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diff --git a/old/spvrt10.txt b/old/spvrt10.txt new file mode 100644 index 0000000..6f7b9b2 --- /dev/null +++ b/old/spvrt10.txt @@ -0,0 +1,19802 @@ +The Project Gutenberg EBook of Species and Varieties, Their Origin by +Mutation, by Hugo DeVries + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: Species and Varieties, Their Origin by Mutation + +Author: Hugo DeVries + +Release Date: January, 2005 [EBook #7234] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on March 30, 2003] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK SPECIES AND VARIETIES *** + + + + +Produced by Dave Gowan <dgowan@bio.fsu.edu> + + + + + + +Species and Varieties +Their Origin by Mutation + +Lectures delivered at the University of California + +By +Hugo DeVries +Professor of Botany in the University of Amsterdam + +Edited by +Daniel Trembly MacDougal +Director Department of Botanical Research +Carnegie Institution of Washington + +Second Edition +Corrected and Revised + + +CHICAGO +The Open Court Publishing Company +LONDON +Kegan Paul, Trench, Trubner and Co., Ltd. +1906 + +- - - - - + + +COPYRIGHT 1904 +BY +The Open Court Pub. Co. +CHICAGO + +- - - - - + + +THE ORIGIN OF SPECIES + +The origin of species is a natural phenomenon. +LAMARCK + +The origin of species is an object of inquiry. +DARWIN + +The origin of species is an object of experimental investigation. +DeVRIES. + +- - - - - + +PREFACE BY THE AUTHOR + +THE purpose of these lectures is to point out the means and methods by +which the origin of species and varieties may become an object for +experimental inquiry, in the interest of agricultural and horticultural +practice as well as in that of general biologic science. Comparative +studies have contributed all the evidence hitherto adduced for the +support of the Darwinian theory of descent and given us some general +ideas about the main lines of the pedigree of the vegetable kingdom, but +the way in which one species originates from another has not been +adequately explained. The current belief assumes that species are slowly +changed into new types. In contradiction to this conception the theory +of mutation assumes that new species and varieties are produced from +existing forms by sudden leaps. The parent-type itself remains unchanged +throughout this process, and may repeatedly give birth to new forms. +These may arise simultaneously and in groups or separately at more or +less widely distant periods. + +The principal features of the theory of mutation have been dealt with at +length in my book "Die Mutationstheorie" (Vol. I., 1901, Vol. II., 1903. +Leipsic, Veit & Co.), in which I have endeavored to present as +completely as possible the detailed evidence obtained from trustworthy +historical records, and from my own experimental researches, upon which +the theory is based. + +The University of California invited me to deliver a series of lectures +on this subject, at Berkeley, during the [vii] summer of 1904, and these +lectures are offered in this form to a public now thoroughly interested +in the progress of modern ideas on evolution. Some of my experiments and +pedigree-cultures are described here in a manner similar to that used in +the "Mutationstheorie," but partly abridged and partly elaborated, in +order to give a clear conception of their extent and scope. New +experiments and observations have been added, and a wider choice of the +material afforded by the more recent current literature has been made in +the interest of a clear representation of the leading ideas, leaving the +exact and detailed proofs thereof to the students of the larger book. + +Scientific demonstration is often long and encumbered with difficult +points of minor importance. In these lectures I have tried to devote +attention to the more important phases of the subject and have avoided +the details of lesser interest to the general reader. + +Considerable care has been bestowed upon the indication of the lacunae +in our knowledge of the subject and the methods by which they may be +filled. Many interesting observations bearing upon the little known +parts of the subject may be made with limited facilities, either in the +garden or upon the wild flora. Accuracy and perseverance, and a warm +love for Nature's children are here the chief requirements in such +investigations. + +In his admirable treatise on Evolution and Adaptation (New York, +Macmillan & Co., 1903), Thomas Hunt Morgan has dealt in a critical +manner with many of the speculations upon problems subsidiary to the +theory of descent, in so convincing and complete a manner, that I think +myself justified in neglecting these questions here. His book gives an +accurate survey of them all, and is easily understood by the general +reader. + +In concluding I have to offer my thanks to Dr. D.T. MacDougal and Miss +A.M. Vail of the New York Botanical Garden for their painstaking work in +the preparation of the manuscript for the press. Dr. MacDougal, by +[viii] his publications, has introduced my results to his American +colleagues, and moreover by his cultures of the mutative species of the +great evening-primrose has contributed additional proof of the validity +of my views, which will go far to obviate the difficulties, which are +still in the way of a more universal acceptation of the theory of +mutation. My work claims to be in full accord with the principles laid +down by Darwin, and to give a thorough and sharp analysis of some of the +ideas of variability, inheritance, selection, and mutation, which were +necessarily vague at his time. It is only just to state, that Darwin +established so broad a basis for scientific research upon these +subjects, that after half a century many problems of major interest +remain to be taken up. The work now demanding our attention is +manifestly that of the experimental observation and control of the +origin of species. The principal object of these lectures is to secure a +more general appreciation of this kind of work. + + +HUGO DE VRIES. +Amsterdam, October, 1904. + +[ix] + +PREFACE BY THE EDITOR + +PROFESSOR DE VRIES has rendered an additional service to all naturalists +by the preparation of the lectures on mutation published in the present +volume. A perusal of the lectures will show that the subject matter of +"Die Mutationstheorie" has been presented in a somewhat condensed form, +and that the time which has elapsed since the original was prepared has +given opportunity for the acquisition of additional facts, and a +re-examination of some of the more important conclusions with the result +that a notable gain has been made in the treatment of some complicated +problems. + +It is hoped that the appearance of this English version of the theory of +mutation will do much to stimulate investigation of the various phases +of the subject. This volume, however, is by no means intended to +replace, as a work of reference, the larger book with its detailed +recital of facts and its comprehensive records, but it may prove a +substitute for the use of the general reader. + +The revision of the lectures has been a task attended with no little +pleasure, especially since it has given the editor the opportunity for +an advance consideration of some of the more recent results, thus +materially facilitating investigations which have been in progress at +the New York Botanical Garden for some time. So far as the ground has +been covered the researches in question corroborate the conclusions of +de Vries in all important particulars. The preparation of the manuscript +for the printer has consisted chiefly in the adaptation of oral [xii] +discussions and demonstrations to a form suitable for permanent record, +together with certain other alterations which have been duly submitted +to the author. The original phraseology has been preserved as far as +possible. The editor wishes to acknowledge material assistance in this +work from Miss A.M. Vail, Librarian of the New York Botanical Garden. + + +D.T. MacDougal. +New York Botanical Garden, October, 1904. + + +PREFACE TO THE SECOND EDITION. + +THE constantly increasing interest in all phases of evolution has made +necessary the preparation of a second edition of this book within a few +months after the first appeared. The opportunity has been used to +eliminate typographical errors, and to make alterations in the form of a +few sentences for the sake of clearness and smoothness. The subject +matter remains practically unchanged. An explanatory note has been added +on page 575 in order to avoid confusion as to the identity of some of +the plants which figure prominently in the experimental investigations +in Amsterdam and New York. + +The portrait which forms the frontispiece is a reproduction of a +photograph taken by Professor F.E. Lloyd and Dr. W.A. Cannon during the +visit of Professor de Vries at the Desert Botanical Laboratory of the +Carnegie Institution, at Tucson, Arizona, in June, 1904. + + +D. T. MACDOUGAL. +December 15, 1905. + + + +CONTENTS + +A. INTRODUCTION. + +LECTURE PAGE + +I. Descent: theories of evolution and methods of investigation. 1 + The theory of descent and of natural selection. Evolution and +adaptation. Elementary species and varieties. Methods of scientific +pedigree-culture. + +B. ELEMENTARY SPECIES. + +II. Elementary species in nature. 32 + _Viola tricolor_, _Draba verna_, _Primula acaulis_, and other +examples. _Euphorbia pecacuanha_. _Prunus maritima_. _Taraxacum_ and +_Hieracium_. + +III. Elementary species of cultivated plants. 63 + Beets, apples, pears, clover, flax and coconut. + +IV. Selection of elementary species. 92 + Cereals. Le Couteur. Running out of varieties. Rimpau and +Risler, _Avena fatua_. Meadows. Old Egyptian cereals. Selection by the +Romans. Shirreff. Hays. + +C. RETROGRADE VARIETIES. + +V. Characters of retrograde varieties. 121 + Seed varieties of pure, not hybrid origin. Differences from +elementary species. Latent characters. Ray-florets of composites. +[xiii] Progressive red varieties. Apparent losses. _Xanthium +canadense_. Correlative variability. Laciniate leaves and petals. +Compound characters. + +VI. Stability and real atavism. 154 + Constancy of retrograde varieties. Atavism in _Ribes sanguineum +Albidum_, in conifers, in _Iris pallida_. Seedlings of _Acacia_. +Reversion by buds. + +VII. Ordinary or false atavism. 185 + Vicinism or variation under the influence of pollination by +neighboring individuals. Vicinism in nurseries. Purifying new and +old varieties. A case of running out of corn in Germany. + +VIII. Latent characters. 216 + Leaves of seedlings, adventitious buds, systematic latency and +retrogressive evolution. Degressive evolution. Latency of specific +and varietal characters in wheat-ear carnation, in the green dahlias, +in white campanulas and others. Systematic latency of flower colors. + +IX. Crossing of species and varieties. 247 + Balanced and unbalanced, or species and variety crosses. +Constant hybrids of _Oenothera muricata_ and _O. biennis_. _Aegilops_, +_Medicago_, brambles and other instances. + +X. Mendel's law of balanced crosses. 276 + Pairs of antagonistic characters, one active and one latent. +_Papaver somniferum_. [xiv] _Mephisto Danebrog_. Mendel's laws. Unit- +characters. + +D. EVERSPORTING VARIETIES. + +XI. Striped flowers. 309 + _Antirrhinum majus luteum rubro-striatum_ with pedigree. Striped +flowers, fruits and radishes. Double stocks. + +XII. "Five leaved" clover. 340 + Origin of this variety. Periodicity of the anomaly. Pedigree- +cultures. Ascidia. + +XIII. Polycephalic poppies. 369 + Permanency and high variability. Sensitive period of the +anomaly. Dependency on external conditions. + +XIV. Monstrosities. 400 + Inheritance of monstrosities. Half races and middle races. +Hereditary value of atavists. Twisted stems and fasciations. Middle +races of tricotyls and syncotyls. Selection by the hereditary +percentage among the offspring. + +XV. Double adaptations. 430 + Analogy between double adaptations and anomalous middle races. +_Polygonum amphibium_. Alpine plants. _Othonna crassifolia_. Leaves +in sunshine and shadow. Giants and dwarfs. Figs and ivy. Leaves of +seedlings. + +E. MUTATIONS. + +XVI. Origin of the peloric toad-flax. 459 + Sudden and frequent origin in the wild state. Origin in the +experiment-garden. Law of repeated mutations. Probable origin of +other pelories. + +[xv] +XVII. The production of double flowers. 488 + Sudden appearance of double flowers in horticulture. Historical +evidence. Experimental origin of _Chrysanthemum segetum plenum_. +Dependency upon nourishment. Petalody of stamens. + +XVIII New species of _Oenothera_. 516 + Mutations of _Oenothera lamarckiana_ in the wild state near +Hilversum. New varieties of _O. laevifolia_, _O. brevistylis_, and +_O. nanella_. New elementary species, _O. gigas_, _O. rubrinervis_, +_albida_, and _oblonga_. _O. lata_, a pistillate form. +Inconstancy of _O. scintillans_. + +XIX. Experimental pedigree-cultures. 547 + Pedigree of the mutative products of _Oenothera lamarckiana_ in +the Botanical Garden at Amsterdam. Laws of mutability. Sudden and +repeated leaps from an unchanging main strain. Constancy of the new +forms. Mutations in all directions. + +XX. Origin of wild species and varieties. 576 + Problems to solve. _Capsella heegeri_. _Oenothera biennis cruciata_. +_Epilobium hirsutum cruciatum_. _Hibiscus Moscheutos_. Purple beech. +Monophyllous strawberries. Chances of success with new mutations. + +XXI. Mutations in horticulture. 604 + _Chelidonium majus lacinatum_. Dwarf and spineless varieties. +Laciniate leaves. Monophyllous and broom-like varieties. [xvi] Purple +leaves. _Celosia_. Italian poplar. Cactus dahlia. Mutative origin of +_Dahlia fistulosa_, and _Geranium praetense_ in the experiment-garden. + +XXII. Systematic atavism. 630 +Reappearance of ancestral characters. _Primula acaulis umbellata_. +Bracts of crucifers. _Zea Mays cryptosperma_. Equisetum, _Dipsacus +sylvestris torsus_. Tomatoes. + +XXIII. Taxonomic anomalies. 658 + Specific characters occurring in other cases as casual +anomalies. _Papaver bracteatum monopetalum_. _Desmodium gyrans_ and +monophyllous varieties. Peltate leaves and ascidia. Flowers on +leaves. Leaves. _Hordeum trifurcatum_. + +XXIV. Hypothesis of periodical mutations. 686 + Discovering mutable strains. Periods of mutability and constancy. +Periods of mutations. Genealogical trees. Limited life-time of the +organic kingdom. + + +F. FLUCTUATIONS. + +XXV. General laws of fluctuations. 715 + Fluctuating variability. Quetelet's law. Individual and partial +fluctuations. Linear variability. Influence of nutrition. +Periodicity curves. + +XXVI. Asexual multiplication of extremes. 742 + Selection between species and intra-specific selection. +Excluding individual [xvii] embryonic variability. Sugar-canes. +Flowering cannas. Double lilacs. Other instances. Burbank's method +of selection. + +XXVII. Inconstancy of improved races 770 + Larger variability in the case of propagation by seed, +progression and regression after a single selection, and after +repeated selections. Selection experiments with corn. Advantages +and effect of repeated selection. + +XXVIII. Artificial and natural selection. 798 + Conclusions. Specific and intra-specific selection. Natural +selection in the field. Acclimatization. Improvement-selection of +sugar-beets by various methods. Rye. Hereditary percentage and +centgener power as marks by which intraspecific selection may be +guided. + +Index 827 + + +[1] +A. INTRODUCTION + +LECTURE I + +DESCENT: THEORIES OF EVOLUTION +AND METHODS OF INVESTIGATION + +Newton convinced his contemporaries that natural laws rule the whole +universe. Lyell showed, by his principle of slow and gradual evolution, +that natural laws have reigned since the beginning of time. To Darwin we +owe the almost universal acceptance of the theory of descent. + +This doctrine is one of the most noted landmarks in the advance of +science. It teaches the validity of natural laws of life in its broadest +sense, and crowns the philosophy founded by Newton and Lyell. + +Lamarck proposed the hypothesis of a common origin of all living beings +and this ingenious and thoroughly philosophical conception was warmly +welcomed by his partisans, but was not widely accepted owing to lack of +supporting evidence. To Darwin was reserved the task of [2] bringing the +theory of common descent to its present high rank in scientific and +social philosophy. + +Two main features in his work have contributed to this early and +unexpected victory. One of them is the almost unlimited amount of +comparative evidence, the other is his demonstration of the possibility +of a physiological explanation of the process of descent itself. + +The universal belief in the independent creation of living organisms was +revised by Linnaeus and was put upon a new foundation. Before him the +genera were supposed to be created, the species and minor forms having +arisen from them through the agency of external conditions. In his first +book Linnaeus adhered to this belief, but later changed his mind and +maintained the principle of the separate creation of species. The weight +of his authority soon brought this conception to universal acceptance, +and up to the present time the prevailing conception of a species has +been chiefly based on the definition given by Linnaeus. His species +comprised subspecies and varieties, which were in their turn, supposed +to have evolved from species by the common method. + +Darwin tried to show that the links which bind species to genera are of +the same nature as those which determine the relationship of [3] +subspecies and varieties. If an origin by natural laws is conceded for +the latter, it must on this ground be granted for the first also. In +this discussion he simply returned to the pre-Linnean attitude. But his +material was such as to allow him to go one step further, and this step +was an important and decisive one. He showed that the relation between +the various genera of a family does not exhibit any features of a nature +other than that between the species of a genus. What has been conceded +for the one must needs be accepted for the other. The same holds good +for the large groups. + +The conviction of the common origin of closely allied forms necessarily +leads to the conception of a similar descent even in remote +relationships. + +The origin of subspecies and varieties as found in nature was not +proved, but only generally recognized as evident. A broader knowledge +has brought about the same state of opinion for greater groups of +relationships. Systematic affinities find their one possible explanation +by the aid of this principle; without it, all similarity is only +apparent and accidental. Geographic and paleontologic facts, brought +together by Darwin and others on a previously unequalled scale, point +clearly in the same direction. The vast amount of evidence of all [4] +comparative sciences compels us to accept the idea. To deny it, is to +give up all opportunity of conceiving Nature in her true form. + +The general features of the theory of descent are now accepted as the +basis of all biological science. Half a century of discussion and +investigation has cleared up the minor points and brought out an +abundance of facts; but they have not changed the principle. Descent +with modification is now universally accepted as the chief law of nature +in the organic world. In honor of him, who with unsurpassed genius, and +by unlimited labor has made it the basis of modern thought, this law is +called the "Darwinian theory of descent." + +Darwin's second contribution to this attainment was his proof of the +possibility of a physiological explanation of the process of descent +itself. Of this possibility he fully convinced his contemporaries, but +in indicating the particular means by which the change of species has +been brought about, he has not succeeded in securing universal +acceptation. Quite on the contrary, objections have been raised from the +very outset, and with such force as to compel Darwin himself to change +his views in his later writings. This however, was of no avail, and +objections and criticisms have since steadily accumulated. Physiologic +facts concerning the origin of [5] species in nature were unknown in the +time of Darwin. It was a happy idea to choose the experience of the +breeders in the production of new varieties, as a basis on which to +build an explanation of the processes of nature. In my opinion Darwin +was quite right, and he has succeeded in giving the desired proof. But +the basis was a frail one, and would not stand too close an examination. +Of this Darwin was always well aware. He has been prudent to the utmost, +leaving many points undecided, and among them especially the range of +validity of his several arguments. Unfortunately this prudence has not +been adopted by his followers. Without sufficient warrant they have laid +stress on one phase of the problem, quite overlooking the others. +Wallace has even gone so far in his zeal and ardent veneration for +Darwin, as to describe as Darwinism some things, which in my opinion, +had never been a part of Darwin's conceptions. + +The experience of the breeders was quite inadequate to the use which +Darwin made of it. It was neither scientific, nor critically accurate. +Laws of variation were barely conjectured; the different types of +variability were only imperfectly distinguished. The breeders' +conception was fairly sufficient for practical purposes, but science +needed a clear understanding of the [6] factors in the general process +of variation. Repeatedly Darwin tried to formulate these causes, but the +evidence available did not meet his requirements. + +Quetelet's law of variation had not yet been published. Mendel's claim +of hereditary units for the explanation of certain laws of hybrids +discovered by him, was not yet made. The clear distinction between +spontaneous and sudden changes, as compared with the ever-present +fluctuating variations, is only of late coming into recognition by +agriculturists. Innumerable minor points which go to elucidate the +breeders' experience, and with which we are now quite familiar, were +unknown in Darwin's time. No wonder that he made mistakes, and laid +stress on modes of descent, which have since been proved to be of minor +importance or even of doubtful validity. + +Notwithstanding all these apparently unsurmountable difficulties, Darwin +discovered the great principle which rules the evolution of organisms. +It is the principle of natural selection. It is the sifting out of all +organisms of minor worth through the struggle for life. It is only a +sieve, and not a force of nature, not a direct cause of improvement, as +many of Darwin's adversaries, and unfortunately many of his followers +also, have so often asserted. + +It is [7] only a sieve, which decides what is to live, and what is to +die. But evolutionary lines are of great length, and the evolution of a +flower, or of an insectivorous plant is a way with many sidepaths. It is +the sieve that keeps evolution on the main line, killing all, or nearly +all that try to go in other directions. By this means natural selection +is the one directing cause of the broad lines of evolution. + +Of course, with the single steps of evolution it has nothing to do. Only +after the step has been taken, the sieve acts, eliminating the unfit. +The problem, as to the manner in which the individual steps are brought +about, is quite another side of the question. + +On this point Darwin has recognized two possibilities. One means of +change lies in the sudden and spontaneous production of new forms from +the old stock. The other method is the gradual accumulation of those +always present and ever fluctuating variations which are indicated by +the common assertion that no two individuals of a given race are exactly +alike. The first changes are what we now call "mutations," the second +are designated as "individual variations," or as this term is often used +in another sense, as "fluctuations." Darwin recognized both lines of +evolution; Wallace disregarded the sudden changes and proposed +fluctuations [8] as the exclusive factor. Of late, however, this point +of view has been abandoned by many investigators, especially in America. + +The actual occurrence of mutations is recognized, and the battle rages +about the question, as to whether they are be regarded as the principal +means of evolution, or whether slow and gradual changes have not also +played a large and important part. + +The defenders of the theory of evolution by slow accumulation of slight +fluctuations are divided into two camps. One group is called the +Neo-Lamarckians; they assume a direct modifying agency of the +environment, producing a corresponding and useful change in the +organization. The other group call themselves Darwinians or +selectionists, but to my mind with no other right beyond the arbitrary +restriction of the Darwinian principles by Wallace. They assume +fluctuating variations in all directions and leave the choice between +them to the sieve of natural selection. + +Of course we are far from a decision between these views, on the sole +ground of the facts as known at present. Mutations under observation are +as yet very rare; enough to indicate the possible and most probable +ways, but no more. On the other hand the accumulation of fluctuations +does not transgress relatively narrow [9] limits as far as the present +methods of selection go. But the question remains to be solved, whether +our methods are truly the right ones, and whether by the use of new +principles, new results might not cause the balance of opinion to favor +the opposite side. + +Of late, a thorough and detailed discussion of the opposing views has +been given by Morgan in his valuable book on evolution and adaptation. +He has subjected all the proposed theories to a severe criticism both on +the ground of facts and on that of their innate possibility and logical +value. He decides in favor of the mutation theory. His arguments are +incisive and complete and wholly adapted to the comprehension of all +intelligent readers, so that his book relieves me entirely of the +necessity of discussing these general questions, as it could not be done +in a better or in a clearer way. + +I intend to give a review of the facts obtained from plants which go to +prove the assertion, that species and varieties have originated by +mutation, and are, at present, not known to originate in any other way. +This review consists of two parts. One is a critical survey of the facts +of agricultural and horticultural breeding, as they have accumulated +since the time of Darwin. This body of evidence is to be combined with +some corresponding experiments [10] concerning the real nature of +species in the wild state. The other part rests on my own observations +and experiments, made in the botanical garden of the University of +Amsterdam. + +For many years past I have tried to elucidate the hereditary conditions +of species and varieties, and the occasional occurrence of mutations, +that suddenly produce new forms. + +The present discussion has a double purpose. On one side it will give +the justification of the theory of mutations, as derived from the facts +now at hand. On the other hand it will point out the deficiencies of +available evidence, and indicate the ways by which the lacunae may +gradually be filled. Experimental work on heredity does not require vast +installments or costly laboratory equipment. It demands chiefly +assiduity and exactitude. Any one who has these two qualities, and who +has a small garden at his disposal is requested to take part in this +line of investigation. + +In order to observe directly the birth of new forms it is necessary, in +the first place, to be fully clear concerning the question as to what +forms are to be expected to arise from others, and before proceeding to +a demonstration of the origin of species, it is pertinent to raise the +question as to what constitutes a species. + +Species is a word, which always has had a [11] double meaning. One is +the systematic species, which is the unit of our system. But these units +are by no means indivisible. Long ago Linnaeus knew them to be compound +in a great number of instances, and increasing knowledge has shown that +the same rule prevails in other instances. Today the vast majority of +the old systematic species are known to consist of minor units. These +minor entities are called varieties in systematic works. However, there +are many objections to this usage. First, the term variety is applied in +horticulture and agriculture to things so widely divergent as to convey +no clear idea at all. Secondly, the subdivisions of species are by no +means all of the same nature, and the systematic varieties include units +the real value of which is widely different in different cases. Some of +these varieties are in reality as good as species, and have been +"elevated," as it is called by some writers, to this rank. This +conception of the elementary species would be quite justifiable, and +would at once get rid of all difficulties, were it not for one practical +obstacle. The number of the species in all genera would be doubled and +tripled, and as these numbers are already cumbersome in many cases, the +distinction of the native species of any given country would lose most +of its charm and interest. + +[12] In order to meet this difficulty we must recognize two sorts of +species. The systematic species are the practical units of the +systematists and florists, and all friends of wild nature should do +their utmost to preserve them as Linnaeus has proposed them. These units +however, are not really existing entities; they have as little claim to +be regarded as such as genera and families. The real units are the +elementary species; their limits often apparently overlap and can only +in rare cases be determined on the sole ground of field observations. +Pedigree-culture is the method required and any form which remains +constant and distinct from its allies in the garden is to be considered +as an elementary species. + +In the following lectures we shall consider this point at length, to +show the compound nature of systematic species in wild and in cultivated +plants. In both cases, the principle is becoming of great importance, +and many papers published recently indicate its almost universal +acceptation. + +Among the systematic subdivisions of species, not all have the same +claim to the title of elementary species. In the first place the cases +in which the differences may occur between parts of the same individual +are to be excluded. Dividing an alpine plant into two halves and [13] +planting one in a garden, varietal differences at once arise and are +often designated in systematic works under different varietal names. +Secondly all individual differences which are of a fluctuating nature +are to be combined into a group. But with these we shall deal later. + +Apart from these minor points the subdivisions of the systematic species +exhibit two widely different features. I will now try to make this clear +in a few words, but will return in another lecture to a fuller +discussion of this most interesting contrast. + +Linnaeus himself knew that in some cases all subdivisions of a species +are of equal rank, together constituting the group called species. No +one of them outranks the others; it is not a species with varieties, but +a group, consisting only of varieties. A closer inquiry into the cases +treated