recessive in all ordinary cases, F2 from the cross colour x albino contains i albino to 3 coloured members. As regards the characters of the dominant or coloured mem- bers various complications have to be considered. In the simplest cases the coloured /% individuals are all of the same colour. For example on crossing a grey rabbit with an albino, Fl is grey and I\ may be 3 greys : i albino. But frequently it is found that in addition to the greys and albinos blacks appear in F.2. Repeated experiments, for example those of Hurst, have shown that in such families the F2 ratio is 9 greys : 3 blacks : 4 albinos The relation of this ratio to the ordinary 9:3:3:1 was first pointed out by Cue"not (86)*. As represented by him two pairs of allelomorphs are concerned, namely : Dominant. Recessive. 1. Colour (C). Albinism (A). 2. Grey determiner (G). Black determiner (£}. The presence of one or other of the determiners G or B is only perceptible when it exists in combination with the colour- factor. If G is present together with C, the colour is grey ; if B is present with C but without G, the colour is black. If a coloured individual contain both G and B, being thus heterozygous* in the second pair of factors, the colour is grey, for the effects of grey dominate. But since in the absence of colour (C) neither determiner produces a perceptible effect, albinos may exist of the forms AAGG, AAGB, or A ABB, and without breeding tests it will not be possible to distinguish between these several forms. On crossing with a black of course each albino can be known by its effects. For GG will then give greys only, GB will give equal numbers of greys and blacks, while BB albinos will give only blacks. * Shortly before the publication of Cuenot's paper Mr R. H. Lock wrote to me from Ceylon with the same suggestion. j6 Presence and Absence [CH. The "Presence and Absence" Hypothesis applied to the Case of Colour. So long as attention is restricted to crosses like these involving only two sorts of colours besides the albinos, the system suggested by Cuenot is adequate, but when a third colour has to be considered, as in the case of mice, some modification is required. The simplest notation by which these and other complex Mendelian phenomena can be expressed is provided by what is spoken of as the Presence and Absence hypothesis already illustrated in the case of the combs of fowls. Mendel himself probably conceived of allelomorphism as depending on the separation of a definite something responsible for the dominant character from another some- thing responsible for the production of the recessive character. It is however evidently simpler to imagine that the dominant character is due to the presence of something which in the case of the recessive is absent. As yet there is no absolute proof that this mode of de- scribing the facts is correct, but everything points that way, and no phenomena have yet been encountered which cannot be thus formulated when their nature is understood. In cases where the pure dominants are recognizably distinct from the heterozygous dominants, it must naturally be sup- posed that two " doses " of the active factor are required, one from the paternal, and another from the maternal side, in order to produce the full effect. Applying the presence and absence system to the case of the colours of rabbits, the first pair of allelomorphs can obviously be represented as Dominant. Recessive. i. Presence of Colour (€}. Absence of Colour (c). The second pair we have so far spoken of as the grey determiner and the black determiner, regarding these two as allelomorphic to each other. But it is equally possible to describe them thus 2. Grey determiner (G]. Absence of ditto ( g). IV] Presence and Absence 77 Then in the case where grey x albino gives in /% 9 grey : 3 black : 4 albino, we simply have to regard B, the black determiner, as common to both parents, and the same numerical result is produced. Such a case may usefully be represented in a tabular form, thus : Parents Grey x Albino Gametic Composition... CGB cgB Grey CcGgBB ! 9 Grey 3 Black 12 3 — i 4 Albino 4 i CGB CGB cGB CGB CgB CGB cgB CGB grey grey grey grey CGB cGB cGB cGB CgB cGB cgB cGB grey albino ' grey albino CGB CgB cGB CgB CgB CgB cgB CgB grey grey black black CGB cgB cGB cgB CgB cgB cgB cgB grey albino black albino Fig. 15. Distribution of grey, black, and albino individuals in Fz from the cross grey, CGB, with albino cgB, showing the meaning of the ratio 9 grey : 3 black : 4 albino. 