in this manner by the great master of systematic science, shows +that here his varieties were exactly what we now call elementary +species. + +In other cases the varieties are of a derivative nature. The species +constitutes a type that is pure in a race which ordinarily is still +growing somewhere, though in some cases it may have died out. From this +type the varieties are derived, and the way of this derivation is +usually quite manifest to the botanist. It is ordinarily [14] by the +disappearance of some superficial character that a variety is +distinguished from its species, as by the lack of color in the flowers, +of hairs on stems and foliage, of the spines and thorns, &c. Such +varieties are, strictly speaking, not to be treated in the same way as +elementary species, though they often are. We shall designate them by +the term of "retrograde varieties," which clearly indicates the nature +of their relationship to the species from which they are assumed to have +sprung. In order to lay more stress on the contrast between elementary +species and retrograde varieties, it should be stated at once, that the +first are considered to have originated from their parent-form in a +progressive way. They have succeeded in attaining something quite new +for themselves, while retrograde varieties have only thrown off some +peculiarity, previously acquired by their ancestors. + +The whole vegetable kingdom exhibits a constant struggle between +progression and retrogression. Of course, the great lines of the general +pedigree are due to progression, many single steps in this direction +leading together to the great superiority of the flowering plants over +their cryptogamous ancestors. But progression is nearly always +accompanied by retrogression in the principal lines of evolution, [15] +as well as in the collateral branches of the genealogical tree. +Sometimes it prevails, and the monocotyledons are obviously a reduced +branch of the primitive dicotyledons. In orchids and aroids, in grasses +and sedges, reduction plays a most important part, leaving its traces on +the flowers as well as on the embryo of the seed. Many instances could +be given to prove that progression and retrogression are the two main +principles of evolution at large. Hence the conclusion, that our +analysis must dissect the complicated phenomena of evolution so far as +to show the separate functions of these two contrasting principles. +Hundreds of steps were needed to evolve the family of the orchids, but +the experimenter must take the single steps for the object of his +inquiry. He finds that some are progressive and others retrogressive and +so his investigation falls under two heads, the origin of progressive +characters, and the subsequent loss of the same. Progressive steps are +the marks of elementary species, while retrograde varieties are +distinguished by apparent losses. They have equal claim to our interest +and our study. + +As already stated I propose to deal first with the elementary species +and afterwards with the retrograde varieties. I shall try to depict them +to you in the first place as they are seen in [16] nature and in +culture, leaving the question of their origin to a subsequent +experimental treatment. + +The question of the experimental origin of new species and varieties has +to be taken up from two widely separated starting points. This may be +inferred from what we have already seen concerning the two opposing +theories, derived and isolated from Darwin's original broad conception. +One of them considers mutations as the origin of new forms, while the +other assumes fluctuations to be the source of all evolution. + +As mentioned above, my own experience has led me to accept the first +view. Therefore I shall have to show that mutations do yield new and +constant forms, while fluctuations are not adequate to do so. Retrograde +varieties and elementary species may both be seen to be produced by +sudden mutations. Varieties have often been observed to appear at once +and quite unexpectedly in horticulture and agriculture, and a survey of +these historical facts will be the subject of one of my lectures. In +some instances I have succeeded in repeating these observations in my +garden under the strict conditions of a scientific experiment, and these +instances teach us the real nature of the process of mutation in all its +visible features. New elementary [17] species are far more rare, but I +have discovered in the great evening-primrose, or _Oenothera +lamarckiana_ a strain which is producing them yearly in the wild state +as well as in my garden. These observations and pedigree-experiments +will be dealt with at due length in subsequent lectures. + +Having proved the existence and importance of mutations, it remains to +inquire how far the improvements may go which are due only to +fluctuating variability. As the term indicates, this variability is +fluctuating to and fro, oscillating around an average type. It never +fails nor does it, under ordinary circumstances, depart far from the +fixed average. + +But the deviation may be enlarged by a choice of extremes. In sowing +their seed, the average of the strain is seen to be changed, and in +repeating the experiment the change may be considerable. It is not +clear, whether theoretically by such an accumulation, deviations might +be reached which could not be attained at once in a single sowing. This +question is hardly susceptible of an experimental answer, as it would +require such an enormous amount of seed from a few mother plants as can +scarcely ever be produced. + +The whole character of the fluctuations shows them to be of an opposite +nature, contrasting [18] manifestly with specific and varietal +characters. By this method they may be proved to be inadequate ever to +make a single step along the great lines of evolution, in regard to +progressive as well as to retrograde development. + +First of all fluctuations are linear, amplifying or lessening the +existing qualities, but not really changing their nature. They are not +observed to produce anything quite new, and evolution of course, is not +restricted to the increase of the already existing peculiarities, but +depends chiefly on the continuous addition of new characters to the +stock. Fluctuations always oscillate around an average, and if removed +from this for some time, they show a tendency to return to it. This +tendency, called retrogression, has never been observed to fail, as it +should, in order to free the new strain from the links with the average, +while new species and new varieties are seen to be quite free from their +ancestors and not linked to them by intermediates. + +The last few lectures will be devoted to questions concerning the great +problem of the analogy between natural and artificial selection. As +already stated, Darwin made this analogy the foundation stone of his +theory of descent, and he met with the severest objections and +criticisms precisely on this point. But I hope to [19] show that he was +quite right, and that the cause of the divergence of opinions is due +simply to the very incomplete state of knowledge concerning both +processes. If both are critically analyzed they may be seen to comprise +the same factors, and further discussion may be limited to the +appreciation of the part which each of them has played in nature and +among cultivated plants. + +Both natural and artificial selection are partly specific, and partly +intra-specific or individual. Nature of course, and intelligent men +first chose the best elementary species from among the swarms. In +cultivation this is the process of variety-testing. In nature it is the +survival of the fittest species, or, as Morgan designates it, the +survival of species in the struggle for existence. The species are not +changed by this struggle, they are only weighed against each other, the +weak being thrown aside. + +Within the chosen elementary species there is also a struggle. It is +obvious, that the fluctuating variability adapts some to the given +circumstances, while it lessens the chances of others. A choice results, +and this choice is what is often exclusively called selection, either +natural or artificial. In cultivation it produces the improved and the +local races; in nature little is known about improvement in this way, +but [19] local adaptations with slight changes of the average character +in separate localities, seem to be of quite normal occurrence. + +A new method of individual selection has been used in recent years in +America, especially by W.M. Hays. It consists in judging the hereditary +worth of a plant by the average condition of its offspring, instead of +by its own visible characters. If this determination of the "centgener +power," as Hays calls it, should prove to be the true principle of +selection, then indeed the analogy between natural and artificial +selection would lose a large part of its importance. We will reserve +this question for the last lecture, as it pertains more to the future, +than to our present stock of knowledge. + +Something should be said here concerning hybrids and hybridism. This +problem has of late reached such large proportions that it cannot be +dealt with adequately in a short survey of the phenomena of heredity in +general. It requires a separate treatment. For this reason I shall limit +myself to a single phase of the problem, which seems to be indispensable +for a true and at the same time easy distinction between elementary +species and retrograde varieties. According to accepted terminology, +some crosses are to be considered as unsymmetrical, while others are +symmetrical. The first are one-sided, [21] some peculiarity being found +in one of the parents and lacking in the other. The second are balanced, +as all the characters are present in both parents, but are found in a +different condition. Active in one of them, they are concealed or +inactive in the other. Hence pairs of contrasting units result, while in +unbalanced crosses no pairing of the particular character under +consideration is possible. This leads to the principal difference +between species and varieties, and to an experimental method of deciding +between them in difficult and doubtful cases. + +Having thus indicated the general outlines of the subjects I shall deal +with, something now may be said as to methods of investigation. + +There are two points in which scientific investigation differs from +ordinary pedigree-culture in practice. First the isolation of the +individuals and the study of individual inheritance, instead of +averages. Next comes the task of keeping records. Every individual must +be entered, its ancestry must be known as completely as possible, and +all its relations must be noted in such a form, that the most complete +reference is always possible. Mutations may come unexpectedly, and when +once arisen, their parents and grand-parents should be known. Records +must be available which will allow of a most complete knowledge of the +whole ancestral [22] line. This, and approximately this only, is the +essential difference between experimental and accidental observation. + +Mutations are occurring from time to time in the wild state as well as +in horticulture and agriculture. A selection of the most interesting +instances will be given later. But in all such cases the experimental +proof is wanting. The observations as a rule, only began when the +mutation had made its appearance. A more or less vague remembrance about +the previous state of the plants in question might be available, though +even this is generally absent. But on doubtful points, concerning +possible crosses or possible introduction of foreign strains, mere +recollection is insufficient. The fact of the mutation may be very +probable, but the full proof is, of course, wanting. Such is the case +with the mutative origin of _Xanthium commune_ Wootoni from New Mexico +and of _Oenothera biennis cruciata_ from Holland. The same doubt exists +as to the origin of the _Capsella heegeri_ of Solms-Laubach, and of the +oldest recorded mutation, that of _Chelidonium laciniatum_ in Heidelberg +about 1600. + +First, we have doubts about the fact itself. These, however, gradually +lose their importance in the increasing accumulation of evidence. +Secondly, the impossibility of a closer [23] inquiry into the real +nature of the change. For experimental purposes a single mutation does +not suffice; it must be studied repeatedly, and be produced more or less +arbitrarily, according to the nature of the problems to be solved. And +in order to do this, it is evidently not enough to have in hand the +mutated individual, but it is indispensable to have also the mutable +parents, or the mutable strain from which it sprang. + +All conditions previous to the mutation are to be considered as of far +higher importance than all those subsequent to it. + +Now mutations come unexpectedly, and if the ancestry of an accidental +mutation is to be known, it is of course necessary to keep accounts of +all the strains cultivated. It is evident that the required knowledge +concerning the ancestry of a supposed mutation, must necessarily nearly +all be acquired from the plants in the experimental garden. + +Obviously this rule is as simple in theory, as it is difficult to carry +out in practice. First of all comes the book-keeping. The parents, +grandparents and previous ancestors must be known individually. Accounts +of them must be kept under two headings. A full description of their +individual character and peculiarities must always be available on the +one hand, and on the other, all facts concerning their hereditary [24] +qualities. These are to be deduced from the composition of the progeny, +and in order to obtain complete evidence on this point, two successive +generations are often required. The investigation must ascertain the +average condition of this offspring and the occurrence of any deviating +specimens, and for both purposes it is necessary to cultivate them in +relatively large numbers. It is obvious that, properly speaking, the +whole family of a mutated individual, including all its nearer and more +remote relatives, should be known and recorded. + +Hence pedigree-book-keeping must become the general rule. Subordinate to +this are two further points, which should likewise be stated here. One +pertains to the pure or hybrid nature of the original strain, and the +other to the life-conditions and all other external influences. It is +manifest that a complete understanding of a mutation depends upon full +information upon these points. + +All experiments must have a beginning. The starting-point may be a +single individual, or a small group of plants, or a lot of seeds. In +many cases the whole previous history is obscure, but sometimes a little +historical evidence is at hand. Often it is evident that the initial +material belongs to a pure species, but with respect to the question of +elementary species it is [25] not rarely open to doubt. Large numbers of +hybrid plants and hybrid races are in existence, concerning the origin +of which it is impossible to decide. It is impossible in many instances +to ascertain whether they are of hybrid or of pure origin. Often there +is only one way of determining the matter; it is to guess at the +probable parents in case of a cross and to repeat the cross. This is a +point which always requires great care in the interpretation of unusual +facts. + +Three cases are to be distinguished as to heredity. Many plants are so +constituted as to be fertilized with their own pollen. In this case the +visits of insects have simply to be excluded, which may be done by +covering plants with iron gauze or with bags of prepared paper. +Sometimes they fertilize themselves without any aid, as for instance, +the common evening-primrose; in other cases the pollen has to be placed +on the stigma artificially, as with Lamarck's evening-primrose and its +derivatives. Other plants need cross-fertilization in order to produce a +normal yield of seeds. Here two individuals have always to be combined, +and the pedigree becomes a more complicated one. Such is the case with +the toad-flax, which is nearly sterile with its own pollen. But even in +these cases the visits of insects bringing pollen [26] from other +plants, must be carefully excluded. A special lecture will be devoted to +this very interesting source of impurity and of uncertainty in ordinary +cultures. + +Of course, crosses may lie in the proposed line of work, and this is the +third point to be alluded to. They must be surrounded with the same +careful isolation and protection against bees, as any other +fertilizations. And not only the seed-parent, but also the pollen must +be kept pure from all possible foreign admixtures. + +A pure and accurately recorded ancestry is thus to be considered as the +most important condition of success in experimental plant breeding. Next +to this comes the gathering of the seeds of each individual separately. +Fifty or sixty, and often more, bags of seeds are by no means uncommon +for a single experiment, and in ordinary years the harvest of my garden +is preserved in over a thousand separate lots. + +Complying with these conditions, the origin of species may be seen as +easily as any other phenomenon. It is only necessary to have a plant in +a mutable condition. Not all species are in such a state at present, and +therefore I have begun by ascertaining which were stable and which were +not. These attempts, of course, had to be made in the experimental +garden, and large quantities of seed had to be procured and [27] sown. +Cultivated plants of course, had only a small chance to exhibit new +qualities, as they have been so strictly controlled during so many +years. Moreover their purity of origin is in many cases doubtful. Among +wild plants only those could be expected to reward the investigator +which were of easy cultivation. For this reason I have limited myself to +the trial of wild plants of Holland, and have had the good fortune to +find among them at least one species in a state of mutability. It was +not really a native plant, but one that had been introduced from America +and belongs to an American genus. I refer to the great evening-primrose +or the evening-primrose of Lamarck. A strain of this beautiful species +is growing in an abandoned field in the vicinity of Hilversum, at a +short distance from Amsterdam. Here it has escaped from a park and +multiplied. In doing so it has produced and is still producing quite a +number of new types, some of which may be considered as retrograde +varieties, while others evidently are of the nature of progressive +elementary species. + +This interesting plant has afforded me the means of observing directly +how new species originate, and of studying the laws of these changes. My +researches have followed a double line of inquiry. On one side, I have +limited [28] myself to direct field observations, and to tests of seed, +collected from the wild plants in their native locality. Obviously the +mutations are decided within the seed, and the culture of young plants +from them had no other aim than that of ascertaining what had occurred +in the field. And then the many chances of destruction that threaten +young plants in a wild state, could be avoided in the garden, where +environmental factors can be controlled. + +My second line of inquiry was an experimental repetition of the +phenomena which were only partly discerned at the native locality. It +was not my aim to intrude into the process, nor to try to bring out new +features. My only object was to submit to the precepts just given +concerning pure treatment, individual seed gathering, exclusion of +crosses and accurate recording of all the facts. The result has been a +pedigree which now permits of stating the relation between all the +descendants of my original introduced plant. This pedigree at once +exhibits the laws followed by the mutating species. The main fact is, +that it does not change itself gradually, but remains unaffected during +all succeeding generations. It only throws off new forms, which are +sharply contrasted with the parent, and which are from the very +beginning as perfect and as constant, as narrowly [29] defined and as +pure of type as might be expected of any species. + +These new species are not produced once or in single individuals, but +yearly and in large numbers. The whole phenomenon conveys the idea of a +close group of mutations, all belonging to one single condition of +mutability. Of course this mutable state must have had a beginning, as +it must sometime come to an end. It is to be considered as a period +within the life-time of the species and probably it is only a small part +of it. + +The detailed description of this experiment, however, I must delay to a +subsequent lecture, but I may be allowed to state, that the discovery of +this period of mutability is of a definite theoretical importance. One +of the greatest objections to the Darwinian theory of descent arose from +the length of time it would require, if all evolution was to be +explained on the theory of slow and nearly invisible changes. This +difficulty is at once met and fully surmounted by the hypothesis of +periodical but sudden and quite noticeable steps. This assumption +requires only a limited number of mutative periods, which might well +occur within the time allowed by physicists and geologists for the +existence of animal and vegetable life on the earth. + +[30] Summing up the main points of these introductory remarks, I propose +to deal with the subjects mentioned above at some length, devoting to +each of them, if possible at least an entire lecture. The decisive facts +and discussions upon which the conclusions are based will be given in +every case. Likewise I hope to point out the weak places and the lacunae +in our present knowledge, and to show the way in which each of you may +try to contribute his part towards the advancement of science in this +subject. Lastly I shall try to prove that sudden mutation is the normal +way in which nature produces new species and new varieties. These +mutations are more readily accessible to observation and experiment than +the slow and gradual changes surmised by Wallace and his followers, +which are entirely beyond our present and future experience. + +The theory of mutations is a starting-point for direct investigation, +while the general belief in slow changes has held back science from such +investigations during half a century. + +Coming now to the subdivisions and headings under which my material is +to be presented, I propose describing first the real nature of the +elementary species and retrograde varieties, both in normal form and in +hybridizations. A discussion of other types of varieties, including [31] +monstrosities will complete the general plan. The second subdivision +will deal with the origin of species and varieties as taught by +experiment and observation, treating separately the sudden variations +which to my mind do produce new forms, and subsequently the fluctuations +which I hold to be not adequate to this purpose. + + +[32] +B. ELEMENTARY SPECIES + +LECTURE II + +ELEMENTARY SPECIES IN NATURE + +What are species? Species are considered as the true units of nature by +the vast majority of biologists. They have gained this high rank in our +estimation principally through the influence of Linnaeus. They have +supplanted the genera which were the accepted units before Linnaeus. +They are now to be replaced in their turn, by smaller types, for reasons +which do not rest upon comparative studies but upon direct experimental +evidence. + +Biological studies and practical interests alike make new demands upon +systematic botany. Species are not only the subject-material of herbaria +and collections, but they are living entities, and their life-history +and life-conditions command a gradually increasing interest. One phase +of the question is to determine the easiest manner to deal with the +collected forms of a country, and another feature is the problem [33] as +to what groups are real units and will remain constant and unchanged +through all the years of our observations. + +Before Linnaeus, the genera were the real units of the system. De +Candolle pointed out that the old common names of plants, such as roses +and clover, poplars and oaks, nearly all refer to genera. The type of +the clovers is rich in color, and the shape of the flower-heads and the +single flowers escape ordinary observation; but notwithstanding this, +clovers are easily recognized, even if new types come to hand. White and +red clovers and many other species are distinguished simply by +adjectives, the generic name remaining the same for all. + +Tournefort, who lived in the second half of the 17th century +(1656-1708), is generally considered as the author of genera in +systematic botany. He adopted, what was at that time the general +conception and applied it throughout the vegetable kingdom. He grouped +the new and the rare and the previously overlooked forms in the same +manner in which the more conspicuous plants were already arranged by +universal consent. Species were distinguished by minor marks and often +indicated by short descriptions, but they were considered of secondary +importance. + +Based on the idea of a direct creation of all [34] living beings, the +genera were then accepted as the created forms. They were therefore +regarded as the real existing types, and it was generally surmised that +species and varieties owed their origin to subsequent changes under the +influence of external conditions. Even Linnaeus agreed with this view in +his first treatises and in his "Philosophical Botany" he still kept to +the idea that all genera had been created at once with the beginning of +life. + +Afterwards Linnaeus changed his opinion on this important point, and +adopted species as the units of the system. He declared them to be the +created forms, and by this decree, at once reduced the genera to the +rank of artificial groups. Linnaeus was well aware that this conception +was wholly arbitrary, and that even the species are not real indivisible +entities. But he simply forbade the study of lesser subdivisions. At his +time he was quite justified in doing so, because the first task of the +systematic botanists was the clearing up of the chaos of forms and the +bringing of them into connection with their real allies. + +Linnaeus himself designated the subdivisions of the species as +varieties, but in doing so he followed two clearly distinct principles. +In some cases his species were real plants, and the varieties seemed to +be derived from them by [35] some simple changes. They were subordinated +to the parent-species. In other cases his species were groups of lesser +forms of equal value, and it was not possible to discern which was the +primary and which were the derivatives. + +These two methods of subdivision seem in the main, and notwithstanding +their relatively imperfect application in many single examples, to +correspond with two really distinct cases. The derivative varieties are +distinguished from the parent-species by some single, but striking mark, +and often this attribute manifests itself as the loss of some apparent +quality. The loss of spines and of hairs and the loss of blue and red +flower-colors are the most notorious, but in rarer cases many single +peculiarities may disappear, thereby constituting a variety. This +relation of varieties to the parent-species is gradually increasing in +importance in the estimation of botanists, sharply contrasting with +those cases, in which such dependency is not to be met with. + +If among the subdivisions of a species, no single one can be pointed out +as playing a primary part, and the others can not be traced back to it, +the relation between these lesser units is of course of another +character. They are to be considered of equal importance. They are +distinguished from each other by more than [36] one character, often by +slight differences in nearly all their organs and qualities. Such forms +have come to be designated as "elementary species." They are only +varieties in a broad and vague systematic significance of the word, not +in the sense accorded to this term in horticultural usage, nor in a +sharper and more scientific conception. + +Genera and species are, at the present time, for a large part +artificial, or stated more correctly, conventional groups. Every +systematist is free to delimit them in a wider or in a narrower sense, +according to his judgment. The greater authorities have as a rule +preferred larger genera, others of late have elevated innumerable +subgenera to the rank of genera. This would work no real harm, if +unfortunately, the names of the plants had not to be changed each time, +according to current ideas concerning genera. Quite the same inconstancy +is observed with species. In the Handbook of the British Flora, Bentham +and Hooker describe the forms of brambles under 5 species, while +Babington in his Manual of British Botany makes 45 species out of the +same material. So also in other cases. For instance, the willows which +have 13 species in one and 31 species in the other of these manuals, and +the hawkweeds for which the figures are 7 and 32 [37] respectively. +Other authors have made still greater numbers of species in the same +groups. + +It is very difficult to estimate systematic differences on the ground of +comparative studies alone. All sorts of variability occur, and no +individual or small group of specimens can really be considered as a +reliable representative of the supposed type. Many original diagnoses of +new species have been founded on divergent specimens and of course, the +type can afterwards neither be derived from this individual, nor from +the diagnosis given. + +This chaotic state of things has brought some botanists to the +conviction that even in systematic studies only direct experimental +evidence can be relied upon. This conception has induced them to test +the constancy of species and varieties, and to admit as real units only +such groups of individuals as prove to be uniform and constant +throughout succeeding generations. The late Alexis Jordan, of Lyons in +France, made extensive cultures in this direction. In doing so, he +discovered that systematic species, as a rule, comprise some lesser +forms, which often cannot easily be distinguished when grown in +different regions, or by comparing dried material. This fact was, of +course, most distasteful to the systematists of his time and even for a +long period afterwards [38] they attempted to discredit it. Milde and +many others have opposed these new ideas with some temporary success. +Only of late has the school of Jordan received due recognition, after +Thuret, de Bary, Rosen and others tested its practices and openly +pronounced for them. Of late Wittrock of Sweden has joined them, making +extensive experimental studies concerning the real units of some of the +larger species of his country. + +From the evidence given by these eminent authorities, we may conclude +that systematic species, as they are accepted nowadays, are as a rule +compound groups. Sometimes they consist of two or three, or a few +elementary types, but in other cases they comprise twenty, or fifty, or +even hundreds of constant and well differentiated forms. + +The inner constitution of these groups is however, not at all the same +in all cases. This will be seen by the description of some of the more +interesting of them. The European heartsease, from which our +garden-pansies have been chiefly derived, will serve as an example. The +garden-pansies are a hybrid race, won by crossing the _Viola tricolor_ +with the large flowered and bright yellow _V. lutea_. They combine, as +everyone knows, in their wide range of [39] varieties, the attributes of +the latter with the peculiarities of the former species. + +Besides the _lutea_, there are some other species, nearly allied to +tricolor, as for instance, _cornuta_, _calcarata_, and _altaica_, which +are combined with it under the head of _Melanium_ as a subgenus, and +which together constitute a systematic unity of undoubted value, but +ranging between the common conceptions of genus and species. These forms +are so nearly allied to the heartsease that they have of late been made +use of in crosses, in order to widen the range of variability of +garden-pansies. + +_Viola tricolor_ is a common European weed. It is widely dispersed and +very abundant, growing in many localities in large numbers. It is an +annual and ripens its seeds freely, and if opportunity is afforded, it +multiplies rapidly. + +_Viola tricolor_ has three subspecies, which have been elevated to the +rank of species by some authors, and which may here be called, for +brevity's sake, by their binary names. One is the typical _V. tricolor_, +with broad flowers, variously colored and veined with yellow, purple and +white. It occurs in waste places on sandy soil. The second is called _V. +arvensis_ or the field-pansy; it has small inconspicuous flowers, with +pale-yellowish petals which are shorter than the sepals. It pollinates +itself without the [40] aid of insects, and is widely dispersed in +cultivated fields. The third form, _V. alpestris_, grows in the Alps, +but is of lesser importance for our present discussion. + +Anywhere throughout the central part of Europe _V. tricolor_ and _V. +arvensis_ may be seen, each occupying its own locality. They may be +considered as ranging among the most common native plants of the +particular regions they inhabit. They vary in the color of the flowers, +branching of the stems, in the foliage and other parts, but not to such +an extent as to constitute distinct strains. They have been brought into +cultivation by Jordan, Wittrock and others, but throughout Europe each +of them constitutes a single type. + +These types must be very old and constant, fluctuating always within the +same distinct and narrow limits. No slow, gradual changes can have taken +place. In different countries their various habitats are as old as the +historical records, and probably many centuries older. They are quite +independent of one another, the distance being in numerous cases far too +great for the exchange of pollen or of seeds. If slow and gradual +changes were the rule, the types could not have remained so uniform +throughout the whole range of these two species. They would necessarily +have split up into thousands [41] and thousands of minor races, which +would show their peculiar characteristics if tested by cultures in +adjacent beds. This however, is not what happens. As a matter of fact +_V. tricolor_ and _V. arvensis_ are widely distributed but wholly +constant types. + +Besides these, there occur distinct types in numerous localities. Some +of them evidently have had time and opportunity to spread more or less +widely and now occupy larger regions or even whole countries. Others are +narrowly limited, being restricted to a single locality. Wittrock +collected seeds or plants from as many localities as possible in +different parts of Sweden and neighboring states and sowed them in his +garden near Stockholm. He secured seeds from his plants, and grew from +them a second, and in many cases a third generation in order to estimate +the amount of variability. As a rule the forms introduced into his +garden proved constant, notwithstanding the new and abnormal conditions +under which they were propagated. + +First of all we may mention three perennial forms called by him _Viola +tricolor ammotropha_, _V. tricolor coniophila_ and _V. stenochila_. The +typical _V. tricolor_ is an annual plant; sowing itself in summer and +germinating soon afterwards. The young plants thrive throughout [42] the +latter part of the summer and during the fall, reaching an advanced +stage of development of the branched stems before winter. Early in the +spring the flowers begin to open, but after the ripening of the seeds +the whole plant dies. + +The three perennial species just mentioned develop in the same manner in +the first year. During their flowering period, however, and afterwards, +they produce new shoots from the lower parts of the stem. They prefer +dry and sandy soils, often becoming covered with the sand that is blown +on them by the winds. They are prepared for such seemingly adverse +circumstances by the accumulation of food in the older stems and by the +capacity of the new shoots to thrive on this food till they have become +long enough to reach the light. _V. tricolor ammotropha_ is native near +Ystad in Sweden, and the other two forms on Gotland. All three have +narrowly limited habitats. + +The typical tricolored heartsease has remained annual in all its other +subspecies. It may be divided into two types in the first place, _V. +tricolor genuina_ and _V. tricolor versicolor_. Both of them have a wide +distribution and seem to be the prototypes from which the rarer forms +must have been derived. Among these latter Wittrock describes seven +local types, which [43] proved to be constant in his pedigree-cultures. +Some of them have produced other forms, related to them in the way of +varieties. They all have nearly the same general habit and do not +exhibit any marked differences in their growth, in the structure and +branching of the stems, or in the character of their foliage. +Differentiating points are to be found mainly in the colors and patterns +of the flowers. The veins, which radiate from the centre of the corolla +are branched in some and undivided in others; in one elementary species +they are wholly lacking. The purple color may be absent, leaving the +flowers of a pale or a deep yellow. Or the purple may be reddish or +bluish. Of the petals all five may have the purple hue on their tips, or +this attribute may be limited to the two upper ones. Contrasting with +this wide variability is the stability of the yellow spot in the centre, +which is always present and becomes inconspicuous only, when the whole +petals are of the same hue. It is a general conception that colors and +color-markings are liable to great variability and do not constitute +reliable standards. But the cultures of Wittrock have proved the +contrary, at least in the case of the violets. No pattern, however +quaint, appears changeable, if one elementary species only is +considered. Hundreds of plants from seeds [44] from one locality may be +grown, and all will exhibit exactly the same markings. Most of these +forms are of very local occurrence. The most beautiful of all, the +_ornatissima_, is found only in Jemtland, the _aurobadia_ only in +Sodermanland, the anopetala_ in other localities in the same country, +the _roseola_ near Stockholm, and the yellow _lutescens_ in Finmarken. + +The researches of Wittrock included only a small number of elementary +species, but every one who has observed the violets in the central parts +of Europe must be convinced that many dozens of constant forms of the +typical _Viola tricolor_ might easily be found and isolated. + +We now come to the field pansy, the _Viola arvensis_, a very common weed +in the grain-fields of central Europe. I have already mentioned its +small corolla, surpassed by the lobes of the calyx and its capacity of +self-fertilization. It has still other curious differentiating +characters; the pollen grains, which are square in _V. tricolor_, are +five-sided in _V. arvensis_. Some transgressive fluctuating variability +may occur in both cases through the admixture of pollen-grains. Even +three-angled pollen grains are seen sometimes. Other marks are observed +in the form of the anthers and the spur. + +There seem to be very many local subspecies [45] of the field-pansy. +Jordan has described some from the vicinity of Lyons, and Wittrock +others from the northern parts of Europe. They diverge from their common +prototype in nearly all attributes, the flowers not showing the +essential differentiating characters as in the _V. tricolor_. Some have +their flower-stalks erect, and in others the flowers are held nearly at +right angles to the stem. _V. pallescens_ is a small, almost unbranched +species with small pale flowers. _V. segetalis_ is a stouter species +with two dark blue spots on the tips of the upper petals. _V. agrestis_ +is a tall and branched, hairy form. _V. nemausensis_ attains a height of +only 10 cm., has rounded leaves and long flower-stalks. Even the seeds +afford characters which may be made use of in isolating the various +species. + +The above-mentioned elementary forms belong to the flora of southern +France, and Wittrock has isolated and cultivated a number of others from +the fields of Sweden. A species from Stockholm is called _Viola patens_; +_V. arvensis curtisepala_ occurs in Gotland, and _V. arvensis striolata_ +is a distinct form, which has appeared in his cultures without its true +origin being ascertained. + +The alpine violets comprise a more widespread type with some local +elementary species [46] derived exactly in the same way as the +tricolored field pansies. + +Summarizing the general result of this description we see that the +original species _Viola tricolor_ may be split up into larger and lesser +groups of separate forms. These last prove to be constant in +pedigree-cultures, and therefore are to be considered as really existent +units. They are very numerous, comprising many dozens in each of the two +larger subdivisions. + +All systematic grouping of these forms, and their combination into +subspecies and species rests on the comparative study of their +characters. The result of such studies must necessarily depend on +principles which underlie them. According to the choice of these +principles, the construction of the groups will be found to be +different. Wittrock trusts in the first place to morphologic characters, +and considers the development as passing from the more simple to the +more complex types. On the other hand the geographic distribution may be +considered as an indication of the direction of evolution, the +wide-spread forms being regarded as the common parents of the minor +local species. + +However, such considerations are only of secondary importance. It must +be borne in mind that an ordinary systematic species may include [47] +many dozens of elementary forms, each of which remains constant and +unchanged in successive generations, even if cultivated in the same +garden and under similar external conditions. + +Leaving the violets, we may take the vernal whitlow-grass or _Draba +verna_ for a second illustration. This little annual cruciferous plant +is common in the fields of many parts of the United States, though +originally introduced from Europe. It has small basal rosettes which +develop during summer and winter, and produce numerous leafless +flowering stems early in the spring. It is a native of central Europe +and western Asia, and may be considered as one of the most common +plants, occurring anywhere in immense numbers on sandy soils. Jordan was +the first to point out that it is not the same throughout its entire +range. Although a hasty survey does not reveal differences, they show +themselves on closer inspection. De Bary, Thuret, Rosen and many others +confirmed this result, and repeated the pedigree-cultures of Jordan. +Every type is constant and remains unchanged in successive generations. +The anthers open in the flower-buds and pollinate the stigmas before the +expansion of the flowers, thus assuring self-fertilization. Moreover, +these inconspicuous little flowers are only sparingly visited by +insects. Dozens of subspecies [48] may be cultivated in the same garden +without any real danger of their intercrossing. They remain as pure as +under perfect isolation. + +It is very interesting to observe the aspect of such types, when growing +near each other. Hundreds of rosettes exhibit one type, and are +undoubtedly similar. The alternative group is distinguishable at first +sight, though the differentiating marks are often so slight as to be +traceable with difficulty. Two elementary species occur in Holland, one +with narrow leaves in the western provinces and one with broader foliage +in the northern parts. I have cultivated them side by side, and was as +much struck with the uniformity within each group, as with the contrast +between the two sets. + +Nearly all organs show differences. The most marked are those of the +leaves, which may be small or large, linear or elliptic or oblong and +even rhomboidal in shape, more or less hairy with simple or with +stellate branched hairs, and finally of a pure green or of a glaucous +color. The petals are as a rule obcordate, but this type may be combined +with others having more or less broad emarginations at the summit, and +with differences in breadth which vary from almost linear types to +others which touch along their margins. The pods are short and broad, or +long and narrow, or varying in sundry other [49] ways. All in all there +are constant differences which are so great that it has been possible to +distinguish and to describe large numbers of types. + +Many of them have been tested as to their constancy from seed. Jordan +made numerous cultures, some of which lasted ten or twelve years; Thuret +has verified the assertion concerning their constancy by cultures +extending over seven years in some instances; Villars and de Bary made +numerous trials of shorter duration. All agree as to the main points. +The local races are uniform and come true from seed; the variability of +the species is not of a fluctuating, but of a polymorphous nature. A +given elementary species keeps within its limits and cannot vary beyond +them, but the whole group gives the impression of variability by its +wide range of distinct, but nearly allied forms. + +The geographic distribution of these elementary species of the +whitlow-grass is quite distinct from that of the violets. Here +predominant species are limited to restricted localities. Most of them +occupy one or more departments of France, and in Holland two of them are +spread over several provinces. An important number are native in the +centre of Europe, and from the vicinity of Lyons, Jordan succeeded in +establishing about fifty elementary [50] species in his garden. In this +region they are crowded together and not rarely two or even more quite +distinct forms are observed to grow side by side on the same spot. +Farther away from this center they are more widely dispersed, each +holding its own in its habitat. In all, Jordan has distinguished about +two hundred species of _Draba verna_ from Europe and western Asia. +Subsequent authors have added new types to the already existing number +from time to time. + +The constancy of these elementary species is directly proven by the +experiments quoted above, and moreover it may be deduced from the +uniformity of each type within its own domain. These are so large that +most of the localities are practically isolated from one another, and +must have been so for centuries. If the types were slowly changing such +localities would often, though of course not always, exhibit slighter +differences, and on the geographic limits of neighboring species +intermediates would be found. Such however, are not on record. Hence the +elementary species must be regarded as old and constant types. + +The question naturally arises how these groups of nearly allied forms +may originally have been produced. Granting a common origin for all of +them, the changes may have been [51] simultaneous or successive. +According to the geographic distribution, the place of common origin +must probably be sought in the southern part of central Europe, perhaps +even in the vicinity of Lyons. Here we may assume that the old _Draba +verna_ has produced a host or a swarm of new types. Thence they must +have spread over Europe, but whether in doing so they have remained +constant, or whether some or many of them have repeatedly undergone +specific mutations, is of course unknown. + +The main fact is, that such a small species as _Draba verna_ is not at +all a uniform type, but comprises over two hundred well distinguished +and constant forms. + +It is readily granted that violets and whitlowgrasses are extreme +instances of systematic variability. Such great numbers of elementary +species are not often included in single species of the system. But the +numbers are of secondary importance, and the fact that systematic +species consist, as a rule, of more than one independent and constant +subspecies, retains its almost universal validity. + +In some cases the systematic species are manifest groups, sharply +differentiated from one another. In other instances the groups of +elementary forms as they are shown by direct observation, have been +adjudged by many authors [52] to be too large to constitute species. +Hence the polymorphous genera, concerning the systematic subdivisions of +which hardly two authors agree. Brambles and roses are widely known +instances, but oaks, elms, apples, and pears, _Mentha_, _Prunu_s, +_Vitis_, _Lactuca_, _Cucumis_, _Cucurbita_ and numerous others are in +the same condition. + +In some instances the existence of elementary species is so obvious, +that they have been described by taxonomists as systematic varieties or +even as good species. The primroses afford a widely known example. +Linnaeus called them _Primula veris_, and recognized three types as +pertaining to this species, but Jacquin and others have elevated these +subspecies to the full rank of species. They now bear the names of +_Primula elatior_ with larger, _P. officinalis_ with smaller flowers, +and _P. acaulis_. In the last named the common flower-stalk is lacking +and the flowers of the umbel seem to be borne in the arils of the basal +leaves. + +In other genera such nearly allied species are more or less universally +recognized. _Galium Mollugo_ has been divided into _G. elatum_ with a +long and weak stem, and _G. erectum_ with shorter and erect stems; +_Cochlearia danica_, _anglica_ and _officinalis_ are so nearly allied as +to be hardly distinguishable. _Sagina apetala_ and _patula_, [53] +_Spergula media_ and _salina_ and many other pairs of allied species +have differentiating characters of the same value as those of the +elementary species of _Draba verna_. _Filago_, _Plantago_, _Carex_, +_Ficaria_ and a long series of other genera afford proofs of the same +close relation between smaller and larger groups of species. The +European frost-weeds or _Helianthemum_ include a group of species which +are so closely allied, that ordinary botanical descriptions are not +adequate to give any idea of their differentiating features. It is +almost impossible to determine them by means of the common analytical +keys. They have to be gathered from their various native localities and +cultivated side by side in the garden to bring out their differences. +Among the species of France, according to Jordan, _Helianthemum +polifolium_, _H. apenninum_, _H. pilosum_ and _H. pulverulentum_ are of +this character. + +A species of cinquefoil, _Potentilla Tormentilla_, which is +distinguished by its quaternate flowers, occurs in Holland in two +distinct types, which have proved constant in my cultural experiments. +One of them has, broad petals, meeting together at the edges, and +constituting rounded saucer without breaks. The other has narrow petals, +which are strikingly separated from one another and show the sepals +between them. [54] In the same manner bluebells vary in the size and +shape of the corolla, which may be wide or narrow, bell-shaped or +conical, with the tips turned downwards, sidewards or backwards. + +As a rule all of the more striking elementary types have been described +by local botanists under distinct specific names, while they are thrown +together into the larger systematic species by other authors, who study +the distribution of plants over larger portions of the world. Everything +depends on the point of view taken. Large floras require large species. +But the study of local floras yields the best results if the many forms +of the region are distinguished and described as completely as possible. +And the easiest way is to give to each of them a specific name. If two +or more elementary species are united in the same district, they are +often treated in this way, but if each region had its own type of some +given species, commonly the part is taken for the whole, and the sundry +forms are described under the same name, without further distinctions. + +Of course these questions are all of a practical and conventional +nature, but involve the different methods in which different authors +deal with the same general fact. The fact is that systematic species are +compound groups, exactly like the genera and that their real units [55] +can only be recognized by comparative experimental studies. + +Though the evidence already given might be esteemed to be sufficient for +our purpose, I should like to introduce a few more examples; two of them +pertain to American plants. + +The Ipecac spurge or _Euphorbia Ipecacuanha_ occurs from Connecticut to +Florida, mainly near the coast, preferring dry and sandy soil. It is +often found by the roadsides. According to Britton and Brown's +"Illustrated Flora" it is glabrous or pubescent, with several or many +stems, ascending or nearly erect; with green or red leaves, which are +wonderfully variable in outline, from linear to orbicular, mostly +opposite, the upper sometimes whorled, the lower often alternate. The +glands of the involucres are elliptic or oblong, and even the seeds vary +in shape. + +Such a wide range of variability evidently points to the existence of +some minor types. Dr. John Harshberger has made a study of those which +occur in the vicinity of Whitings in New Jersey. His types agree with +the description given above. Others were gathered by him at Brown's +Mills in the pinelands, New Jersey, where they grew in almost pure sand +in the bright sunlight. He observed still other differentiating +characters. The amount of seed [56] produced and the time of flowering +were variable to a remarkable degree. + +Dr. Harshberger had the kindness to send me some dried specimens of the +most interesting of these types. They show that the peculiarities are +individual, and that each specimen has its own characters. It is very +probable that a comparative experimental study will prove the existence +of a large number of elementary species, differing in many points; they +will probably also show differences in the amount of the active chemical +substances, especially of emetine, which is usually recorded as present +in about 1%, but which will undoubtedly be found in larger quantities in +some, and in smaller quantities in other elementary species. In this way +the close and careful distinction of the really existing units might +perhaps prove of practical importance. + +MacFarlane has studied the beach-plum or _Prunus maritima_, which is +abundant along the coast regions of the Eastern States from Virginia to +New Brunswick. It often covers areas from two to two hundred acres in +extent, sometimes to the exclusion of other plants. It is most prolific +on soft drifting sand near the sea or along the shore, where it may at +times be washed with ocean-spray. The fruit usually become ripe about +the middle of August, and show extreme [57] variations in size, shape, +color, taste, consistency and maturation period, indicating the +existence of separate races or elementary species, with widely differing +qualities. The earlier varieties begin to ripen from August 10 to 20, +and a continuous supply can be had till September 10, while a few good +varieties continue to ripen till September 20. But even late in October +some other types are still found maturing their fruits. + +Exact studies were made of fruit and stone variations, and their +characteristics as to color, weight, size, shape and consistency were +fully described. Similar variations have been observed, as is well +known, in the cultivated plums. Fine blue-black fruits were seen on some +shrubs and purplish or yellow fruits on others. Some exhibit a firmer +texture and others a more watery pulp. Even the stones show differences +which are suggestive of distinct races. + +Recently Mr. Luther Burbank of Santa Rosa, California, has made use of +the beach-plum to produce useful new varieties. He observed that it is a +very hardy species, and never fails to bear, growing under the most +trying conditions of dry and sandy, or of rocky and even of heavy soil. +The fruits of the wild shrubs are utterly worthless for anything but +preserving. [58] But by means of crossing with other species and +especially with the Japanese plums, the hardy qualities of the +beach-plum have been united with the size, flavor and other valuable +qualities of the fruit, and a group of new plums have been produced with +bright colors, ovoid and globular forms which are never flattened and +have no suture. The experiments were not finished, when I visited Mr. +Burbank in July, 1904, and still more startling improvements were said +to have been secured. + +I may perhaps be allowed to avail myself of this opportunity to point +out a practical side of the study of elementary species. This always +appears whenever wild plants are subjected to cultivation, either in +order to reproduce them as pure strains, or to cross them with other +already cultivated species. The latter practice is as a rule made use of +whenever a wild species is found to be in possession of some quality +which is considered as desirable for the cultivated forms. In the case +of the beach-plum it is the hardiness and the great abundance of fruits +of the wild species which might profitably be combined with the +recognized qualities of the ordinary plums. Now it is manifest, that in +order to make crosses, distinct individual plants are to be chosen, and +that the variability of the wild species may be of very great +importance. [59] Among the range of elementary species those should be +used which not only possess the desired advantages in the highest +degree, but which promise the best results in other respects or their +earliest attainment. The fuller our knowledge of the elementary species +constituting the systematic groups, the easier and the more reliable +will be the choice for the breeder. Many Californian wild flowers with +bright colors seem to consist of large numbers of constant elementary +forms, as for instance, the lilies, godetias, eschscholtias and others. +They have been brought into cultivation many times, but the minutest +distinction of their elementary forms is required to attain the highest +success. + +In concluding, I will point out a very interesting difficulty, which in +some cases impedes the clear understanding of elementary species. It is +the lack of self-fertilization. It occurs in widely distant families, +but has a special interest for us in two genera, which are generally +known as very polymorphous groups. + +One of them is the hawkweed or _Hieracium_, and the other is the +dandelion or _Taraxacum officinale_. Hawkweeds are known as a genus in +which the delimitation of the species is almost impossible, Thousands of +forms may be cultivated side by side in botanical gardens, exhibiting +[60] slight but undoubted differentiating features, and reproduce +themselves truly by seed. Descriptions were formerly difficult and so +complicated that the ablest writers on this genus, Fries and Nageli are +said not to have been able to recognize the separate species by the +descriptions given by each other. Are these types to be considered as +elementary species, or only as individual differences? The decision of +course, would depend upon their behavior in cultures. Such tests have +been made by various experimenters. In the dandelion the bracts of the +involucre give the best characters. The inner ones may be linear or +linear-lanceolate, with or without appendages below the tip; the outer +ones may be similar and only shorter, or noticeably larger, erect, +spreading or even reflexed, and the color of the involucre may be a pure +green or glaucous; the leaves may be nearly entire or pinnatifid, or +sinuate-dentate, or very deeply runcinate-pinnatifid, or even pinnately +divided, the whole plant being more or less glabrous. + +Raunkiaer, who has studied experimentally a dozen types from Denmark, +found them constant, but observed that some of them have no pollen at +all, while in others the pollen, though present, is impotent. It does +not germinate on the stigma, cannot produce the ordinary tube, [61] and +hence has no fertilizing power. But the young ovaries do not need such +fertilization. They are sufficient unto themselves. One may cut off all +the flowers of a head before the opening of the anthers, and leave the +ovaries untouched, and the head will ripen its seeds quite as well. The +same thing occurs in the hawkweeds. Here, therefore, we have no +fertilization and the extensive widening of the variability, which +generally accompanies this process is, of course, wanting. Only partial +or vegetative variability is present. Unfertilized eggs when developing +into embryos are equivalent to buds, separated from the parent-plant and +planted for themselves. They repeat both the specific and the individual +characters of the parent. In the case of the hawkweed and the dandelion +there is at present no means of distinguishing between these two +contrasting causes of variability. But like the garden varieties which +are always propagated in the vegetative way, their constancy and +uniformity are only apparent and afford no real indication of hereditary +qualities. + +In addition to these and other exceptional cases, seed-cultures are +henceforth to be considered as the sole means of recognizing the really +existing systematic units of nature. All other groups, including +systematic species and [62] genera, are equally artificial or +conventional. In other words we may state "that current misconceptions +as to the extreme range of fluctuating variability of many native +species have generally arisen from a failure to recognize the composite +nature of the forms in question," as has been demonstrated by MacDougal +in the case of the common evening-primrose, _Oenothera biennis_. "It is +evident that to study the behavior of the characters of plants we must +have them in their simplest combinations; to investigate the origin and +movements of species we must deal with them singly and uncomplicated." + + +[63] + +LECTURE III + +ELEMENTARY SPECIES OF CULTIVATED PLANTS + +Recalling the results of the last lecture, we see that the species of +the systematists are not in reality units, though in the ordinary course +of floristic studies they may, as a rule, seem to be so. In some cases +representatives of the same species from different countries or regions, +when compared with one another do not exactly agree. Many species of +ferns afford instances of this rule, and Lindley and other great +systematists have frequently been puzzled by the wide range of +differences between the individuals of a single species. + +In other cases