78 Presence and Absence [CH. In this diagram the 9 squares containing C, G, B, are the 9 greys, the 3 squares containing C and B only are the 3 blacks and the 4 squares containing no C at all are the 4 albinos. Proceeding to the case of mice we write the composition as follows : Grey C, G, B, Ch. Black C, g, B, Ch. Chocolate C, g, d, Ch. We thus regard the black mouse as one from which the grey determiner. G, has been removed. In the chocolate mouse the process of removal has been carried further and the black determiner, B, is also gone. A proof that this system of representation is so far correct is obtained by crossing the grey mouse with the chocolate. Such a cross, if G is not allelomorphic to B, must give blacks in /%. This experiment has been lately carried out by Miss F. M. Durham, to whose work our knowledge of the genetics of mice is largely due. The result is that, as expected, /% does contain blacks, and though the num- bers as yet obtained are small, there can be little doubt that the /% ratio is 12 grey : 3 black : i chocolate. Some interesting questions arise in regard to the greys. Obviously we should expect 9 greys containing G and B + 3 greys without B. Now fanciers are well aware of a dis- tinction between two kinds of greys or "agoutis" as they are called. These are known as "golden agoutis" and "cinna- mon agoutis," the former containing black pigment, the latter being without it. In the F,. from grey x chocolate both these kinds of agoutis appear, and evidently the cinnamon agoutis are the expected greys wanting in the determiner B. Thus far all is clear. Certain difficulties however remain unexplained. These will be described later. At this stage in the discussion it is convenient to notice that in view of the facts now stated the use of the term domi- nance must be more carefully restricted than has hitherto been necessary. When we speak of the colour as being dominant over the absence of colour we mean that if the colour is present it will appear, and that if the factor for iv] Epistatic and Hypostatic 79 colour is absent the individual will be devoid of colour. The term is thus used correctly to denote the relation between allelomorphic features belonging to the same pair. But confusion will be introduced if we extend the same term to the relationship between various determining factors which belong to distinct allelomorphic pairs. Hitherto we have spoken of the determiner for such a colour as grey in rabbits and mice as " dominant " over the colours lower in the scale, such as black or chocolate. Nevertheless we are here dealing with a relationship quite different in order from that subsisting between the coloured and the albino. Pending a more precise knowledge of the nature of this relationship it will be enough to regard those factors which prevent others from manifesting their effects as higher, and the concealed factors as lower. In accord- ance with this suggestion the terms epistatic and hypostatic may conveniently be introduced. We shall then speak of the determiner for grey as epistatic to that for black ; that for black as epistatic to the determiner for chocolate, and so on. When the facts are thus clearly represented we perceive that the variation by which, for example, a black mouse came originally into existence, consisted in the omission of the de- terminer for grey. The chocolate mouse similarly owed its origin to the successive omission of the determiner for black. The important question what the effect of the grey determiner, for example, actually is, remains undecided. A further serious difficulty also arises in regard to the relation of the colour yellow to the other colours. Neither of these points is yet satisfactorily understood in the case of mice. The recent papers of Castle (53) and of Hurst* have made the phenomena in rabbits comparatively clear, though even there, however, an unexplained difficulty remains. The special problems raised by the behaviour of yellow pigment in these animals will be discussed in a sub- sequent section! (see Chap. vn). * Read at the Internal. Congr. Zool. Boston, 1907 : not yet published. t According to the number of factors involved and to the definiteness with which their several combinations can be distinguished, an indefinite variety of ratios may of course be produced in JF"2 families. Some of the most interesting are those in which some of the heterozygous combinations can be distinguished from the homozygous dominants. (See for examples Shull, 242.) 