the differing forms are observed to grow near each other, +sometimes in neighboring provinces, sometimes in the same locality, +growing and flowering in mixtures of two or three or even more +elementary types. The violets exhibit widespread ancient types, from +which the local species may be taken to have arisen. The common +ancestors of the Whitlow-grasses are probably not to be found [64] among +existing forms, but numerous types are crowded together in the southern +part of central Europe and more thinly scattered elsewhere, even as far +as western Asia. There can be little doubt that their common origin is +to be sought in the center of their geographic distribution. + +Numerous other cases exhibit smaller numbers of elementary units within +a systematic species; in fact purely uniform species seem to be +relatively rare. But with small numbers there are of course no +indications to be expected concerning their common origin or the +starting point of their distribution. + +It is manifest that these experiences with wild species must find a +parallel among cultivated plants. Of course cultivated plants were +originally wild and must have come under the general law. Hence we may +conclude that when first observed and taken up by man, they must already +have consisted of sundry elementary subspecies. And we may confidently +assert that some must have been rich and others poor in such types. + +Granting this state of things as the only probable one, we can easily +imagine what must have been the consequences. If a wild species had been +taken into cultivation only once, the cultivated form would have been a +single elementary [65] type. But it is not very likely that such +partiality would occur often. The conception that different tribes at +different times and in distant countries would have used the wild plants +of their native regions seems far more natural than that all should have +obtained plants for cultivation from the same source or locality. If +this theory may be relied upon, the origin of many of the more widely +cultivated agricultural plants must have been multiple, and the number +of the original elementary species of the cultivated types must have +been so much the larger, the more widely distributed and variable the +plants under consideration were before the first period of cultivation. + +Further it would seem only natural to explain the wide variability of +many of our larger agricultural and horticultural stocks by such an +incipient multiformity of the species themselves. Through commercial +intercourse the various types might have become mixed so as to make it +quite impossible to point out the native localities for each of them. + +Unfortunately historical evidence on this point is almost wholly +lacking. The differences in question could not have been appreciated at +that remote period, and interest the common observer but little even +today. The history of most of the cultivated plants is very obscure, +[66] and even the most skillful historians, by sifting the evidence +afforded by the older writers, and that obtained by comparative +linguistic investigations have been able to do little more than frame +the most general outline of the cultural history of the most common and +most widely used plants. + +Some authors assume that cultivation itself might have been the +principal cause of variability, but it is not proved, nor even probable, +that cultivated plants are intrinsically more variable than their wild +prototypes. Appearances in this case are very deceptive. Of course +widely distributed plants are as a rule richer in subspecies than forms +with limited distribution, and the former must have had a better chance +to be taken into cultivation than the latter. In many cases, especially +with the more recent cultivated species, man has deliberately chosen +variable forms, because of their greater promise. Thirdly, wide +variability is the most efficient means of acclimatization, and only +species with many elementary units would have offered the adequate +material for introduction into new countries. + +From this discussion it would seem that it is more reasonable to assert +that variability is one of the causes of the success of cultivation, +than to assume that cultivation is a cause of variability [67] at large. +And this assumption would be equally sufficient to explain the existing +conditions among cultivated plants. + +Of course I do not pretend to say that cultivated plants should be +expected to be less variable than in the wild state, or that swarms of +elementary species might not be produced during cultivation quite as +well as before. However the chance of such an event, as is easily seen, +cannot be very great, and we shall have to be content with a few +examples of which the coconut is a notable one. + +Leaving this general discussion of the subject, we may take up the +example of the beets. The sugar-beet is only one type from among a horde +of others, and though the origin of all the single types is not +historically known, the plant is frequently found in the wild state even +at the present time, and the native types may be compared with the +corresponding cultivated varieties. + +The cultivation of beets for sugar is not of very ancient date. The +Romans knew the beets and used them as vegetables, both the roots and +the leaves. They distinguished a variety with white and one with red +flesh, but whether they cultivated them, or only collected them from +where they grew spontaneously, appears to be unknown. + +[68] Beets are even now found in large quantities along the shores of +Italy. They prefer the vicinity of the sea, as do so many other members +of the beet family, and are not limited to Italy, but are found growing +elsewhere on the littoral of the Mediterranean, in the Canary Islands +and through Persia and Babylonia to India. In most of their native +localities they occur in great abundance. + +The color of the foliage and the size of the roots are extremely +variable. Some have red leafstalks and veins, others a uniform red or +green foliage, some have red or white or yellow roots, or exhibit +alternating rings of a red and of a white tinge on cut surfaces. It +seems only natural to consider the white and the red, and even the +variegated types as distinct varieties, which in nature do not +transgress their limits nor change into one another. In a subsequent +lecture I will show that this at least is the rule with the +corresponding color-varieties in other genera. + +The fleshiness or pulpiness of the roots is still more variable. Some +are as thick as the arm and edible, others are not thicker than a finger +and of a woody composition, and the structure of this woody variety is +very interesting. The sugar-beet consists, as is generally known, of +concentric layers of sugar-tissue and of vascular [69] strands; the +larger the first and the smaller the latter, the greater is, as a rule, +the average amount of sugar of the race. Through the kindness of the +late Mr. Rimpau, a well known German breeder of sugar-beet varieties, I +obtained specimens from seed of a native wild locality near Bukharest. +The plants produced quite woody roots, showing almost no sugar tissue at +all. Woody layers of strongly developed fibrovascular strands were seen +to be separated one from another only by very thin layers of +parenchymatous cells. Even the number of layers is variable; it was +observed to be five in my plants; but in larger roots double this number +and even more may easily be met with. + +Some authors have distinguished specific types among these wild forms. +While the cultivated beets are collected under the head of _Beta +vulgaris_, separate types with more or less woody roots have been +described as _Beta maritima_ and _Beta patula_. These show differences +in the habit of the stems and the foliage. Some have a strong tendency +to become annual, others to become biennial. The first of course do not +store a large quantity of food in their roots, and remain thin, even at +the time of flowering. The biennial types occur in all sizes of roots. +In the annuals the stems may vary from [70] erect to ascending, and the +name _patula_ indicates stems which are densely branching from the base +with widely spreading branches throughout. Mr. Em. von Proskowetz of +Kwassitz, Austria, kindly sent me seeds of this _Beta patula_, the +variability of which was so great in my cultures as to range from nearly +typical sugar-beets to the thin woody type of Bukharest. + +Broad and narrow leaves are considered to be differentiating marks +between _Beta vulgaris_ and _Beta patula_, but even here a wide range of +forms seem to occur. + +Rimpau, Proskowetz, Schindler and others have made cultures of beets +from wild localities in order to discover a hypothetical common ancestor +of all the present cultivated types. These researches point to the _B. +patula_ as the probable ancestor, but of course they were not made to +decide the question as to whether the origination of the several now +existing types had taken place before or during culture. From a general +point of view the variability of the wild species is parallel to that of +the cultivated forms to such a degree as to suggest the multiple origin +of the former. But a close investigation of this highly important +problem has still to be made. + +The varieties of the cultivated beets are commonly [71] included in four +subspecies. The two smallest are the salad-beets and the ornamental +forms, the first being used as food, and ordinarily cultivated in red +varieties, the second being used as ornamental plants during the fall, +when they fill the beds left empty by summer flowers, with a bright +foliage that is exceedingly rich in form and color. Of the remaining +subspecies, one comprises the numerous sorts cultivated as forage-crops +and the other the true sugar-beets. Both of them vary widely as to the +shape and the size of the roots, the quality of the tissue, the foliage +and other characteristics. + +Some of these forms, no doubt, have originated during culture. Most of +them have been improved by selection, and no beet found in the wild +state ever rivals any cultivated variety. But the improvement chiefly +affects the size, the amount of sugar and nutrient substances and some +other qualities which recur in most of the varieties. The varietal +attributes themselves however, are more or less of a specific nature, +and have no relation to the real industrial value of the race. The +short-rooted and the horn-shaped varieties might best be cited as +examples. + +The assertion that the sundry varieties of forage-beets are not the +result of artificial selection, [72] is supported in a large measure by +the historic fact that the most of them are far older than the method of +conscious selection of plants itself. This method is due to Louis +Vilmorin and dates from the middle of the last century. But in the +sixteenth century most of our present varieties of beets were already in +cultivation. Caspar Bauhin gives a list of the beets of his time and it +is not difficult to recognize in it a large series of subspecies and +varieties and even of special forms, which are still cultivated. A more +complete list was published towards the close of the same century by +Olivier de Serres in his world-renowned "Theatre d'Agriculture" (Paris, +1600). + +The red forage-beets which are now cultivated on so large a scale, had +been introduced from Italy into France only a short time before. + +From this historic evidence, the period during which the beets were +cultivated from the time of the Romans or perhaps much later, up to the +time of Bauhin and De Serres, would seem far too short for the +production by the unguided selection of man of all the now existing +types. On the other hand, the parallelism between the characters of some +wild and some cultivated varieties goes to make it very probable that +other varieties have been found in the same way, some in this country +and others in that, [73] and have been taken into cultivation +separately. Afterwards of course all must have been improved in the +direction required by the needs of man. + +Quite the same conclusion is afforded by apples. The facts are to some +extent of another character, and the rule of the derivation of the +present cultivated varieties from original wild forms can be illustrated +in this case in a more direct way. Of course we must limit ourselves to +the varieties of pure ancestry and leave aside all those which are of +hybrid or presumably hybrid origin. + +Before considering their present state of culture, something must be, +said about the earlier history and the wild state of the apples. + +The apple-tree is a common shrub in woods throughout all parts of +Europe, with the only exception of the extreme north. Its distribution +extends to Anatolia, the Caucasus and Ghilan in Persia. It is found in +nearly all forests of any extent and often in relatively large numbers +of individuals. It exhibits varietal characters, which have led to the +recognition of several spontaneous forms, especially in France and in +Germany. + +The differentiating qualities relate to the shape and indumentum of the +leaves. Nothing is known botanically as to differences between [74] the +fruits of these varieties, but as a matter of fact the wild apples of +different countries are not at all the same. + +Alphonse De Candolle, who made a profound study of the probable origin +of most of our cultivated plants, comes to the conclusion that the apple +tree must have had this wide distribution in prehistoric times, and that +its cultivation began in ancient times everywhere. + +This very important conclusion by so high an authority throws +considerable light on the relation between cultivated and wild varieties +at large. If the historic facts go to prove a multiple origin for the +cultivation of some of the more important useful plants, the probability +that different varieties or elementary species have been the starting +points for different lines of culture, evidently becomes stronger. + +Unfortunately, this historic evidence is scanty. The most interesting +facts are those concerning the use of apples by the Romans and by their +contemporaries of the Swiss and middle European lake-dwellings. Oswald +Heer has collected large numbers of the relics of this prehistoric +period. Apples were found in large quantities, ordinarily cut into +halves and with the signs of having been dried. Heer distinguished two +varieties, one with large and one with small fruits. The first about 3 +and [75] the other about 1.5-2 cm. in diameter. Both are therefore very +small compared with our present ordinary varieties, but of the same +general size as the wild forms of the present day. Like these, they must +have been of a more woody and less fleshy tissue. They would scarcely +have been tasteful to us, but in ancient times no better varieties were +known and therefore no comparison was possible. + +There is no evidence concerning the question, as to whether during the +periods mentioned apples were cultivated or only collected in the wild +state. The very large numbers which are found, have induced some writers +to believe in their culture, but then there is no reason why they should +not have been collected in quantity from wild shrubs. The main fact is +that the apple was not a uniform species in prehistoric times but showed +even then at least some amount of variability. + +At the present day the wild apples are very rich in elementary species. +Those of Versailles are not the same as those of Belgium, and still +others are growing in England and in Germany. The botanical differences +derived from the blossoms and the leaves are slight, but the flavor, +size and shape of the fruits diverge widely. Two opinions have been +advanced to explain this high degree of variability, but [76] neither of +them conveys a real explanation; their aim is chiefly to support +different views as to the causes of variability, and the origin of +elementary species at large. + +One opinion, advocated by De Candolle, Darwin and others, claims that +the varieties owe their origin to the direct influence of cultivation, +and that the corresponding forms found in the wild state, are not at all +original, but have escaped from cultivation and apparently become wild. +Of course this possibility cannot be denied, at least in any single +instance, but it seems too sweeping an assertion to make for the whole +range of observed forms. + +The alternative theory is that of van Mons, the Belgian originator of +commercial varieties of apples, who has published his experiments in a +large work called "Arbres fruitiers ou Pomonomie belge." Most of the +more remarkable apples of the first half of the last century were +produced by van Mons, but his greatest merit is not the direct +production of a number of good varieties, but the foundation of the +method, by which new varieties may be obtained and improved. + +According to van Mons, the production of a new variety consists chiefly +of two parts. The first is the discovery of a subspecies with new +desirable qualities. The second is the transformation [77] of the +original small and woody apple into a large, fleshy and palatable +variety. Subspecies, or what we now call elementary species were not +produced by man; nature alone creates new forms, as van Mons has it. He +examined with great care the wild apples of his country, and especially +those of the Ardennes, and found among them a number of species with +different flavors. For the flavor is the one great point, which must be +found ready in nature and which may be improved, but can never be +created by artificial selection. The numerous differences in flavor are +quite original; all of them may be found in the wild state and most of +them even in so limited a region as the Ardennes Mountains. Of course +van Mons preferred not to start from the wild types themselves, when the +same flavor could be met with in some cultivated variety. His general +method was, to search for a new flavor and to try to bring the bearer of +it up to the desired standard of size and edibility. + +The latter improvement, though it always makes the impression of an +achievement, is only the last stone to be added to the building up of +the commercial value of the variety. Without it, the best flavored apple +remains a crab; with it, it becomes a conquest. According to the method +of van Mons it may be reached within [78] two or three generations, and +a man's life is wholly sufficient to produce in this way many new types +of the very best sorts, as van Mons himself has done. It is done in the +usual way, sowing on a large scale and selecting the best, which are in +their turn brought to an early maturation of their fruit by grafting, +because thereby the life from seed to seed may be reduced to a few +years. + +Form, taste, color, flavor and other valuable marks of new varieties are +the products of nature, says van Mons, only texture, fleshiness and size +are added by man. And this is done in each new variety by the same +method and according to the same laws. The richness of the cultivated +apples of the present day was already present in the large range of +original wild elementary species, though unobserved and requiring +improvement. + +An interesting proof of this principle is afforded by the experience of +Mr. Peter M. Gideon, as related by Bailey. Gideon sowed large quantities +of apple-seeds, and one seed produced a new and valuable variety called +by him the "Wealthy" apple. He first planted a bushel of apple-seeds, +and then every year, for nine years, planted enough seeds to produce a +thousand trees. At the end of ten years all seedlings had perished +except one hardy seedling [79] crab. This experiment was made in +Minnesota, and failed wholly. Then he bought a small lot of seeds of +apples and crab-apples in Maine and from these the "Wealthy" came. There +were only about fifty seeds in the lot of crab-apple seed which produced +the "Wealthy," but before this variety was obtained, more than a bushel +of seed had been sown. Chance afforded a species with an unknown taste; +but the growing of many thousands of seedlings of known varieties was +not the best means to get something really new. + +Pears are more difficult to improve than apples. They often require six +or more generations to be brought from the wild woody state to the +ordinary edible condition. But the varieties each seem to have a +separate origin, as with apples, and the wide range of form and of taste +must have been present in the wild state, long before cultivation. Only +recently has the improvement of cherries, plums, currants and +gooseberries been undertaken with success by Mr. Burbank, and the +difference between the wild and cultivated forms has hitherto been very +small. All indications point to the existence, before the era of +cultivation, of larger or smaller numbers of elementary species. + +The same holds good with many of the larger forage crops and other +plants of great industrial [80] value. Clover exhibits many varieties, +which have been cultivated indiscriminately, and often in motley +mixtures. The flower heads may be red or white, large or small, +cylindric or rounded, the leaves are broader or narrower, with or +without white spots of a curious pattern. They may be more or less hairy +and so forth. Even the seeds exhibit differences in size, shape or +color, and of late Martinet has shown, that by the simple means of +picking out seeds of the same pattern, pure strains of clover may be +obtained, which are of varying cultural value. In this way the best +subspecies or varieties may be sought out for separate cultivation. Even +the white spots on the leaflets have proved to be constant characters +corresponding with noticeable differences in yield. + +Flax is another instance. It was already cultivated, or at least made +use of during the period of the lake-dwellers, but at that time it was a +species referred to as _Linum angustifolium_, and not the _Linum +usitatissimum_, which is our present day flax. There are now many +subspecies, elementary species, and varieties under cultivation. The +oldest of them is known as the "springing flax," in opposition to the +ordinary "threshing flax." It has capsules which open of themselves, in +order to disseminate the seeds, while the ordinary heads of the [81] +flax remain closed until the seeds are liberated by threshing. It seems +probable that the first form or _Linum crepitans_ might thrive in the +wild state as well as any other plant, while in the common species those +qualities are lacking which are required for a normal dissemination of +the seeds. White or blue flowers, high or dwarf stems, more or less +branching at the base and sundry other qualities distinguish the +varieties, aside from the special industrial difference of the fibres. +Even the life-history varies from annual and biennial, to perennial. + +It would take us too long to consider other instances. It is well known +that corn, though considered as a single botanical species, is +represented by different subspecies and varieties in nearly every region +in which it is grown. Of course its history is unknown and it is +impossible to decide whether all the tall and dwarf forms, or starchy +and sweet varieties, dented or rounded kernels, and hundreds of others +are older than culture or have come into existence during historic +times, or as some assume, through the agency of man. But our main point +now is not the origin, but only the existence of constant and sharply +differentiated forms within botanical species. Nearly every cultivated +plant affords instances of such diversity. Some include a few types +only, while [82] others show, a large number of forms clearly separated +to a greater or lesser degree. + +In some few instances it is obvious that this variability is of later +date than culture. The most conspicuous case is that of the coconut. +This valuable palm is found on nearly all tropical coasts, in America, +as well as in Asia, but in Africa and Australia there are many hundreds +of miles of shore line, where it is not found. Its importance is not at +all the same everywhere. On the shores and islands of the Indian Ocean +and the Malay Archipelago, man is chiefly dependent upon it, but in +America it is only of subordinate usefulness. + +In connection with these facts, it abounds in subspecies and varieties +in the East Indian regions, but on the continent of America little +attention has as yet been given to its diverging qualities. In the +Malayan region it affords nearly all that is required by the +inhabitants. The value of its fruit as food, and the delicious beverage +which it yields, are well known. The fibrous rind is not less useful; it +is manufactured into a kind of cordage, mats and floor-cloths. An +excellent oil is obtained from the kernel by compression. The hard +covering of the stem is converted into drums and used in the +construction of huts; the lower part is so hard as to take on a +beautiful polish [83] when it resembles agate. Finally the unexpanded +terminal bud is a delicate article of food. Many other uses could be +mentioned, but these may suffice to indicate how closely the life of the +inhabitants is bound up with the culture of this palm, and how sharply, +in consequence, its qualities must have been watched by early man. Any +divergence from the ordinary type must have been noted; those which were +injurious must have been rejected, but the useful ones must have been +appreciated and propagated. In a word any degree of variability afforded +by nature must have been noticed and cultivated. + +More than fifty different sorts of the coconut are described from the +Indian shores and islands, with distinct local and botanical names. +Miquel, who was one of the best systematists of tropical plants, of the +last century, described a large number of them, and since, more have +been added. Nearly all useful qualities vary in a higher or lesser +degree in the different varieties. The fibrous strands of the rind of +the nut are developed in some forms to such a length and strength as to +yield the industrial product known as the coir-fibre. Only three of them +are mentioned by Miquel that have this quality, the _Cocos nucifera +rutila_, _cupuliformis_ and _stupposa_. Among them the _rutila_ [84] +yields the best and most supple fibres, while those of the _stupposa_ +are stiff and almost unbending. + +The varieties also differ greatly in size, color, shape and quality, and +the trees have also peculiar characteristics. One variety exhibits +leaves which are nearly entire, the divisions being only imperfectly +separated, as often occurs in the very first leaves of the seedlings of +other varieties. The flavor of the flesh, oil and milk likewise yield +many good varietal marks. + +In short, the coconut-palm comes under the general rule, that botanical +species are built up of a number of sharply distinguishable types, which +prove their constancy and relative independence by their wide +distribution in culture. In systematic works all these forms are called +varieties, and a closer investigation of their real systematic value has +not yet been made. But the question as to the origin of the varieties +and of the coconut itself has engrossed the attention of many botanists, +among whom are De Candolle in the middle of the last century, and Cook +at its close. + +Both questions are closely connected. De Candolle claimed an Asiatic +origin for the whole species, while Cook's studies go to prove that its +original habitat is to be sought in the northern countries of South +America. Numerous [85] varieties are growing in Asia and have as yet not +been observed to occur in America, where the coconut is only of +subordinate importance, being one of many useful plants, and not the +only one relied upon by the natives for their subsistence. If therefore, +De Candolle's opinion is the right one, the question as to whether the +varieties are older or younger than the cultivated forms of the species, +must always remain obscure. But if the proofs of an American origin +should be forthcoming, the possibility, and even the probability that +the varieties are of later date than the beginning of their culture, and +have originated while in this condition must at once be granted. An +important point in the controversy is the manner in which the coconuts +were disseminated from shore to shore, from island to island. De +Candolle, Darwin and most of the European writers claim that the +dispersal was by natural agencies, such as ocean-currents. They point +out that the fibrous rind or husk would keep the fruits afloat, and +uninjured, for many days or even many weeks, while being carried from +one country to another in a manner that would explain their geographic +distribution. But the probability of the nuts being thrown upon the +strand, and far enough from the shore to find suitable conditions for +their germination, is a very small one. To insure [86] healthy and +vigorous seedlings the nuts must be fully ripe, after which planting +cannot be safely delayed for more than a few weeks. If kept too moist +the nuts rot. If once on the shore, and allowed to lie in the sun, they +become overheated and are thereby destroyed; if thrown in the shade of +other shrubs and trees, the seedlings do not find the required +conditions for a vigorous growth. + +Some authors have taken the fibrous rind to be especially adapted to +transport by sea, but if this were so, this would argue that water is +the normal or at least the very frequent medium of dissemination, which +of course it is not. We may, claim with quite as much right that the +thick husk is necessary to enable the heavy fruit to drop from tall +trees with safety. But even for this purpose the protection is not +sufficient, as the nuts often suffer from falling to such a degree as to +be badly injured as to their germinating qualities. It is well known +that nuts, which are destined for propagation, are as a rule not allowed +to fall off, but are taken from the trees with great care. + +Summing up his arguments, Cook concludes that there is little in the way +of known facts to support the poetic theory of the coconut palm dropping +its fruits into the sea to float away to barren islands and prepare them +for [87] human habitation. Shipwrecks might furnish a successful method +of launching viable coconuts, and such have no doubt sometimes +contributed to their distribution. But this assumption implies a +dissemination of the nuts by man, and if this principal fact is granted, +it is far more natural to believe in a conscious intelligent +dissemination. + +The coconut is a cultivated tree. It may be met with in some spots +distant from human dwellings, but whenever such cases have been +subjected to a closer scrutiny, it appears that evidently, or at least +probably, huts had formerly existed in their neighborhood, but having +been destroyed by some accident, had left the palm trees uninjured. Even +in South America, where it may be found in forests at great distances +from the sea-shore, it is not at all certain that true native localities +occur, and it seems to be quite lost in its natural condition. + +Granting the cultivated state of the palms as the only really important +one, and considering the impossibility or at least great improbability +of its dissemination by natural means, the distribution by man himself, +according to his wants, assumes the rank of an hypothesis fully adequate +to the explanation of all the facts concerning the life-history of the +tree. + +We now have to inquire into the main question, [88] whether it is +probable that the coconut is of American or of Asiatic origin, leaving +aside the historic evidence which goes to prove that nothing is known +about the period in which its dissemination from one hemisphere to +another took place, we will now consider only the botanic and geographic +evidence, brought forward by Cook. He states that the whole family of +coconut-palms, consisting of about 20 genera and 200 species, are all +strictly American with the exception of the rather aberrant African +oilpalm, which has, however, an American relative referred to the same +genus. The coconut is the sole representative of this group which is +connected with Asia and the Malayan region, but there is no manifest +reason why other members of the same group could not have established +themselves there, and maintained an existence under conditions, which +are not at all unfavorable to them. The only obvious reason is the +assumption already made, that the distribution was brought about by man, +and thus only affected the species, chosen by him for cultivation. That +the coconut cannot have been imported from Asia into America seems to be +the most obvious conclusion from the arguments given. It should be +briefly noted, that it was known and widely distributed in tropical +America at the time of the discovery of that continent [89] by Columbus, +according to accounts of Oviedo and other contemporary Spanish writers. + +Concluding we may state that according to the whole evidence as it has +been discussed by De Candolle and especially by Cook, the coconut-palm +is of American origin and has been distributed as a cultivated tree by +man through the whole of its wide range. This must have happened in a +prehistoric era, thus affording time enough for the subsequent +development of the fifty and more known varieties. But the possibility +that at least some of them have originated before culture and have been +deliberately chosen by man for distribution, of course remains +unsettled. + +Coconuts are not very well adapted for natural dispersal on land, and +this would rather induce us to suppose an origin within the period of +cultivation for the whole group. There are a large number of cultivated +varieties of different species which by some peculiarity do not seem +adapted for the conditions of life in the wild state. These last have +often been used to prove the origin of varietal forms during culture. +One of the oldest instances is the variety or rather subspecies of the +opium-poppy, which lacks the ability to burst open its capsules. The +seeds, which are thrown out by the wind, in the common forms, through +the apertures underneath [90] the stigma, remain enclosed. This is +manifestly a very useful adaptation for a cultivated plant, as by this +means no seeds are lost. It would be quite a disadvantage for a wild +species, and is therefore claimed to have been connected from the +beginning with the cultivated form. + +The large kernels of corn and grain, of beans and peas, and even of the +lupines were considered by Darwin and others to be unable to cope with +natural conditions of life. Many valuable fruits are quite sterile, or +produce extremely few seeds. This is notoriously the case with some of +the best pears and grapes, with the pine-apples, bananas, bread-fruits, +pomegranate and some members of the orange tribe. It is open to +discussion as to what may be the immediate cause of this sterility, but +it is quite evident, that all such sterile varieties must have +originated in a cultivated condition. Otherwise they would surely have +been lost. + +In horticulture and agriculture the fact that new varieties arise from +time to time is beyond all doubt, and it is not this question with which +we are now concerned. Our arguments were only intended to prove that +cultivated species, as a rule, are derived from wild species, which obey +the laws discussed in a previous lecture. The botanic units are compound +entities, and [91] the real systematic units in elementary species play +the same part as in ordinary wild species. The inference that the origin +of the cultivated plants is multiple, in most cases, and that more than +one, often many separate elementary forms of the same species must +originally have been taken into cultivation, throws much light upon many +highly important problems of cultivation and selection. This aspect of +the question will therefore be the subject of the next lecture. + + +[92] + +LECTURE IV + +SELECTION OF ELEMENTARY SPECIES + +The improvement of cultivated plants must obviously begin with already +existing forms. This is true of old cultivated sorts as well as for +recent introductions. In either case the starting-point is as important +as the improvement, or rather the results depend in a far higher degree +on the adequate choice of the initial material than on the methodical +and careful treatment of the chosen varieties. This however, has not +always been appreciated as it deserves, nor is its importance at present +universally recognized. The method of selecting plants for the +improvement of the race was discovered by Louis Vilmorin about the +middle of the last century. Before his time selection was applied to +domestic animals, but Vilmorin was the first to apply this principle to +plants. As is well known, he used this method to increase the amount of +sugar in beets and thus to raise their value as forage-crops, with such +success, that his plants have since been used for the production [93] of +sugar. He must have made some choice among the numerous available sorts +of beets, or chance must have placed in his hands one of the most +appropriate forms. On this point however, no evidence is at hand. + +Since the work of Vilmorin the selection-principle has increased +enormously in importance, for practical purposes as well as for the +theoretical aspect of the subject. It is now being applied on a large +scale to nearly all ornamental plants. It is the one great principle now +in universal practice as well as one of preeminent scientific value. Of +course, the main arguments of the evolution theory rest upon +morphologic, systematic, geographic and paleontologic evidence. But the +question as to how we can coordinate the relation between existing +species and their supposed ancestors is of course one of a physiologic +nature. Direct observation or experiments were not available for Darwin +and so he found himself constrained to make use of the experience of +breeders. This he did on a broad scale, and with such success that it +was precisely this side of his arguments that played the major part in +convincing his contemporaries. + +The work of the breeders previous to Darwin's time had not been very +critically performed. Recent analyses of the evidence obtained [94] from +them show that numerous types of variability were usually thrown +together. What type in each case afforded the material, which the +breeder in reality made use of, has only been inquired into in the last +few decades. Among those who have opened the way for thorough and more +scientific treatment are to be mentioned Rimpau and Von Rumker of +Germany and W.M. Hays of America. + +Von Rumker is to be considered as the first writer, who sharply +distinguished between two phases of methodical breeding-selection. One +side he calls the production of new forms, the other the improvement of +the breed. He dealt with both methods extensively. New forms are +considered as spontaneous variations occurring or originating without +human aid. They have only to be selected and isolated, and their progeny +at once yields a constant and pure race. This race retains its character +as long as it is protected against the admixture of other minor +varieties, either by cross-pollination, or by accidental seeds. + +Improvement, on the other hand, is the work of man. New varieties of +course can only be isolated if chance offers them; the improvement is +not incumbent on chance. It does not create really anything new, but +develops characters, which were already existing. It brings [95] the +race above its average, and must guard constantly against the regression +towards this average which usually takes place. + +Hays has repeatedly insisted upon the principle of the choice of the +most favorable varieties as the foundation for all experiments in +improving races. He asserts that half the battle is won by choosing the +variety which is to serve as a foundation stock, while the other half +depends upon the selection of parent-plants within the chosen variety. +Thus the choice of the variety is the first principle to be applied in +every single case; the so-called artificial selection takes only a +secondary place. Calling all minor units within the botanic species by +the common name of varieties, without regard to the distinction between +elementary species and retrograde varieties, the principle is designated +by the term of "variety-testing." This testing of varieties is now, as +is universally known, one of the most important lines of work of the +agricultural experiment stations. Every state and every region, in some +instances even the larger farms, require a separate variety of corn, or +wheat, or other crops. They must be segregated from among the hundreds +of generally cultivated forms, within each single botanic species. Once +found, the type may be ameliorated according to the local conditions +[96] and needs, and this is a question of improvement. + +The fact that our cultivated plants are commonly mixtures of different +sorts, has not always been known. The first to recognize it seems to +have been the Spanish professor of botany, Mariano Lagasca, who +published a number of Spanish papers dealing with useful plants and +botanical subjects between 1810 and 1830, among them a catalogue of +plants cultivated in the Madrid Botanical Garden. Once when he was on a +visit to Colonel Le Couteur on his farm in Jersey, one of the Channel +Islands off the coast of France, in discussing the value of the fields +of wheat, he pointed out to his host, that they were not really pure and +uniform, as was thought at that time, and suggested the idea that some +of the constituents might form a larger part in the harvest than others. +In a single field he succeeded in distinguishing no less than 23 +varieties, all growing together. Colonel Le Couteur took the hint, and +saved the seeds of a single plant of each supposed variety separately. +These he cultivated and multiplied till he got large lots of each and +could compare their value. From among them he then chose the variety +producing the greatest amount of the finest, whitest and most nutritious +flour. This he eventually placed in the [97] market under the name of +"Talavera de Bellevue." It is a tall, white variety, with long and +slender white heads, almost without awns, and with fine white pointed +kernels. It was introduced into commerce about 1830, and is still one of +the most generally cultivated French wheats. It was highly prized in the +magnificent collection of drawings and descriptions of wheats, published +by Vilmorin under the title "Les meilleurs bles" and is said to have +quite a number of valuable qualities, branching freely and producing an +abundance of good grain and straw. It is however, sensitive to cold +winters in some degree and thereby limited in its distribution. Hallett, +the celebrated English wheat-breeder, tried in vain to improve the +peculiar qualities of this valuable production of Le Couteur's. + +Le Couteur worked during many years along this line, long before the +time when Vilmorin conceived the idea of improvement by race selections, +and he used only the simple principle of distinguishing and isolating +the members of his different fields. Later he published his results in a +work on the varieties, peculiarities and classification of wheat (1843), +which though now very rare, has been the basis and origin of the +principle of variety-testing. + +The discovery of Lagasca and Le Couteur was [98] of course not +applicable to the wheat of Jersey alone. The common cultivated sorts of +wheat and other grains were mixtures then as they are even now. Improved +varieties are, or at least should be, in most cases pure and uniform, +but ordinary sorts, as a rule, are mixtures. Wheat, barley and oats are +self-fertile and do not mix in the field through cross-pollination. +Every member of the assemblage propagates itself, and is only checked by +its own greater or less adaptation to the given conditions of life. +Rimpau has dealt at large with the phenomenon as it occurs in the +northern and middle parts of Germany. Even Rivett's "Bearded wheat," +which was introduced from England as a fine improved variety, and has +become widely distributed throughout Germany, cannot keep itself pure. +It is found mingled almost anywhere with the old local varieties, which +it was destined to supplant. Any lot of seed exhibits such impurities, +as I have had the opportunity of observing myself in sowings in the +experimental-garden. But the impurities are only mixtures, and all the +plants of Rivett's "Bearded wheat," which of course constitute the large +majority, are of pure blood. This may be confirmed when the seeds are +collected and sown separately in cultures that can be carefully guarded. + +[99] In order to get a closer insight into the causes of this confused +condition of ordinary races, Rimpau made some observations on Rivett's +wheat. He found that it suffers from frost during winter more than the +local German varieties, and that from various causes, alien seeds may +accidentally, and not rarely, become mixed with it. The +threshing-machines are not always as clean as they should be and may be +the cause of an accidental mixture. The manure comes from stables, where +straw and the dust from many varieties are thrown together, and +consequently living kernels may become mixed with the dung. Such stray +grains will easily germinate in the fields, where they find more +congenial conditions than does the improved variety. If winter arrives +and kills quantities of this latter, the accidental local races will +find ample space to develop. Once started, they will be able to multiply +so rapidly, that in one or two following generations they will +constitute a very considerable portion of the whole harvest. In this way +the awnless German wheat often prevails over the introduced English +variety, if the latter is not kept pure by continuous selection. + +The Swiss wheat-breeder Risler made an experiment which goes to prove +the certainty of the explanation given by Rimpau. He observed on his +farm at Saleves near the lake of Geneva that after a lapse of time the +"Galland wheat" deteriorated and assumed, as was generally believed, the +characters of the local sorts. In order to ascertain the real cause of +this apparent change, he sowed in alternate rows in a field, the +"Galland" and one of the local varieties. The "Galland" is a race with +obvious characters and was easily distinguished from the other at the +time when the heads were ripe. They are bearded when flowering, but +afterwards throw off the awns. The kernels are very large and yield an +extraordinarily good, white flour. + +During the first summer all the heads of the "Galland" rows had the +deciduous awns but the following year these were only seen on half of +the plants, the remainder having smooth heads, and the third year the +"Galland" had nearly disappeared, being supplanted by the competing +local race. The cause of this rapid change was found to be twofold. +First the "Galland," as an improved variety, suffers from the winter in +a far higher degree than the native Swiss sorts, and secondly it ripens +its kernels one or two weeks later. At the time of harvest it may not +have become fully ripe, while the varieties mixed with it had reached +maturity. The wild oat, _Avena fatua_, is very common in [101] Europe +from whence it has been introduced in the United States. In summers +which are unfavorable to the development of the cultivated oats it may +be observed to multiply with an almost incredible rapidity. It does not +contribute to the harvest, and is quite useless. If no selection were +made, or if selection were discontinued, it would readily supplant the +cultivated varieties. + +From these several observations and experiments it may be seen, that it +is not at all easy to keep the common varieties of cereals pure and that +even the best are subject to the encroachment of impurities. Hence it is +only natural that races of cereals, when cultivated without the utmost +care, or even when selected without an exact knowledge of their single +constituents, are always observed to be more or less in a mixed +condition. Here, as everywhere with cultivated and wild plants, the +systematic species consist of a number of minor types, which pertain to +different countries and climates, and are growing together in the same +climate and under the same external conditions. They do not mingle, nor +are their differentiating characters destroyed by intercrossing. They +each remain pure, and may be isolated whenever and wherever the +desirability for such a proceeding should arise. The purity of [102] the +races is a condition implanted in them by man, and nature always strives +against this arbitrary and one-sided improvement. Numerous slight +differences in characters and numerous external influences benefit the +minor types and bring them into competition with the better ones. +Sometimes they tend to supplant the latter wholly, but ordinarily sooner +or later a state of equilibrium is reached, in which henceforth the +different sorts may live together. Some are favored by warm and others +by cool summers, some are injured by hard winters while others thrive +then and are therefore relatively at an advantage. The mixed condition +is the rule, purity is the exception. + +Different sorts of cereals are not always easily distinguishable by the +layman and therefore I will draw your attention to conditions in +meadows, where a corresponding phenomenon can be observed in a much +simpler way. + +Only artificial pasture-grounds are seen to consist of a single species +of grass or clover. The natural condition in meadows is the occurrence +of clumps of grasses and some clovers, mixed up with perhaps twenty or +more species of other genera and families. The numerical proportion of +these constituents is of great interest, and has been studied at +Rothamstead in England and on a number of other farms. It is [103] +always changing. No two successive years show exactly the same +proportions. At one time one species prevails, at another time one or +two or more other species. The weather during the spring and summer +benefits some and hurts others, the winter may be too cold for some, but +again harmless for others, the rainfall may partly drown some species, +while others remain uninjured. Some weeds may be seen flowering +profusely during some years, while in other summers they are scarcely to +be found in the same meadow. The whole population is in a fluctuating +state, some thriving and others deteriorating. It is a continuous +response to the ever changing conditions of the weather. Rarely a +species is wholly annihilated, though it may apparently be so for years; +but either from seeds or from rootstocks, or even from neighboring +lands, it may sooner or later regain its foothold in the general +struggle for life. + +This phenomenon is a very curious and interesting one. The struggle for +life, which plays so considerable a part in the modern theories of +evolution, may be seen directly at work. It does not alter the species +themselves, as is commonly supposed, but it is always changing their +numerical proportion. Any lasting change in the external conditions will +of course alter the average oscillation and the influence [104] of such +alterations will manifest itself in most cases simply in new numerical +proportions. Only extremes have extreme effects, and the chance for the +weaker sorts to be completely overthrown is therefore very small. + +Any one, who has the opportunity of observing a waste field during a +series of years, should make notes concerning the numerical proportions +of its inhabitants. Exact figures are not at all required; approximate +estimates will ordinarily prove to be sufficient, if only the standard +remains the same during the succeeding years. + +The entire mass of historic evidence goes to prove that the same +conditions have always prevailed, from the very beginning of cultivation +up to the present time. The origin of the cultivation of cereals is to +be sought in central Asia. The recent researches of Solms Laubach show +it to be highly probable that the historic origin of the wheat +cultivated in China, is the same as that of the wheat of Egypt and +Europe. Remains of cereals are found in the graves of Egyptian mummies, +in the mounds of waste material of the lake-dwellings of Central Europe, +and figures of cereals are to be seen on old Roman coins. In the +sepulchre of King Ra-n-Woser of the Fifth Dynasty of Egypt, who lived +about 2000 years B.C., two [105] tombs have recently been opened by the +German Oriental Society. In them were found quantities of the tares of +the _Triticum dicoccum_, one of the more primitive forms of wheat. In +other temples and pyramids and among the stones of the walls of Dashur +and El Kab studied by Unger, different species and varieties of cereals +were discovered in large quantities, that showed their identity with the +present prevailing cultivated races of Egypt. + +The inhabitants of the lake-dwellings in Switzerland possessed some +varieties of cereals, which have entirely disappeared. They are +distinguished by Heer under special names. The small barley and the +small wheat of the lake-dwellers are among them. All in all there were +ten well distinguished varieties of cereals, the Panicum and the Setaria +or millet being of the number. Oats were evidently introduced only +toward the very last of the lake-dwelling period, and rye is of far +later introduction into western Europe. Similar results are attained by +the examination of the cereals figured by the Romans of the same period. + +All these are archaeologic facts, and give but slight indications +concerning the methods of cultivation or the real condition of the +cultivated races of that time. Virgil has left us some knowledge of the +requirements of methodical [106] culture of cereals of his time. In his +poem _Georgics_ (I. 197) the following lines are found: + + + _Vidi lecta din, et multo spectata labore + Degenerare tamen, ni vis humana quotannis + Maxima quaeque manu legeret_. + + (The chosen seed, through years and labor improved, + Was seen to run back, unless yearly + Man selected by hand the largest and fullest of ears.) + +Elsewhere Virgil and also some lines of Columella and Varro go to prove +in the same way that selection was applied by the Romans to their +cereals, and that it was absolutely necessary to keep their races pure. +There is little doubt, but that it was the same principle as that which +has led, after many centuries, to the complete isolation and improvement +of the very best races of the mixed varieties. It further proves that +the mixed conditions of the cereals was known to man at that time, +although distinct ideas of specific marks and differences were of course +still wholly lacking. It is proof also that cultivated cereals from the +earliest times must have been built up of numerous elementary forms. +Moreover it is very probable, that in the lapse of centuries a goodly +number of such types must have disappeared. [107] Among the vanished +forms are the special barley and wheat of the lake-dwellings, the +remains of which have been accidentally preserved, but most of the forms +must have disappeared without leaving any trace. + +This inference is supported by the researches of Solms-Laubach, who +found that in Abyssinia numerous primitive types of cereals are still in +culture. They are not adequate to compete with our present varieties, +and would no doubt also have disappeared, had they not been preserved by +such quite accidental and almost primitive isolation. + +Closing this somewhat long digression into history we will now resume +our discussion concerning the origin of the method of selecting cereals +for isolation and segregate-cultivation. Some decades after Le Couteur, +this method was taken up by the celebrated breeder Patrick Sheriff of +Haddington in Scotland. His belief, which was general at that time, was +"That cultivation has not been found to change well defined kinds, and +that improvement can be best attained by selecting new and superior +varieties, which nature occasionally produces, as if inviting the +husbandman to stretch forth his hand and cultivate them." + +Before going into the details of Sheriff's work it is as well to say +something concerning [108] the use of the word "selection." This word +was used by Sheriff as seen in the quotation given, and it was obviously +designed to convey the same idea as the word "lecta" in the quotation +from Virgil. It was a choice of the best plants from among known mixed +fields, but the chosen individuals were considered to be representatives +of pure and constant races, which could only be isolated, but not +ameliorated. Selection therefore, in the primitive sense of the word, is +the choice of elementary species and varieties, with no other purpose +than that of keeping them as pure as possible from the admixture of +minor sorts. The Romans attained this end only imperfectly, simply +because the laws governing the struggle for life and the competition of +numerous sorts in the fields were unsuspected by them. + +Le Couteur and Sheriff succeeded in the solution of the problem, because +they had discovered the importance of isolation. The combination of a +careful choice with subsequent isolation was all they knew about it, and +it was one of the great achievements to which modern agriculture owes +its success. + +The other great principle was that of Vilmorin. It was the improvement +within the race, or the "amelioration of the race" as it was termed by +him. It was introduced into [109] England by F.F. Hallett of Brighton in +Sussex, who at once called it "pedigree-culture," and produced his first +new variety under the very name of "Pedigree-wheat." This principle, +which yields improved strains, that are not constant but dependent on +the continued and careful choice of the best plants in each succeeding +generation, is now generally called "selection." But it should always be +remembered that according to the historic evolution of the idea, the +word has the double significance of the distinction and isolation of +constant races from mixtures, and that of the choice of the best +representatives of a race during all the years of its existence. Even +sugar-beets, the oldest "selected" agricultural plants, are far from +having freed themselves from the necessity of continuous improvement. +Without this they would not remain constant, but would retrograde with +great rapidity. + +The double meaning of the word selection still prevailed when Darwin +published his "Origin of Species." This was in the year 1859, and at +that time Shirreff was the highest authority and the most successful +breeder of cereals. Vilmorin's method had been applied only to beets, +and Hallett had commenced his pedigree-cultures only a few years before +and his first publication of the "Pedigree-wheat" [110] appeared some +years later at the International Exhibition of London in 1862. Hence, +whenever Darwin speaks of selection, Shirreff's use of the word may as +well be meant as that of Vilmorin. + +However, before going deeper into such theoretical questions, we will +first consider the facts, as given by Shirreff himself. + +During the best part of his life, in fact during the largest part of the +first half of the nineteenth century, Shirreff worked according to a +very simple principle. When quite young he had noticed that sometimes +single plants having better qualities than the average were seen in the +fields. He saved the grains, or sometimes the whole heads of such plants +separately, and tried to multiply them in such manner as to avoid +intermixtures. + +His first result was the "Mungoswell's wheat." In the spring of 1819 he +observed quite accidentally in a field of the farm of that name, a +single plant which attracted his attention by a deeper green and by +being more heavily headed out. Without going into further details, he at +once chose this specimen as the starting point of a new race. He +destroyed the surrounding plants so as to give it more space, applied +manure to its roots, and tended it with special care. It yielded 63 +heads and nearly [111] 2500 grains. All of these were sown the +following fall, and likewise in the succeeding years the whole harvest +was sown in separate lots. After two years of rapid multiplication it +proved to be a good new variety and was brought into commerce. It has +become one of the prominent varieties of wheat in East Lothian, that +county of Scotland of which Haddington is the principal borough. + +The grains of "Mungoswell's wheat" are whiter than those of the allied +"Hunter's wheat," more rounded but otherwise of the same size acid +weight. The straw is taller and stronger, and each plant produces more +culms and more heads. + +Shirreff assumed, that the original plant of this variety was a sport +from the race in which he had found it, and that it was the only +instance of this sport. He gives no details about this most interesting +side of the question, omitting even to tell the name of the parent +variety. He only asserts that it was seen to be better, and afterwards +proved so by the appreciation of other breeders and its success in +trade. He observed it to be quite constant from the beginning, no +subsequent selection being needed. This important feature was simply +assumed by him to be true as a matter of course. + +[112] Some years afterwards, in the summer of 1824, he observed a large +specimen of oats in one of the fields of the same farm. Being at that +time occupied in making a standard collection of oats for a closer +comparison of the varieties, he saved the seeds of that plant and sowed +them in a row in his experiment-field. It yielded the largest culms of +the whole collection and bore long and heavy kernels with a red streak +on the concave side and it excelled all other sorts by the fine +qualities of its very white meal. In the unequal length of its stalks it +has however a drawback, as the field appears thinner and more meager +than it is in reality. "Hopetown oats," as it is called, has found its +way into culture extensively in Scotland and has even been introduced +with success into England, Denmark and the United States. It has been +one of the best Scottish oats for more than half a century. + +The next eight years no single plant judged worthy of selection on his +own farm attracted Shirreff's attention. But in the fall of 1832 he saw +a beautiful plant of wheat on a neighboring farm and he secured a head +of it with about 100 grains. From this he produced the "Hopetown wheat." +After careful separation from the kernels this original ear was +preserved, and was afterwards exhibited at the Stirling Agricultural +[113] Museum. The "Hopetown wheat" has proved to be a constant variety, +excelling the ordinary "Hunter's wheat" by larger grains and longer +heads; it yields