8o Saturated and Dihite Colours [CH. The FZ ratio 9:3:4, the significance of which we have been considering, is one which very frequently recurs in Mendelian analysis. For example, as Tschermak found, when a pink-and-white flowered eating pea (Pisum sativum] is crossed with a white flowered type, F^ is often of the original purple flowered type. Then F.2 will be 9 purple : 3 pink-and-white : 4 white. Similarly pink Salvia Horminum x white may give Fl purple, and Fz 9 purple : 3 pink : 4 white. In these cases the factor for the purpleness is of course brought in by the albino, but exactly the same Fz may result from a cross between the purple type and an albino not carrying the factor for purpleness. All that is essential for the produc- tion of this ratio in Fz is that F^ should be heterozygous for two factors, of which one is perceptible whenever present, while the other needs the presence of the first in order that its own effects may be manifested. Such cases are very numerous and in practical breeding are to be looked out for continually. Care must be taken to distin- guish them from families like those of the Andalusian fowl (p. 52) in which the commonest term in the Fz series is a heterozygous type. There the numbers will be 1:2:1, which in a practical example may give results not obviously distinguishable from 3 : 9 : 4. To decide between the two possibilities it is necessary to breed the Fz types again. If neither of the scarcer types when bred inter se can throw the other, and the commoner type cannot be bred pure, the latter is a heterozygous type; but if one of the scarcer types can throw the other, then the ratio is presumably 9:3:4, and in such a case it will be possible to raise a breed true to the type occurring as 9. Saturation and Dilution of Colours. Omitting yellow from our consideration, we thus re- cognize that in the mouse the colours, grey (agouti), black, chocolate, which the fur visibly presents, result from the interaction of several factors, and that these factors can in great measure be shown to be distributed in gametogenesis according to Mendelian allelomorphic systems. The ex- i. Agouti. 2. Black. 3. Chocolate. Plate II 4, Cinnamon Agouti, viz. Agouti without black. 5. Blue : = dilute black. 6. Silver fawn : = dilute chocolate. iv] Colours of Mice 81 periments of Miss Durham (116) have shown that not only the particular pigment is thus constituted, but also that the intensity or degree of saturation in which it is formed can be represented as determined by similar factors. For example the black colour may exist in the saturated condition, when the mouse is called black, or in a more dilute form, when it gives the "blue" appearance. Similarly the chocolate colour when diluted gives what fanciers call "silver-fawn." Experiment shows that the cross black x silver fawn gives exactly the same result (in F^ and /%) as blue x chocolate. (See Plate II.) The following are experimental results illustrating these points. The allelomorphs concerned may be represented as B, the black determiner, b the absence of B, leaving the colour chocolate. D the dense or saturated condition of the colour, d, the absence of D, leaving the colour dilute. (In the case of the introduction of the albino we should have also to take cognizance of C, the presence of colour, c, its absence.) The actual results may then be expressed in a tabular form. Blue x Chocolate (Bd) \ (bD) A Black (BbDd) r -J —l F2... Black Blue Chocolate Silver-fawn (BD) (Bd) (bD) (bd) Observed 42 16 14 8 Calculated 45 15 15 5 Black x Silver-fawn (BD) \ (bd) FI Black (BbDd) F2... Black (BD) Observed 9 Calculated zo'i Blue (Bd) 4 3'4 Chocolate (bD) 3 3'4 Silver-fa (bd) 2 I-I It is thus immaterial whether the factor for saturation is brought in together with the black determiner or with B. H. 6 82 Colours of Mice [CH. the chocolate (more strictly, with the "absence of the black determiner"). So long as the same factors are introduced, the consequences in F^ and the results of re-combination in Fn_ are the same. But when the dilution is introduced from each side, Fz is of course the usual 3 dominants : i recessive, thus : Blue x Silver-fawn (Bd) | (bd) FI ........................ Blue (Bbdd) ^a...Blue (Bd) Observed 33 Calculated 33'75 Silver-fawn (bd) 12 It is evident that an extracted albino cannot be carrying a determiner for a colour higher in the scale than that of its coloured parents. Moreover if the parents from which an albino is extracted are alike, and if they throw no offspring with colours other than their own (besides the albinos), then the albinos so extracted must be all bearers of the determiner for their parental colour. If such albinos are crossed with forms of a colour lower in the scale than that borne by the albinos, F^ must be of the colour deter- mined by the albinos. For example, Miss Durham obtained the following : Silver-fawn x Albino (extracted from chocolates) (Cbd) | (cbD) FI ........................ Chocolate (CObDd) 1 Fz . . . Chocolate (CbD) Observed 19 Calculated 16-3 Silver-fawn (Cbd) 4 5'4 -\ Albino (various) 6 7-2 F