likewise a straw of superior quality and has become +quite popular in large districts of England and Scotland, where it is +known by the name of "White Hunter's" from its origin and the brilliant +whiteness of its heads. + +In the same way Shirreff's oats were discovered in a single plant in a +field where it was isolated in order to be brought into commerce after +multiplication. It has won the surname of "Make-him-rich." Nothing is on +record about the details of its origin. + +Four valuable new varieties of wheat and oats were obtained in this way +in less than forty years. Then Shirreff changed his ideas and his method +of working. Striking specimens appeared to be too rare, and the +expectation of a profitable result too small. Therefore he began work on +a larger scale. He sought and selected during the summer of 1857 seventy +heads of wheat, each from a single plant showing some marked and +presumably favorable peculiarity. These were not gathered on one field, +but were brought together from all the fields to which he had access in +his vicinity. The grains of each of these selected heads were [114] sown +separately, and the lots compared during their whole life-period and +chiefly at harvest time. Three of the lots were judged of high +excellence, and they alone were propagated, and proving to be constant +new varieties from the outset were given to the trade under the names of +"Shirreff's bearded white," "Shirreff's bearded red," and "Pringle's +wheat." They have found wide acceptance, and the first two of them are +still considered by Vilmorin as belonging to the best wheats of France. + +This second method of Shirreff evidently is quite analogous to the +principle of Lagasca and Le Couteur. The previous assumption that new +varieties with striking features were being produced by nature from time +to time, was abandoned, and a systematic inquiry into the worth of all +the divergent constituents of the fields was begun. Every single ear at +once proved to belong to a constant and pure race, but most of these +were only of average value. Some few however, excelled to a degree, +which made them worth multiplying, and to be introduced into trade as +separate varieties. + +Once started, this new method of comparison, selection and isolated +multiplication was of course capable of many improvements. The culture +in the experiment-field was improved, so as to insure a fuller and more +rapid growth. + +[115] The ripe heads had to be measured and counted and compared with +respect to their size and the number of their kernels. Qualities of +grain and of meal had to be considered, and the influence of climate and +soil could not be overlooked. + +Concerning the real origin of his new types Shirreff seems never to have +been very inquisitive. He remarks that only the best cultivated +varieties have a chance to yield still better types, and that it is +useless to select and sow the best heads of minor sorts. He further +remarks that it is not probable that he found a new sport every time; on +the contrary he assumes that his selections had been present in the +field before, and during a series of succeeding generations. How many +years old they were, was of course impossible to determine. But there is +no reason to believe that the conditions in the fields of Scotland were +different from those observed on the Isle of Jersey by Le Couteur. + +In the year 1862 Shirreff devoted himself to the selection of oats, +searching for the best panicles from the whole country, and comparing +their offspring in his experimental garden. "Early Fellow," "Fine +Fellow," "Longfellow" and "Early Angus" are very notable varieties +introduced into trade in this way. + +[116] Some years later Patrick Shirreff described his experiments and +results in a paper entitled, "On the improvement of cereals," but the +descriptions are very short, and give few details of systematic value. +The leading principle, however, is clearly indicated, and anyone who +studies with care his method of working, may confidently attempt to +improve the varieties of his own locality in the same way. + +This great principle of "variety-testing," as it has been founded by Le +Couteur and Patrick Shirreff, has increased in importance ever since. +Two main features are to be considered here. One is the production of +local races, the other the choice of the best starting-point for +hybridizing experiments, as is shown in California by the work of Luther +Burbank in crossing different elementary species of _Lilium pardalinum_ +and others. + +Every region and locality has its own conditions of climate and soil. +Any ordinary mixed race will contain some elementary forms which are +better adapted to a given district, while others are more suitable to +divergent conditions. Hence it can readily be inferred that the choice +cannot be the same for different regions. Every region should select its +own type from among the various forms, and variety testing therefore +becomes a task which every [117] one must undertake under his own +conditions. Some varieties will prove, after isolation, to be profitable +for large districts and perhaps for whole states. Others will be found +to be of more local value, but in such localities to excel all others. + +As an example we may take one of the varieties of wheat originated by +the Minnesota Experiment Station. Hays described it as follows. It was +originated from a single plant. From among 400 plants of "Blue stem" +several of the best were chosen, each growing separately, a foot apart +in every direction. Each of the selected plants yielded 500 or more +grains of wheat, weighing 10 or more grams. The seeds from these +selected plants were raised for a few years until sufficient was +obtained to sow a plot. Then for several years the new strains were +grown in a field beside the parent-variety. One of them was so much +superior that all others were discarded. It was the one named "Minnesota +No. 169." For a large area of Minnesota this wheat seems capable of +yielding at least 1 or 2 bushels more grain per acre than its parent +variety, which is the best kind commonly and almost universally found on +the farms in southern and central Minnesota. + +It would be quite superfluous for our present purpose to give more +instances. The fact of [118] the compound nature of so-called species of +cultivated plants seems to be beyond all doubt, and its practical +importance is quite obvious. + +Acclimatization is another process, which is largely dependent on the +choice of adequate varieties. This is shown on a large scale by the slow +and gradual dispersion of the varieties of corn in this country. The +largest types are limited to temperate and subtropical regions, while +the varieties capable of cultivation in more northern latitudes are +smaller in size and stature and require a smaller number of days to +reach their full development from seed to seed. Northern varieties are +small and short lived, but the "Forty-day-corn" or "Quarantino maize" is +recorded to have existed in tropical America at the time of Columbus. In +preference, or rather to the entire exclusion of taller varieties, it +has thriven on the northern boundaries of the corn-growing states of +Europe since the very beginning of its cultivation. + +According to Naudin, the same rule prevails with melons, cucumbers and +gherkins, and other instances could easily be given. + +Referring now to the inferences that may be drawn from the experience of +the breeders in order to elucidate the natural processes, we will return +to the whitlow-grasses and pansies. + +[119] Nature has constituted them as groups of slightly different +constant forms, quite in the same way as wheat and oats and corn. +Assuming that this happened ages ago somewhere in central Europe, it is +of course probable that the same differences in respect to the influence +of climatic conditions will have prevailed as with cereals. Subsequent +to the period which has produced the numerous elementary species of the +whitlow-grass came a period of widespread distribution. The process must +have been wholly comparable with that of acclimatization. Some species +must have been more adapted to northern climates, others to the soils of +western or eastern regions and so on. These qualities must have decided +the general lines of the distribution, and the species must have been +segregated according to their respective climatic qualities, and their +adaptability to soil and weather. A struggle for life and a natural +selection must have accompanied and guided the distribution, but there +is no reason to assume that the various forms were changed by this +process, and that we see them now endowed with other qualities than they +had at the outset. + +Natural selection must have played, in this and in a large number of +other cases, quite the same part as the artificial method of variety +testing. + +[120] Indeed it may be surmised that this has been its chief and +prominent function. Taking up again our metaphor of the sieve we can +assert that in such cases climate and soil exercise sifting action and +in this way the application of the metaphor becomes more definite. Of +course, next to the climate and soil in importance, come ecological +conditions, the vegetable and animal enemies of the plants and other +influences of the same nature. + +In conclusion it is to be pointed out that this side of the problem of +natural selection and the struggle for life appears to offer the best +prospects for experimental, or for continued statistical inquiry. Direct +observations are possible and any comparison of numerical proportions of +species in succeeding years affords clear proof of the part it plays. +And above all, such observations can be made quite independently of +doubtful theoretical considerations about presumed changes of character. + +The fact of natural selection is plain and it should be studied in its +most simple conditions. + + +[121] +C. RETROGRADE VARIETIES + +LECTURE V + +CHARACTERS OF RETROGRADE VARIETIES + +Every one admires the luxuriance of garden-flowers, and their diversity +of color and form. All parts of the world have contributed to their +number and every taste can find its preference among them. New forms +produced by the skill of the breeder are introduced every year. This has +been done mostly by crossing and intermingling the characters of +introduced species of the same genus. In some of the cases the history +of our flowers is so old that their hybrid origin is forgotten, as in +the case of the pansies. Hybridizations are still going on in other +groups on a large scale, and new forms are openly claimed to be of +hybrid origin. + +Breeders and amateurs generally have more interest in the results than +in the way in which they have been brought about. Excellent flowers and +fruit recommend themselves and there seems to be no reason for inquiring +[122] about their origin. In some cases the name of the originator may +be so widely known that it adds weight to the value of the new form, and +therefore may advantageously be coupled with it. The origin and history +of the greater part of our garden-flowers, fruits and vegetables are +obscure; we see them as they are, and do not know from whence they came. +The original habitat for a whole genus or for a species at large, may be +known, but questions as to the origin of the single forms, of which it +is built up, ordinarily remain unanswered. + +For these reasons we are restricted in most cases to the comparison of +the forms before us. This comparison has led to the general use of the +term "variety" in opposition to "species." The larger groups of forms, +which are known to have been introduced as such are called species. All +forms which by their characters belong to such a species are designated +as varieties, irrespective of their systematic relation to the form, +considered as the ancestor of the group. + +Hence, we distinguish between "hybrid varieties" and "pure varieties" +according to their origin from different parents or from a single line +of ancestors. Moreover, in both groups the forms may be propagated by +seeds, or in the vegetative way by buds, by grafting or [123] by +cutting, and this leads to the distinction of "seed-varieties" and +"vegetative varieties." In the first case the inheritance of the special +characters through the seeds decides the status of the variety, in the +latter case this point is left wholly out of consideration. + +Leaving aside all these different types, we are concerned here only with +the "seed-varieties" of pure origin, or at least with those, that are +supposed to be so. Hybridization and vegetative multiplication of the +hybrids no doubt occur in nature, but they are very rare, when compared +with the ordinary method of propagation by seed. "Seed-varieties" may +further be divided into constant and inconstant ones. The difference is +very essential, but the test is not always easy to apply. Constant +varieties are as sharply defined and as narrowly limited as are the best +wild species, while inconstant types are cultivated chiefly on account +of their wide range of form and color. This diversity is repeated +yearly, even from the purest seed. We will now discuss the constant +seed-varieties, leaving the inconstant and eversporting types to a +subsequent lecture. + +In this way we may make an exact inquiry into the departures from the +species which are ordinarily considered to constitute the essential +character of such a constant and pure seed-variety [124] and need only +compare these differences with those that distinguish the elementary +species of one and the same group from each other. + +Two points are very striking. By far the greatest part of the ordinary +garden-varieties differ from their species by a single sharp character +only. In derivative cases two, three or even more such characters may be +combined in one variety, for instance, a dwarfed variety of the larkspur +may at the same time bear white flowers, or even double white flowers, +but the individuality of the single characters is not in the least +obscured by such combinations. + +The second point is the almost general occurrence of the same variety in +extended series of species. White and double flowers, variegated leaves, +dwarfs and many other instances may be cited. It is precisely this +universal repetition of the same character that strikes us as the +essential feature of a variety. + +And again these two characteristics may now be considered separately. +Let us begin with the sharpness of the varietal characters. In this +respect varieties differ most obviously from elementary species. These +are distinguished from their nearest allies in almost all organs. There +is no prominent distinctive feature between the single forms of _Draba_ +[125] _Verna_, _Helianthemum_ or of _Taraxacum_; all characters are +almost equally concerned. The elementary species of _Draba_ are +characterized, as we have seen, by the forms and the hairiness of the +leaves, the number and height of the flower-stalks, the breadth and +incision of the petals, the forms of the fruits, and so on. Every one of +the two hundred forms included in this collective species has its own +type, which it is impossible to express by a single term. Their names +are chosen arbitrarily. Quite the contrary is the case with most of the +varieties, for which one word ordinarily suffices to express the whole +difference. + +White varieties of species with red or blue flowers are the most common +instances. If the species has a compound color and if only one of the +constituents is lost, partially colored types arise as in _Agrostemma +Coronaria bicolor_. Or the spots may disappear and the color become +uniform as in _Gentiana punctata concolor_ and the spotless Arum or +_Arum maculatum immaculatum_. Absence of hairs produces forms as +_Biscutella laevigata glabra_; lack of prickles gives the varieties +known as _inermis, as for instance, _Ranunculus arvensis inermis_. +_Cytisus prostratus_ has a variety _ciliata_, and _Solanum Dulcamara_, +or the bitter-sweet, has a variety called _tomentosum_. The curious +monophyllous [126] variety of the strawberry and many other forms will +be discussed later. + +To enlarge this list it would only be necessary to extract from a flora, +or from a catalogue of horticultural plants, the names of the varieties +enumerated therein. In nearly every instance, where true varieties and +not elementary species are concerned, a single term expresses the whole +character. + +Such a list would also serve to illustrate the second point since the +same names would recur frequently. Long lists of varieties are called +alba, or inermis, or canescens or lutea, and many genera contain the +same appellations. In some instances the systematists use a diversity of +names to convey exactly the same idea, as if to conceal the monotony of +the character, as for instance in the case of the lack of hairs, which +is expressed by the varietal names of _Papaver dubium glabrum_, _Arabis +ciliata glabrata_, _Arabis hirsuta glaberrima_, _Veronica spicata +nitens_, _Amygdalus persica laevis_, _Paeonia corallina Leiocarpa_, &c. + +On the contrary we find elementary species in different genera based on +the greatest possible diversity of features. The forms of _Taraxacum_ or +_Helianthemum_ do not repeat those of _Draba_ or _Viola_. In roses and +brambles the distinguishing features are characteristic of the type, as +[127] they are evidently derived from it and limited to it. And this is +so true that nobody claims the grade of elementary species for white +roses or white brambles, but everyone recognizes that forms diverging +from the nearest species by a single character only, are to be regarded +as varieties. + +This general conviction is the basis on which we may build up a more +sharply defined distinction between elementary species and varieties. It +is an old rule in systematic botany, that no form is to be constituted a +species upon the basis of a single character. All authors agree on this +point; specific differences are derived from the totality of the +attributes, not from one organ or one quality. This rule is intimately +connected with the idea that varieties are derived from species. The +species is the typical, really existing form from which the variety has +originated by a definite change. In enumerating the different forms the +species is distinguished by the term of genuine or typical, often only +indicated as _a_ or the first; then follow the varieties sometimes in +order of their degree of difference, sometimes simply in alphabetical +order. In the case of elementary species there is no real type; no one +of them predominates because all are considered to be equal in rank, and +the systematic species to which they [128] are referred is not a really +existing form, but is the abstraction of the common type of all, just as +it is in the case of a genus or of a family. + +Summarizing the main points of this discussion, we find that elementary +species are of equal rank and together build up the collective or +systematic ideal species. Varieties on the other hand are derived from a +real and commonly, still existing type. + +I hope that I have succeeded in showing that the difference between +elementary species, or, as they are often called, smaller or subspecies, +on the one hand and varieties on the other, is quite a marked one. +However, in order to recognize this principle it is necessary to limit +the term variety, to those propagating themselves by seed and are of +pure and not of hybrid origin. + +But the principle as stated here, does not involve an absolute contrast +between two groups of characters. It is more a difference in our +knowledge and appreciation of them than a difference in the things +themselves. The characters of elementary species are, as a rule, new to +us, while those of varieties are old and familiar. It seems to me that +this is the essential point. + +And what is it that makes us familiar with them? Obviously the +continuous recurrence of the same changes, because by a constant +repetition they must of course lose their novelty. + +[129] Presently we shall look into these characters more in detail and +then we shall find that they are not so simple as might be supposed at +first sight; but precisely because we are so familiar with them, we +readily see that their different features really belong to a single +character; while in elementary species everything is so new that it is +impossible for us to discern the unities of the new attributes. + +If we bear in mind all these difficulties we cannot wonder at the +confusion on this question that seems to prevail everywhere. Some +authors following Linnaeus simply call all the subdivisions of species, +varieties; others follow Jordan and avoid the difficulty by designating +all smaller forms directly as species. The ablest systematists prefer to +consider the ordinary species as collective groups, calling their +constituents "The elements of the species," as was done by A.P. De +Candolle, Alph. De Candolle and Lindley. + +By this method they clearly point out the difference between the +subdivisions of wild species as they ordinarily occur, and the varieties +in our gardens, which would be very rare, were they not singled out and +preserved. + +Our familiarity with a character and our grounds for calling it an old +acquaintance may result from two causes, which in judging a new [130] +variety are essentially different. The character in question may be +present in the given species or it may be lacking, but present in the +other group. In the first case a variety can only be formed by the loss +of the character, in the second case it arises by the addition of a new +one. + +The first mode may be called a negative process, while the second is +then to be designated as positive. And as it is more easy to lose what +one has than to obtain something new, negative varieties are much more +common than are positive ones. + +Let us now take an instance of a character that is apt to vary in both +ways, for this is obviously the best way of making clear what is meant +by a negative and a positive change. + +In the family of the composites we find a group of genera with two forms +of florets on each flower-head. The hermaphrodite ones are tubular with +5, or rarely 4, equal teeth, and occupy the center of the head. These +are often called the flosculous florets or disk-florets. Those of the +circumference are ligulate and ordinarily unisexual, without stamens. In +many cases they are sterile, having only an imperfect ovary. They are +large and brightly colored and are generally designated as ray-florets. +As instances we may cite the camomile (_Anthemis nobilis_), the wild +camomile (_Matricaria Chamomilla_), [131] the yarrow (_Achillea +Millefolium_), the daisies, the Dahlia and many others. Species occur in +this group of plants from time to time that lack the ray-florets, as in +the tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus of +the marigolds or _Bidens_ is noted for containing both of these types. +The smaller and the three-toothed marigold (_B. cernua_ and _B. +tripartita_) are very common plants of wet soil and swamps, ordinarily +lacking the ray-florets, and in some countries they are very abundant +and wholly constant in this respect, never forming radiate flower-heads. +On the other hand the white-flowered and the purple marigold (_B. +leucantha_ and _B. atropurpurea_) are cultivated species of our gardens, +prized for their showy flower-heads with large white or deeply colored, +nearly black-purple florets. + +Here we have opportunity to observe positive and negative varieties of +the same character. The smaller, and the three-toothed marigold occur +from time to time, provided with ray florets, showing a positive +variation. And the white marigold has produced in our gardens a variety +without rays. Such varieties are quite constant, never returning to the +old species. Positive and negative varieties of this kind are by no +means rare among the compositae. + +[132] In systematic works the positive ones are as a rule called +"radiate," and the negative ones "discoid." Discoid forms of the +ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and +of some centauries have been described. Radiate forms have been observed +in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada +fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio +vulgaris_). Taken broadly the negative varieties seem to be somewhat +more numerous than the positive ones, but it is very difficult to come +to a definite conclusion on this point. + +Quite the contrary is the case with regard to the color-varieties of red +and blue flowers. Here the loss of color is so common that every one +could give long lists of examples of it. Linnaeus himself supposed that +no blue or red-colored wild species would be without a white variety. It +is well known that he founded his often criticized prescript never to +trust to color in recognizing or describing a species, on this belief. + +On the other hand there are some red varieties of white-flowered +species. But they are very rare, and little is known about their +characters or constancy. Blue varieties of white species are not found. +The yarrow (_Achillea Millefolium_) has a red-flowered form, which +occurs [133] from time to time in sunny and sandy localities. I have +isolated it and cultivated it during a series of years and during many +generations. It is quite true to its character, but the degree of its +coloring fluctuates between pink and white and is extremely variable. +Perhaps it can be considered as an inconstant variety. A redflowered +form of the common _Begonia semperflorens_ is cultivated under the name +of "Vernon," the white hawthorn (_Crataegus Oxyacantha_) is often seen +with red flowers, and a pink-flowered variety of the "Silverchain" or +"Bastard acacia" (_Robinia Pseud-Acacia_) is not rarely cultivated. The +"Crown" variety of the yellow wall-flower and the black varieties, are +also to be considered as positive color variations, the black being due +in the latter cases to a very great amount of the red pigment. + +Among fruits there are also some positive red varieties of greenish or +yellowish species, as for instance the red gooseberry (_Ribes +Grossularia_) and the red oranges. The red hue is far more common in +leaves, as seen among herbs, in cultivated varieties of _Coleus_ and in +the brown leaved form of the ordinary white clover, among trees and +shrubs in the hazelnut (_Corylus_), the beech (_Fagus_), the birch +(_Betula_), the barberry (_Berberis_) and many others. But though most +of these forms are very ornamental and abundant [134] in parks and +gardens, little is as yet known concerning the origin of their varietal +attributes and their constancy, when propagated by seeds. Besides the +ray-florets and the colors, there are of course a great many other +characters in which varieties may differ from their species. In most of +the cases it is easy to discern whether the new character is a positive +or a negative one. And it is not at all necessary to scrutinize very +narrowly the list of forms to become convinced that the negative form is +the one which prevails nearly everywhere, and that positive aberrations +are in a general sense so rare that they might even be taken for +exceptions to the rule. + +Many organs and many qualities may be lost in the origination of a +variety. In some instances the petals may disappear, as in _Nigella_, or +the stamens, as in the Guelder-rose (_Viburnum Opulus_) and the +_Hortensia_ and in some bulbs even the whole flowers may be wanting, as +in the beautiful "Plumosa" form of the cultivated grape-hyacinth or +_Muscari comosum_. Fruits of the pineapples and bananas without seeds +are on record as well as some varieties of apples and pears, of raisins +and oranges. And some years ago Mr. Riviere of Algeria described a date +growing in his garden that forms fruit without pits. The stoneless plum +of Mr. [135] Burbank of Santa Rosa, California, is also a very curious +variety, the kernel of which is fully developed but naked, no hard +substance intervening between it and the pulp. + +More curious still are the unbranched varieties consisting of a single +stem, as may be seen sometimes in the corn or maize and in the fir. +Fir-trees of some three or four meters in height without a single +branch, wholly naked and bearing leaves only on the shoots of the last +year's growth at the apex of the tree, may be seen. Of course they +cannot bear seed, and so it is with the sterile maize, which never +produces any seed-spikes or staminate flowers. Other seedless varieties +can be propagated by buds; their origin is in most cases unknown, and we +are not sure as to whether they should be classified with the constant +or with the inconstant varieties. + +A very curious loss is that of starch in the grains of the sugar-corn +and the sugar-peas. It is replaced by sugar or some allied substance +(dextrine). Equally remarkable is the loss of the runners in the +so-called "Gaillon" strawberries. + +Among trees the pendulous or weeping, and the broomlike or fastigiate +forms are very marked varieties, which occur in species belonging to +quite different orders. The ash, the beach, some willows, many other +trees and some [136] finer species of garden-plants, as _Sophora +japonica_, have given rise to weeping varieties, and the yew-tree or +_Taxus_ has a fastigiate form which is much valued because of its +ascending branches and pyramidal habit. So it is with the pyramidal +varieties of oaks, elms, the bastard-acacia and some others. + +It is generally acknowledged that these forms are to be considered as +varieties on the ground of their occurrence in so wide a range of +species, and because they always bear the same attributes. The pendulous +forms owe their peculiarity to a lengthening of the branches and a loss +of their habit of growing upwards; they are too weak to retain a +vertical position and the response to gravity, which is ordinarily the +cause of the upright growth, is lacking in them. As far as we know, the +cause of this weeping habit is the same in all instances. The fastigiate +trees and shrubs are a counterpart of the weeping forms. Here the +tendency to grow in a horizontal direction is lacking, and with it the +bilateral and symmetric structure of the branches has disappeared. In +the ordinary yew-tree the upright stem bears its needles equally +distributed around its circumference, but on the branches the needles +are inserted in two rows, one to the left and one to the right. All the +needles turn their upper surfaces upwards, [137] and their lower +surfaces downwards, and all of them are by this means placed in a single +horizontal plane, and branching takes place in the same plane. Evidently +this general arrangement is another response to gravity, and it is the +failure of this reaction which induces the branches to grow upwards and +to behave like stems. + +Both weeping and fastigiate characters are therefore to be regarded as +steps in a negative direction, and it is highly important that even such +marked departures occur without transitions or intermediate forms. If +these should occur, though ever so rarely, they would probably have been +brought to notice, on account of the great prospect the numerous +instances would offer. The fact that they are lacking, proves that the +steps, though apparently great, are in reality to be considered as +covering single units, that cannot be divided into smaller parts. +Unfortunately we are still in the dark as to the question of the +inheritance of these forms, since in most cases it is difficult to +obtain pure seed. + +We now consider the cases of the loss of superficial organs, of which +the nectarines are example. These are smooth peaches, lacking the soft +hairy down, that is a marked peculiarity of the true peaches. They occur +in different [138] races of the peach. As early as the beginning of the +past century, Gallesio described no less than eight subvarieties of +nectarines, each related to a definite race of peach. Most of them +reproduce themselves truly from seed, as is well known in this country +concerning the clingstones, freestones and some other types. Nectarines +have often varied, giving rise to new sorts, as in the case of the white +nectarine and many others differing greatly in appearance and flavor. On +the other hand it is to be remarked, that the trees do not differ in +other respects and cannot be distinguished while young, the varietal +mark being limited to the loss of the down on the fruit. Peaches have +been known to produce nectarines, and nectarines to yield true peaches. +Here we have another instance of positive and negative steps with +reference to the same character, but I cannot withhold an expression of +some doubt as to the possibility of crossing and subsequently splitting +up of the hybrids as a more probable explanation of at least some of the +cases quoted by various writers. + +Smooth or glabrous varieties often occur, and some of them have already +been cited as instances of the multiplication of varietal names. +Positive aberrations are rather rare, and are mostly restricted to a +greater density of the [139] pubescence in some hairy species, as in +_Galeopsis Ladanum canescens_, _Lotus corniculatus hirsutus_ and so on. +But _Veronica scutellata_ is smooth and has a pubescent variety, and +Cytisus prostratus and _C. spinescens_ are each recorded to have a +ciliate form. + +Comparable with the occurrence and the lack of hairs, is the existence +or deficiency of the glaucous effect in leaves, as is well known in the +common _Ricinus_. Here the glaucous appearance is due to wax distributed +in fine particles over the surface of the leaves, and in the green +variety this wax is lacking. Other instances could be given as in the +green varieties of _Papaver alpinum_ and _Rumex scutatus_. No positive +instances are recorded in this case. + +Spines and prickles may often disappear and give rise to unarmed and +defenceless types. Of the thorn-apples both species, the whiteflowered +_Datura Stramonium_ and the purple _D. Tatula_ have such varieties. +Spinach has a variety called the "Dutch," which lacks the prickles of +the fruit; it is a very old form and absolutely constant, as are also +the thornless thorn-apples. Last year a very curious instance of a +partial loss of prickles was discovered by Mr. Cockerell of East Las +Vegas in New Mexico. It is a variety of the American cocklebur, often +called sea-burdock, or the [140] hedgehog-burweed, a stout and common +weed of the western states. Its Latin name is _Xanthium canadense_ or +_X. commune_ and the form referred to is named by Mr. Cockerell, _X. +Wootoni_, in honor of Professor E.o. Wooton who described the first +collected specimens. + +The burs of the common species are densely covered with long prickles, +which are slightly hooked at the apex. In the new form, which is similar +in all other respects to the common cocklebur, the burs are more slender +and the prickles much less numerous, about 25 to the bur and mostly +stouter at the base. It occurs abundantly in New Mexico, always growing +with the common species, and seems to be quite constant from seed. Mr. +Cockerell kindly sent me some burs of both forms, and from these I +raised in my garden last year a nice lot of the common, as well as of +the _Wootoni_ plants. + +Spineless varieties are recorded for the bastard-acacia, the holly and +the garden gooseberry (_Ribes Grossularia_, or _R. Uva-crispa_). A +spineless sport of the prickly Broom (_Ulex europaeus_) has been seen +from time to time, but it has not been propagated. + +Summarizing the foregoing facts, we have excellent evidence of varieties +being produced either by the loss of some marked peculiarity or by the +acquisition of others that are already [141] present in allied species. +There are a great many cases however, in which the morphologic cause of +the dissimilarity is not so easily discerned. But there is no reason to +doubt that most of them will be found to conform to the rule on closer +investigation. Therefore we can consider the following as the principal +difference between elementary species and varieties; that the first +arise by the acquisition of entirely new characters, and the latter by +the loss of existing qualities or by the gain of such peculiarities as +may already be seen in other allied species. + +If we suppose elementary species and varieties originated by sudden +leaps or mutations, then the elementary species have mutated in the line +of progression, some varieties have mutated in the line of +retrogression, while others have diverged from their parental types in a +line of depression, or in the way of repetition. This conception agrees +quite well with the current idea that in the building up of the +vegetable kingdom according to the theory of descent, it is species that +form the links of the chain from the lower forms to the more highly +organized later derivatives. Otherwise expressed, the system is built up +of species, and varieties are only local and lateral, but never of real +importance for the whole structure. + +[142] Heretofore we have generally assumed, that varieties differ from +the parent-species in a single character only, or at least that only one +need be considered. We now come to the study of those varieties, which +differ in more than one character. Of these there are two types. In the +first the points of dissimilarity are intimately connected with one +another, in the second they are more or less independent. + +The mutually related peculiarities may be termed correlative, and we +therefore speak, in such cases, of correlative variability. This +phenomenon is of the highest importance and is of general occurrence. +But before describing some examples, it is as well to note that in the +lecture on fluctuating variability, cases of a totally different nature +will be dealt with, which unfortunately are designated by the same term. +Such merely fluctuating variations are therefore to be left out of the +present discussion. + +The purple thorn-apple, which is considered by some writers as a variety +of the white-flowered species or _Datura Stramonium_, and by others as a +separate species, _D. Tatula_, will serve as an illustration. But as its +distinguishing attributes, as far as we are concerned with them here, +are of the nature described above as characteristic of varietal +peculiarities no objection [143] can be made to our using them as a case +of correlative variability. + +The essential character of the purple thornapple lies in the color of +the flowers, which are of a very beautiful pale blue. But this color is +not limited to the corolla. It is also to be seen in the stems and in +the stalks and veins of the leaves, which are stained with a deep +purple, the blue color being added to the original green. Even on the +surface of the leaves it may spread into a purplish hue. On the stems it +is to be met with everywhere, and even the young seedlings show it. This +is of some importance, as the young plants when unfolding their +cotyledons and primary leaves, may be distinguished by this means from +the seedlings of the white flowered species. + +In crossing experiments it is therefore possible to distinguish the +whites and the blues, even in young seedlings, and experience shows that +the correlation is quite constant. The color can always be relied upon; +if lacking in the seedlings, it will be lacking in the stems and flowers +also; but if the axis of the young plant is ever so slightly tinged, the +color will show itself in its beauty in the later stages of the life of +the plant. + +This is what we term correlation. The colors of the different organs are +always in agreement. It is true that they require the concurrence of +[144] light for development, and that in the dark or in a faint light +the seedlings are apt to remain green when they should become purple, +but aside from such consideration all organs always come true to their +color, whether pure green and white, or whether these are combined with +the blue tinge. This constancy is so absolute that the colors of the +different organs convey the suggestion, that they are only separate +marks of a single character. + +It is on this suggestion that we must work, as it indicates the cause of +the correlation. Once present, the faculty of producing the anthocyan, +the color in question, will come into activity wherever and whenever +opportunity presents itself. It is the cell-sap of the ordinary cell +tissue or parenchyma, which is colored by the anthocyan, and for this +reason all organs possessing this tissue, may exhibit the color in +question. + +Thus the color is not a character belonging to any single organ or cell, +nor is it bound to a morphologic unit; it is a free, physiologic +quality. It is not localized, but belongs to the entire plant. If we +wish to assume for its basis material representative particles, these +particles must be supposed to be diffused throughout the whole body of +the plant. + +This conception of a physiologic unit as the [145] cause of colors and +other qualities is evidently opposed to the current idea of the cells +and tissues as the morphologic units of the plants. But I do not doubt, +that in the long run it will recommend itself as much to the scientist +as to the breeder. For the breeder, when desiring to keep his varieties +up to their standard, or when breeding to a definite idea, obviously +keeps his standard and his ideal for the whole plant, even if he breeds +only for flowers or for fruit. + +I have chosen the color of the purple thornapple as a first example, but +the colors of other plants show so many diverging aspects, all pointing +so clearly to the same conclusion, that it would be well to take a more +extensive view of this interesting subject. + +First we must consider the correlation in the colors of flowers and +fruits. If both are colored in the species, whether red or brown or +purple or nearly black, and a variety lacking this hue is known, it will +be lacking in both organs. If the color is pure, the flowers and berries +will become white, but such cases are rare. Ordinarily a yellowish or +greenish tinge underlies the ornamental color, and if this latter +disappears, the yellowish ground will become manifest. So for instance +in the Belladonna, a beautiful perennial herb with great shiny black, +but very poisonous, fruits. Its flowers are brown, but in [146] some +woods a variety with greenish flowers and bright yellow berries occurs, +which is also frequently seen in botanic gardens. The anthocyan dye is +lacking in both organs, and the same is the case with the stems and the +leaves. The lady's laurel or _Daphne Mezereum_ has red corollas, purple +leaves and red fruits; its white flowered variety may be distinguished +by lack of the red hue in the stems and leaves, and by their beautiful +yellow berries. Many other instances could be given, since the loss of +color in berries is a very common occurrence, so common that for +instance, in the heath-family or Ericaceae, with only a few exceptions, +all berry-bearing species have white-fruited varieties. + +The same correlation is observed in the seeds. The white-flowered flax +may be seen to yield yellow and not brown seeds as in the blue species. +Many varieties of flowers may be recognized by the color of their seeds, +as in the poppies, stocks and others. Other white-flowered varieties may +be distinguished when germinating, their young axes being of a pure +instead of a purplish green. It is a test ordinarily used by gardeners, +to purify their flower beds long before the blooming time, when thinning +or weeding them. Even in wild plants, as in _Erodium_, _Calluna_, +_Brunella_ and others, a botanist may recognize the rare white-flowered +[147] variety by the pure green color of the leaves, at times when it is +not in flower. Some sorts of peas bear colored flowers and a red mark on +the stipules of their leaves. Among bulbous plants many varieties may be +recognized even in the dry bulbs by the different tinges of the outer +scales. + +Leaving the colors, we come now to another instance of correlation, +which is still more astonishing. For it is as rare, as color-varieties +are common. It is afforded by some plants the leaves of which, instead +of being entire or only divided into large parts, are cleft to a greater +extent by repeated fissures of the marginal lobes. Such foliar +variations are often seen in gardens, where they are cultivated for +their beauty or singularity, as the laciniated alders, fern-leaved, +beeches and limes, oakleaved laburnums, etc. Many of them are described +under the varietal name of _laciniata_. In some cases this fissure +extends to the petals of the flowers, and changes them in a way quite +analogous to the aberrancy of the leaves. + +This is known to occur with a variety of brambles, and is often seen in +botanic gardens in one of the oldest and most interesting of all +anomalies, the laciniated variety of the greater celandine or +_Chelidonium majus_. Many other instances could be given. Most of them +belong to the [148] group of negative variations, as we have defined +them. But the same thing occurs also with positive varieties, though of +course, such cases are very rare. The best known instance is that of the +ever-flowering begonia, _Begonia semperflorens_, which has green leaves +and white flowers, but which has produced garden varieties with a brown +foliage and pink flowers. Here also the new quality manifests itself in +different organs. + +Enough has now been said on correlative changes, to convince us that +they are as a rule to be considered as the expression of some general +internal or physiologic quality, which is not limited to a single organ, +but affects all parts of the organism, provided they are capable of +undergoing the change. Such characters are therefore to be considered as +units, and should be referred to the group of single characters. + +Opposed to these are the true compound characters, which consist of +different units. These may be segregated by the production of varieties, +and thereby betray the separate factors of the complex group. + +The most beautiful instances of such complex characters are offered by +the colors of some of the most prized garden-flowers. Rarely these are +of a single hue, often two or three shades contribute to the effect, and +in some cases special [149] spots or lines or tracings are to be seen on +a white or on a colored background. That such spots and lines are +separate units is obvious and is demonstrated by the fact that sometimes +spotless varieties occur, which in all other respects have kept the +colors of the species. The complexity of the color is equally evident, +whenever it is built up of constituents of the anthocyan and of the +yellow group. The anthocyan dye is limited to the sap-cavity of the +cells, while the yellow and pure orange colors are fixed in special +organs of the protoplasm. The observation under the microscope shows at +once the different units, which though lying in the same cell and in +almost immediate vicinity of each other are always wholly separated from +one another by the wall of the vacuole or sapfilled cell-cavity. + +The combination of red and yellow gives a brown tinge, as in the +cultivated wall-flower, or those bright hues of a dark orange-red, which +are so much sought in tulips. By putting such flowers for a short time +in boiling water, the cells die and release the red pigment, which +becomes diffused in the surrounding fluids and the petals are left +behind with their yellow tinge. In this way it is easy to separate the +constituents, and demonstrate the compound nature of the original +colors. + +[150] But the diversity of the color patterns is far from being +exhausted with these simple instances. Apart from them, or joined to +them, other complications are frequently seen, which it is impossible to +analyze in such an artificial way. Here we have to return to our former +principle, the comparison of different varieties. Assuming that single +units may be lost, irrespective of the others, we may expect to find +them segregated by variation, wherever a sufficiently wide range of +color-varieties is in cultivation. In fact, in most cases a high degree +of dissimilarity may be reached in the simplest way by such a separation +of the components, and by their combination into most diverse smaller +groups. A very nice instance of such an analysis of flower-colors is +afforded by the ordinary snapdragon. The beautiful brown red color of +this common garden-plant is composed on one side of yellow elements, on +the other of red units. Of the yellow there are two, one staining the +whole corolla with a light hue, as is to be seen in the pure yellow +variety called _luteum. This form has been produced by the loss of the +whole group of the red constituents. If the yellow tinge is also lost, +there arises a white variety, but this is not absolutely colorless, but +shows the other yellow constituent. This last stains only some small +parts [151] of the lips of the flower around the throat, brightening, as +it seems, the entrance for the visiting insects. In many of the red or +reddish varieties this one yellow patch remains, while the general +yellow hue fails. In the variety called "Brilliant" the yellow ground +makes the red color more shiny, and if it is absent the pure carmine +tinge predominates. + +It is readily seen, that in the ordinary form the lips are of a darker +red than the tube. This evident dissimilarity indicates some complexity. +And in fact we have two varieties which exhibit the two causes of this +attribute separately. One of them is called "Delila," and has the red +color limited to the lips, whilst the tube is pure white. The other is +called "Fleshy," and is of a pale pink throughout the whole corolla. +Adding these two units to one another, we get the original dark red of +the wild type, and it may be briefly stated here, that the way of +effecting such an addition is given us in the crossing of the "Fleshy" +and the "Delila" variety, the hybrid showing the two colors and +returning thereby to the old prototype. + +Other cases of compound flower colors or of color patterns might be +given as in the _Mimulus_ and the poppy, and in most of these cases some +varieties are to be seen in our gardens which show only the single +constituents of the group. + +[152] Many dark flowers have an intermediate bright hued form besides +the white variety, as in the case of roses, asters, _Nicandra_ and so +on. + +Intermediate forms with respect to stature may also be seen. The +opium-poppy, the snapdragon, peas, the _Nicandra_, and many other +garden-plants have not only dwarf varieties, but also some of +intermediate height. These, though they are intermediate between the +tall and dwarf types, cannot be considered as transitions, as between +them and the extremes, intermediates are, as a rule wholly lacking. +Instances of the same occurrence of three types may be seen in the seeds +of maize ("Cuzco," "Horse-dent" and "Gracillima") of beans and some +other plants. The _Xanthium Wootoni_, above referred to, with only part +of the prickles of Xanthium commune is also a very curious instance of +the demonstration of the compound nature of a character. + +Summarizing the conclusions that may be drawn from the evidence given in +this lecture, we have seen that varieties differ from elementary species +in that they do not possess anything really new. They originate for the +greater part in a negative way, by the apparent loss of some quality, +and rarely in a positive manner by acquiring a character, already seen +in allied species. These characters are not of the nature of [153] +morphologic entities, but are to be considered as physiologic units, +present in all parts of the organisms, and manifesting themselves where +ever occasion is afforded. They are units in the sense that they may +appear and disappear singly. But very often they are combined to yield +compound characters, which are capable of analysis. Opportunities for +such an analysis are afforded by these groups of cultivated varieties, +of which some members show a single distinguishing quality, or a number +of them. + + +[154] + +LECTURE VI + +STABILITY AND REAL ATAVISM + +It is generally believed that varieties are principally distinguished +from species by their inconstancy. This conception is derived from some +special cases and transferred to others, and in its common form this +belief must have originated from the confusion which exists as to the +meaning of the term variety. It is true that vegetative varieties as a +rule run back, when propagated by seeds; they are an obvious instance of +inconstancy. In the second place we have considered the group of +inconstant or sporting varieties, which of course we must exclude when +studying the stability of other types. However, even these sporting +varieties are unstable only to a certain degree, and in a broader sense +will prove to be as true to their character as the most constant types. + +Having separated these two groups, which include also the wide range of +hybrid forms, we may next consider only those varieties of pure origin, +and ordinarily propagated by seeds, [155] which have been discussed in +former chapters. Their general character lies in their fidelity to type, +and in the fact that this is single, and not double, as in the sporting +varieties. + +But the current belief is, that they are only true to their +peculiarities to a certain degree, and that from time to time, and not +rarely, they revert to the type from which they have arisen. Such +reversion is supposed to prove that they are mere varieties, and at the +same time to indicate empirically the species from which they have +sprung. + +In the next lecture we shall examine critically the evidence on which +this assumption rests. Before doing so however, it will be necessary to +collate the cases in which there is no reversion at all, or in which the +reversion is absent at least in experimental and pure sowings. + +In the present state of our knowledge it is very difficult to decide, +whether or not true reversion occurs in constant varieties. If it does +occur, it surely does so very rarely and only under unusual +circumstances, or in particular individuals. However when such +individuals are multiplied by buds and especially when they are the only +representatives of their type, the reversion, though theoretically rare, +will be shown by nearly every specimen of the variety. Examples of this +will be given below. + +[156] They are generally called atavists or reversionists, but even +these terms are sometimes used in a different sense. + +Lastly it is to be said that the empirical and experimental evidence as +to the question of constancy is not as extensive as it should be. The +experimental conditions are seldom described, and it is only recently +that an interest in the matter has been awakened. Much remains to be +done. Among other things the innumerable varieties of trees, shrubs and +perennial herbs should be tested as to their constancy when grown from +purely fertilized seeds. Many of them may be included among the number +that sport constantly. + +Leaving aside the doubtful or insufficiently studied cases, we may now +turn our attention to the facts that prove the absolute stability of a +large number of varieties, at least as far as such completeness can be +attained by experiment or observation. + +The best proof is afforded by the varieties which grow wild in +localities where they are quite isolated from the species, and where for +this reason, no possibility of crossing disturbs the significance of the +proof. As one instance the rayless form of the wild camomile, or the +_Matricaria Chamomilla discoidea_ may be mentioned. Many systematists +have been so strongly [157] impressed with its absolute constancy and +its behavior as an ordinary species, that they have elevated it, as it +is called, to the rank of a species. As such it is described under the +name of _Matricaria discoidea_ DC. It is remarkable for its rapid and +widespread distribution, as of late years it has become naturalized in +different parts of America and of Europe, where it is to be seen +especially in France and in Norway. Experimentally I raised in +succeeding years between 1000 and 2000 seedlings, but observed no trace +of reversion, either in the strongest or in the numerous very small and +weak individuals which appeared in the cultures. + +The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a second +instance. It is a perennial herb with short rootstocks and stout stems +bearing numerous short-peduncled heads in large compact corymb; it +multiplies itself abundantly by seeds and is very common on the sand +dunes of Holland. It has two forms, differing only in the occurrence or +the lack of the ray florets. But these two varieties occupy different +localities and are even limited to different provinces. As far as I have +been able to ascertain on numerous excursions during a series of years, +they never sport, and are only intermingled on the outskirts of their +habitats. The rayless form is generally considered as the [158] variety +but it is quite as stable as the radiate species. + +The radiate varieties of marigold, quoted in a former lecture, seem to +be equally constant, when growing far away from their prototypes. I +sowed the seeds of a single plant of the radiate form of _Bidens +cernua_, and found all of the seedlings came true, and in the next year +I had from their seed between 2,000 and 3,000 flowering individuals, all +equally radiate. Many species of composites have been tried, and they +are all constant. On the other hand rare sports of this kind have been +observed by Murr and other authors. + +Many kinds of vegetables and of fruits give instances of stability. +White strawberries, green grapes, white currants, crisped lettuce, +crisped parsley and some other crisped forms may be cited. The spinage +without prickles is a widely known instance. White-flowered flax never +reverts to the blue prototype, if kept pure. Sugar-peas and sugar-corn +afford further instances. Strawberries without runners have come true +from seed ever since their first appearance, over a hundred years ago. + +Many garden-varieties, the stability of which under ordinary +circumstances is doubtful, because of their being sown too close to +other varieties of the same species, have been tested in [159] respect +to their stability by different writers and at different times. In doing +this it is plain that it is very essential to be sure of the purity of +the seed. Specimens must be grown in positions isolated from their +allies, and if possible be pollinated artificially with the exclusion of +the visits of insects. This may be done in different ways. If it is a +rare species, not cultivated in the neighborhood, it is often sufficient +to make sure of this fact. Pollen may be conveyed by bees from distances +of some ten or twenty meters, or in rare cases from some hundred meters +and more, but a greater distance is ordinarily sufficient for isolation. +If the flowers fertilize themselves, as is more often the case than is +generally supposed, or if it is easy to pollinate them artificially, +with their own pollen or in small groups of similar individuals, the +best way is to isolate them by means of close coverings. When flowering, +the plants are as a rule too large to be put under bell-glasses, and +moreover such coverings would keep the air moist, and cause the +flower-buds to be thrown off. The best coverings are of netting, or of +canvas of sufficiently wide mesh, although after a long experience I +greatly prefer cages of fine iron-wire, which are put around and over +the whole plant or group of plants, and fastened securely and tightly to +the ground. + +[160] Paper bags also may be made use of. They are slipped over the +flowering branches, and bound together around the twigs, thus enclosing +the flowers. It is necessary to use prepared papers, in order that they +may resist rain and wind. The best sort, and the one that I use almost +exclusively in my fertilization-experiments, is made of parchment-paper. +This is a wood-pulp preparation, freed artificially from the so-called +wood-substance or lignin. Having covered the flowers with care, and +having gathered the seeds free from intermixtures and if possible +separately for each single individual, it only remains to sow them in +quantities that will yield the greatest possible number of individuals. +Reversions are supposed to be rare and small groups of seedlings of +course would not suffice to bring them to light. Only sowings of many +hundreds or thousands of individuals are decisive. Such sowings can be +made in one year, or can be extended over a series of years and of +generations. Hildebrand and Hoffman have preferred the last method, and +so did Hofmeister and many others. Hildebrand sowed the white hyacinth, +and the white varieties of the larkspur, the stock and the sweet pea. +Hoffman cultivated the white flax and many other varieties and +Hofmeister extended his sowings [161] over thirty years with the white +variety of the yellow foxglove (_Digitalis parviflora_). White-flowered +varieties of perennial garden plants were used in my own experiments. I +bought the plants, flowered them under isolation in the way described +above, gathered the seeds from each individual separately and sowed them +in isolated groups, keeping many hundreds and in some cases above a +thousand plants up to the time of flowering. Among them I found only one +inconstant variety, the white form of the yellow columbine, _Aquilegia +chrysantha_. It evidently belonged to the group of sporting varieties +already referred to. All others came absolutely true to type without any +exception. The species experimented with, were _Campanula persicifolia_, +_Hyssopus officinalis_, _Lobelia syphilitica_, _Lychnis chalcedonica_, +_Polemonium dissectum_, _Salvia sylvestris_ and some others. Tested in +the same way I found the white varieties of the following annual plants +also quite true: _Chrysanthemum coronarium_, _Godetia amoena_, _Linum +usitatissimum_, _Phlox drummondi_, and _Silene Armeria_. To these may be +added the white hemlock stork's-bill (_Erodium cicutarium album_) which +grows very abundantly in some parts of my fatherland, and is easily +recognizable by its pure green leaves and stems, even when not +flowering. I cultivated it, in large numbers [162] during five +succeeding generations, but was never able to find even the slightest +indication of a reversion to the red prototype. The scarlet pimpernel or +_Anagallis arvensis_ has a blue variety which is absolutely constant. +Even in Britton and Brown's "Flora," which rarely enumerates varieties, +it is mentioned as being probably a distinct species. Eight hundred +blooming seedlings were obtained from isolated parents, all of the same +blue color. The New Zealand spinage (_Tetragonia expansa_) has a +greenish and a brownish variety, the red color extending over the whole +foliage, including the stems and the branches. I have tried both of them +during several years, and they never sported into each other. I raised +more than 5,000 seedlings, from the different seeds of one lot of the +green variety in succeeding years, but neither those germinating in the +first year, nor the others coming into activity after two, three or four +years of repose gave any sign of the red color of the original species. + +It is an old custom to designate intermediate forms as hybrids, +especially when both the types are widely known and the intermediates +rare. Many persons believe that in doing so, they are giving an +explanation of the rarer forms. But since the laws of hybridism are +coming to be known we shall have to break with [163] all such usages. So +for instance there are numerous flowers which are of a dark red or a +dark blue color, and which, besides a white variety, have a pink or a +pale blue form. Such pale varieties are of exactly the same value as +others, and on testing they are found to be equally stable. So for +instance the pink variety of the Sweet William (_Silene Armeria rosea_), +the _Clarkia pulchella carnea_ and the pale variety of the corn-cockle, +called usually _Agrostemma Githago nicaeensis_ or even simply _A. +nicaeensis_. The latter variety I found pure during ten succeeding +generations. Another notable stable intermediate form is the poppy +bearing the Danish flag (_Papaver somniferum Danebrog_). It is an old +variety, and absolutely pure when cultivated separately. A long list of +other instances might easily be given. + +Many garden-varieties, that are still universally prized and cultivated +are very old. It is curious to note how often such forms have been +introduced as novelties. The common foxglove is one of the best +examples. It has a monstrous variety, which is very showy because it +bears on the summit of its raceme and branches, large erect cup-shaped +flowers, which have quite a different aspect from the normal +thimbleshaped side-blossoms. These flowers are ordinarily described as +belonging to the anomaly [164] known as "peloria," or regular form of a +normally symmetric type; they are large and irregular on the stems and +the vigorous branches but slender and quinate on the weaker twigs. Their +beauty and highly interesting anomalous character has been the cause of +their being described many times, and nearly always as a novelty; they +have been recently re-introduced into horticulture as such, though they +were already cultivated before the middle of the last century. About +that time very good descriptions with plates were published in the +journal "Flora" by Vrolik, but afterwards they seem to have been +forgotten. The peloric variety of the foxglove always comes true from +seed, though in the strict sense of the word which we have chosen for +our discussion, it does not seem to be a constant and pure variety. + +It is very interesting to compare old botanical books, or even old +drawings and engravings containing figures of anomalous plants. The +celebrated Pinacothec of Munich contains an old picture by Holbein +(1495-1543) representing St. Sebastian in a flower-garden. Of the plants +many are clearly recognizable, and among others there is one of the +"one-leaved" variety of the strawberry, which may still be met with in +botanical gardens. In the year 1671 a Dutch botanist, Abraham Munting +published [165] a large volume on garden-plants, containing a great +number of very good engravings. Most of them of course show normal +plants, but intermixed with these are varieties, that are still in +cultivation and therefore must be at least two centuries old. Others, +though not figured, are easily recognized by their names and +descriptions. The cockscomb is the most widely known, but many white or +double flowered varieties were already cultivated at that time. The +striped Jalappa, the crested Sedum, the fasciated crown-imperial, white +strawberries, red gooseberries and many others were known to Munting. + +Some varieties are as old as culture itself, and it is generally known +that the Romans cultivated the white form of the opium-poppy and used +the foliage of the red variety of the sugarbeet as a vegetable. + +In our time flowers and fruits are changing nearly as rapidly as the +fancies and tastes of men. Every year new forms are introduced and usurp +the place of older ones. Many are soon forgotten. But if we look at old +country gardens, a goodly number of fine and valued old sorts are still +to be found. It would be worth while to make special collections of +living plants of old varieties, which surely would be a good and +interesting work and bring about a conviction [166] of the stability of +pure strains. Coming now to the other side of the question, we may +consider those cases of reversion which have been recorded from time to +time, and which always have been considered as direct proofs of the +varietal character of the reverting form. Reversion means the falling +back or returning to another type, and the word itself expresses the +idea that this latter type is the form from which the variety has +arisen. + +Some instances of atavism of this kind are well known, as they are often +repeated by individuals that are multiplied by buds or by grafting. +Before looking attentively into the different features of the many cases +of rare reversions it will be advisable to quote a few examples. + +The flowering-currant of the Pacific Coast or North American scarlet +ribes (_Ribes sanguineum_), a very popular ornamental shrub, will serve +as a good example. It is prized because of its brilliant red racemes of +flowers which blossom early in the spring, before the appearance of the +leaves. From this species a white form has arisen, which is an old and +widely cultivated one, but not so highly prized because of its pale +flowers. These are not of a pure white, but have retained a faint +reddish hue. The young twigs and the stalks of the [167] leaves afford +an instance of correlated variability since in the species the red color +shows itself clearly mixed with the green, while in the variety this +tinge is wholly wanting. + +Occasionally this white-flowered currant reverts back to the original +red type and the reversion takes place in the bud. One or two buds on a +shrub bearing perhaps a thousand bunches of white flowers produce twigs +and leaves in which the red pigment is noticeable and the flowers of +which become brightly colored. If such a twig is left on the shrub, it +may grow further, ramify and evolve into a larger group of branches. All +of them keep true to the old type. Once reverted, the branches remain +forever atavistic. It is a very curious sight, these small groups of red +branches among the many white ones. And for this reason attention is +often called to it, and more than once I myself have had the opportunity +of noting its peculiarities. It seems quite certain that by planting +such shrubs in a garden, we may rely upon seeing sooner or later some +new buds reverting to the prototype. + +Very little attention seems hitherto to have been given to this curious +phenomenon, though in many respects it deserves a closer investigation. +The variety is said to have originated from seed in Scotland, many years +ago, and [168] seems to be propagated only by cuttings or by grafting. +If this is true, all specimens must be considered as constituting +together only one individual, notwithstanding their wide distribution in +the gardens and parks of so many countries. This induces me to suppose, +that the tendency to reversion is not a character of the variety as +such, but rather a peculiarity of this one individual. In other words it +seems probable that when the whitish variety arises a second time from +the red species, it is not at all necessary that it should exhibit this +same tendency to revert. Or to put it still in another way, I think that +we may suppose that a variety, which might be produced repeatedly from +the same original stock, would only in rare individuals have a tendency +to revert, and in most cases would be as absolutely constant as the +species itself. + +Such a conception would give us a distinct insight into the cause of the +rarity of these reversions. Many varieties of shrubs and trees have +originated but once or twice. Most of them must therefore, if our +supposition is correct, be expected to be stable and only a few may be +expected to be liable to reversions. + +Among the conifers many very good cases of reversions by buds are to be +found in gardens and glasshouses. They behave exactly like the whitish +currant. But as the varietal characters [169] are chiefly found in the +foliage and in the branches, these aberrations are to be seen on the +plants during the whole year. Moreover they are in some cases much more +numerous than in the first instance. The _Cryptomeria_ of Japan has a +variety with twigs resembling ropes. This is not caused by a twisting, +but only by a curvature of the needles in such a way that they seem to +grow in spiral lines around the twigs. This variety often reverts to the +type with widely spread, straight needles. And on many a specimen four, +five, or more reverted branches may be seen on different parts of the +same shrub. Still more widely cultivated is the shrub called +_Cephalotaxus pedunculata fastigiata_, and more commonly known under its +old name of _Podocarpus koraiana_. It is the broomlike variety of a +species, nearly allied to the common American and European species of +yew, (_Taxus minor_ and _T. baccata_). It is a low shrub, with broadly +linear leaves of a clear green. In the species the leaves are arranged +in two rows, one to the left and one to the right of the horizontally +growing and widely spreading branches. In the variety the branches are +erect and the leaves inserted on all sides. When sporting, it returns to +the bilateral prototype and flat wings of fan-shaped twigs are produced +laterally on its dense broom-like tufts. + +[170] Wherever this variety is cultivated the same reversion may be +seen; it is produced abundantly, and even under seemingly normal +circumstances. But as in the case of the _Ribes_ all the specimens are +derived by buds from a single original plant. The variety was introduced +from Japan about the year 1860, but is probably much older. Nothing is +known as to its real origin. It never bears flowers or fruits. It is +curious to note that the analogous variety of the European yew, _Taxus +baccata fastigiata_, though much more commonly cultivated than the +_Cephalotaxus_, never reverts, at least as far as I have been able to +ascertain. This clearly corroborates the explanation given above. + +After considering these rare instances of more widely known reversions, +we may now examine the question of atavism from a broader point of view. +But in doing so it should once more be remembered, that all cases of +hybridism and also all varieties sporting annually or frequently, are to +be wholly excluded. Only the very rare occurrence of instances of +atavism in varieties that are for the rest known to be absolutely +constant, is to be considered. + +Atavism or reversion is the falling back to a prototype. But what is a +prototype? We may take the word in a physiologic or in a systematic +sense. Physiologically the signification is a [171] very narrowly +restricted one; and includes only those ancestors from which a form is +known to have been derived. But such evidence is of course historic. If +a variety has been observed to spring from a definite species, and if +the circumstances have been sufficiently ascertained not to leave the +slightest doubt as to its pure origin, and if moreover all the evidence +has been duly recorded, we may say that the origin of the variety is +historically known. In most cases we must be content with the testimony, +given somewhat later, and recorded after the new variety had the +opportunity of showing its greater merits. + +If it now happens that such a variety of recorded origin should +occasionally revert to its parent-species, we have all we can wish for, +in the way of a thoroughly proved case of atavism. But such instances +are very rare, as the birth of most varieties has only been very +imperfectly controlled. + +Next to this comes the systematic relation of a variety to its species. +The historic origin of the variety may be obscure, or may simply be +forgotten. But the distinguishing marks are of the order described in +our last lecture, either in the positive or in the negative direction, +and on this ground the rarer form is considered to be a variety of the +more wide-spread one. If [172] now the presumed variety sports and runs +over to the presumed type, the probability of the supposed relation is +evidently enhanced. But it is manifest that the explanation rests upon +the results of comparative studies, and not upon direct observations of +the phenomena themselves. + +The nearer the relations between the two types in question, the less +exposed to doubt and criticism are the conclusions. But the domain of +atavism is not restricted to the cases described. Quite on the contrary +the facts that strike us most forcibly as being reversions are those +that are apt to give us an insight into the systematic affinity of a +higher degree. We are disposed to make use of them in our attempts to +perfect the natural system and to remould it in such a way as to become +a pedigree of the related groups. Such cases of atavism no doubt occur, +but the anomalies referred to them must be interpreted merely on the +ground of our assumptions as to the relative places in the system to be +assigned to the different forms. + +Though such instances cannot be considered as belonging strictly to the +subject we are dealing with, I think it may be as well to give an +example, especially as it affords an occasion for referring to the +highly important researches of Heinricher on the variability and +atavistic [173] tendencies of the pale blue flag or _Iris pallida_. The +flowers of the blue flags have a perianth of six segments united below +into a tube. The three outer parts are dilated and spreading, or +reflexed, while the three inner usually stand erect, but in most species +are broad and colored like the outer ones. Corresponding to the outer, +perianth-segments are the three stamens and the three, petal-like +divisions of the style, each bearing a transverse stigma immediately +above the anther. They are pollinated by bumble-bees, and in some +instances by flies of the genus _Rhingia_, which search for the honey, +brush the pollen out of the anthers and afterwards deposit it on the +stigma. According to systematic views of the monocotyledons the original +prototype of the genus _Iris_ must have had a whorl of six equal, or +nearly equal perianth-segments and six stamens, such as are now seen in +the more primitive types of the family of the lilies, as for instance in +the lilies themselves, the tulips, hyacinths and others. As to the +perianth this view is supported by the existence of one species, the +_Iris falcifolia_, the perianth of which consists of six equal parts. +But species with six stamens are wholly lacking. Heinricher however, in +cultivating some anomalous forms of _Iris pallida_, succeeded in filling +out this gap and in producing [174] flowers with a uniform perianth and +six stamens, recalling thereby the supposed ancestral type. The way in +which he got these was as follows: he started from some slight +deviations observed in the flowers of the pale species, sowed the seeds +in large numbers and selected from the seedlings only those which +clearly showed anomalies in the expected atavistic direction. By +repeating this during several generations he at last reached his goal +and was able to give reality to the prototype, which formerly was only a +hypothetical one. The _Iris kaempferi_, a large-flowered Japanese +species much cultivated in gardens, is very variable in the number of +the different parts of its flowers, and may in some instances be seen +even with six stamens. If studied in the same way as Heinricher's iris, +it no doubt will yield highly interesting and confirmatory results. + +Many other instances of such systematic atavism could be given, and +every botanist can easily add some from memory. Many anomalies, +occurring spontaneously, are evidently due to the same principle, but it +would take too long to describe them. + +Reversion may occur either by buds or by seeds. It is highly probable +that it occurs more readily by sexual than by asexual propagation. But +if we restrict the discussion to the limits [175] hitherto observed, +seed-reversions must be said to be extremely rare. Or rather cases which +are sufficiently certain to be relied upon, are very rare, and perhaps +wholly lacking. Most of the instances, recorded by various writers, are +open to question. Doubts exist as to the purity of the seeds and the +possibility of some unobserved cross disturbing the results. + +In the next lecture we shall deal in general with the ordinary causes +and results of such crosses. We shall then see that they are so common +and occur so regularly under ordinary circumstances that we can never +rely on the absolute purity of any seeds, if the impossibility of an +occasional cross has not been wholly excluded, either by the +circumstances themselves, or by experimental precautions taken during +the flowering period. + +For these reasons cases of atavism given without recording the +circumstances, or the precautions that guarantee the purity of the +fertilization, should always be disregarded. And moreover another proof +should always be demanded. The parent which yielded the seeds might be +itself a hybrid and liable to reversions by the ordinary laws of the +splitting up of hybrids. Such cases should likewise be discarded, since +they bring in confusing elements. If we review the long list of recorded +cases by these [176] strict methods of criticism very few instances will +be found that satisfy legitimate demands. On this ground it is by far +safer in the present state of our knowledge, to accept bud-variations +only as direct proofs of true atavism. And even these may not always be +relied on, as some hybrids are liable to split up in a vegetative way, +and in doing so to give rise to bud-variations that are in many respects +apparently similar to cases of atavism. But fortunately such instances +are as yet very rare. + +After this discussion it would be bold indeed to give instances of +seed-atavism, and I believe that it will be better to refrain wholly +from doing so. + +Many instances of so-called atavism are of purely morphologic nature. +The most interesting cases are those furnished by the forms which some +plants bear only while young, and which evidently connect them with +allied species, in which the same features may be seen in the adult +state. Some species of the genus _Acacia_ bear bipinnate leaves, while +others have no leaves at all, but bear broadened and flattened petioles +instead. The second type is presumed to be descended from the first by +the loss of the leaflets and the modification of the stalks into flat +and simple phyllodes. But many of them are liable to recall this +primitive form [177] when very young, in the first two or three, or +sometimes in eight or ten primary leaves. These leaves are small because +of the weakness of the young plant and therefore often more or less +reduced in structure. But they are usually strictly bipinnate and +thereby give testimony as to their descent from species which bear such +leaves throughout their life. + +Other similar cases could be given, but this will suffice. They once +more show how necessary it is to separate the different cases, thrown +together until now, under this general name of atavism. It would be far +better to give them all special names, and as long as these are not +available we must be cautious not to be misguided by the name, and +especially not to confuse different phenomena with one another, because +at the present time they bear the same names. + +Taking into consideration the relatively numerous restrictions resulting +from this discussion, we will now make a hasty survey of some of the +more notable and generally acknowledged cases of atavism by +bud-propagation. But it should be repeated once more that most of the +highly cultivated plants, grown as vegetables, or for their fruit or +flowers, have so many crosses in their ancestry, that it seems better to +exclude them from all considerations, in which purity of [178] descent +is a requisite. By so doing, we exclude most of the facts which were +until now generally relied upon. For the roses, the hyacinths, the +tulips, the chrysanthemums always have furnished the largest +contributions to the demonstrations of bud-variation. But they have been +crossed so often, that doubt as to the purity of the descent of any +single form may recur, and may destroy the usefulness of their many +recorded cases of bud-variation for the demonstration of real atavism. +The same assertion holds good in many other cases, as with _Azalea_ and +_Camellia_. And the striped varieties of these genera belong to the +group of ever-sporting forms, and therefore will be considered later on. +So it is with carnations and pinks, which occasionally vary by layering, +and of which some kinds are so uncertain in character that they are +called by floriculturists "catch-flowers." On the other hand there is a +larger group of cases of reversion by buds, which is probably not of +hybrid nature, nor due to innate inconstancy of the variety, but must be +considered as pure atavism. I refer to the bud-variations of so many of +our cultivated varieties of shrubs and trees. Many of them are +cultivated because of their foliage. They are propagated by grafting, +and in most cases it is probable that all the numerous specimens [179] +of the same variety have been derived in this way from one primitive, +aberrant individual. We may disregard variegated leaves, spotted or +marked with white or yellow, because they are too inconstant types. + +We may next turn our attention to the varieties of trees with cut +leaves, as the oakleaved _Laburnum_, the parsley-leaved vine and the +fern-leaved birch. Here the margin of the leaves is deeply cut and +divided by many incisions, which sometimes change only the outer parts +of the blade, but in other cases may go farther and reach, or nearly +reach, the midvein, and change the simple leaf into a seemingly compound +structure. The anomaly may even lead to the almost complete loss of all +the chorophyll-tissue and the greater part of the lateral veins, as in +the case of the cut-leaved beech or _Fagus sylvatica pectinata_. + +Such varieties are often apt to revert by buds to the common forms. The +cut-leaved beech sometimes reverts partially only, and the branches +often display the different forms of cut-leaved, fern-like, oak-leaved +and other variously shaped leaves on the same twigs. But this is merely +due to the wide variability of the degree of fissure and is to be +considered only as a fluctuation between somewhat widely distant +extremes, which may even apparently include [180] the form of the common +beech-leaves. It is not a bud-variation at all, and it is to be met with +quite commonly while the true reversions by buds are very rare and are +of the nature of sports appearing suddenly and remaining constant on the +same twig. Analogous phenomena of wide variability with true reversion +may be seen in the variety of the European hornbeam called _Carpinus +Betulus heterophylla_. The leaves of this tree generally show the +greatest diversity in form. Some other cases have been brought together +by Darwin. In the first place a subvariety of the weeping-willow with +leaves rolled up into a spiral coil. A tree of this kind kept true for +twenty-five years and then threw out a single upright shoot bearing flat +leaves. The barberry (_Berberis_) offers another case; it has a well +known variety with seedless fruit, which can be propagated by cuttings +or layers, but its runners are said always to revert to the common form, +and to produce ordinary berries with seeds. Most of the cases referred +to by Darwin, however, seem to be doubtful and cannot be considered as +true proofs of atavism until more is known about the circumstances under +which they were produced. + +Red or brown-leaved varieties of trees and shrubs also occasionally +produce green-leaved branches, and in this way revert to the type [181] +from which they must evidently have arisen. Instances are on record of +the hazel, _Corylus Avellana_, of the allied _Corylus tubulosa_, of the +red beech, the brown birch and of some other purple varieties. Even the +red bananas, which bear fruits without seeds and therefore have no other +way of being propagated than by buds, have produced a green variety with +yellow fruits. The _Hortensia_ of our gardens is another instance of a +sterile form which has been observed to throw out a branch with cymes +bearing in their center the usual small staminate and pistillate flowers +instead of the large radiate and neutral corollas of the variety, +thereby returning to the original wild type. Crisped weeping-willows, +crisped parsley and others have reverted in a similar manner. + +All such cases are badly in need of a closer investigation. And as they +occur only occasionally, or as it is commonly stated, by accident, the +student of nature should be prepared to examine carefully any case which +might present itself to him. Many phases of this difficult problem could +no doubt be solved in this way. First of all the question arises as to +whether the case is one of real atavism, or is only seemingly so, being +due to hybrid or otherwise impure descent of the varying individual, and +secondly whether it may be only an instance of the regularly [182] +occurring so-called atavism of the sporting varieties with which we +shall deal in a later lecture. If it proves to be real atavism and rare, +the case should be accurately described and figured, or photographed if +possible; and the exact position of the reverting bud should be +ascertained. Very likely the so-called dormant or resting buds are more +liable to reversions than the primary ones in the arils of the leaves of +young twigs. Then the characters of the atavistic branches should be +minutely compared with those of the presumed ancestor; they may be quite +identical with them or slightly divergent, as has been asserted in some +instances. The atavism may be complete in one case, but more or less +incomplete in others. By far the most interesting point is the question, +as to what is to be expected from the seeds of such an atavistic branch. +Will they keep true to the reverted character, or return to the +characters of the plant which bears the retrograde branch? Will all of +them do so, or only part of them, and how large a part? It is very +astonishing that this question should still be unsolved where so many +individual trees bear atavistic branches that remain on them through +long series of years. But then many such branches do not flower at all, +or if they flower and bear seed, no care is taken to prevent [183] +cross-fertilization with the other flowers of the same plant, and the +results have no scientific value. For anyone who cares to work with the +precautions prescribed by science, a wide field is here open for +investigation, because old reverted branches may be met with much less +rarely than new ones. + +Finally the possibility is always to be considered that the tendency to +bud-reversions may be a special feature of some individuals, and may not +be met with in others of the same variety. I have spoken of this before. +For the practical student it indicates that a specimen, once observed to +produce atavistic buds, may be expected to do the same thing again. And +then there is a very good chance that by combining this view with the +idea that dormant buds are more apt to revert than young ones, we may +get at a method for further investigation, if we recur to the practice +of pruning. By cutting away the young twigs in the vicinity of dormant +buds, we may incite these to action. Evidently we are not to expect that +in so doing they will all become atavistic. For this result is not at +all assured; on the contrary, all that we might hope to attain would be +the possibility of some of them being induced to sport in the desired +direction. + +Many questions in scientific research can only [184] be answered by long +and arduous work in well equipped laboratories; they are not to be +attempted by every one. But there are other problems which the most +complete of institutions are not able to study if opportunity is not +offered them, and such opportunities are apt to occur more often in +fields, gardens, parks, woods and plains, than in the relatively small +experimental gardens of even the largest institution. Therefore, +whosoever has the good fortune to find such sports, should never allow +the occasion to pass without making an investigation that may bring +results of very great importance to science. + + + +[185] + +LECTURE VII + +FALSE ATAVISM OR VICINISM + +About the middle of the last century Louis de Vilmorin showed that it +was possible to subject plants to the methods of amelioration of races +then in use for domestic animals, and since that time atavism has played +a large part in all breeding-processes. It was considered to be the +greatest enemy of the breeder, and was generally spoken of as a definite +force, working against and protracting the endeavors of the +horticulturist. + +No clear conception as to its true nature had been formulated, and even +the propriety of designating the observed phenomena by the term atavism +seemed doubtful. Duchesne used this word some decades ago to designate +those cases in which species or varieties revert spontaneously, or from +unknown internal causes, to some long-lost characters of their +ancestors. Duchesne's definition was evidently a sharp and useful one, +since it developed for the first time the idea of latent or dormant +qualities, [186] formerly active, and awaiting probably through +centuries an occasion to awaken, and to display the lost characters. + +Cases of apparent reversion were often seen in nurseries, especially in +flower culture, which under ordinary circumstances are rarely wholly +pure, but always sport more or less into the colors and forms of allied +varieties. Such sporting individuals have to be extirpated regularly, +otherwise the whole variety would soon lose its type and its uniformity +and run over to some other form in cultivation in the vicinity. For this +reason atavism in nurseries causes much care and labor, and consequently +is to be dealt with as a very important factor. + +From time to time the idea has suggested itself to some of the best +authorities on the amelioration of plants, that this atavism was not due +to an innate tendency, but, in many cases at least, was produced by +crosses between neighboring varieties. It is especially owing to Verlot +that this side of the question was brought forward. But breeders as a +rule have not attached much importance to this supposition, chiefly +because of the great practical difficulties attending any attempt to +guard the species of the larger cultures against intermixture with other +varieties. Bees and humble-bees fly from bud to bud, and carry the +pollen from one [187 ] sort to another, and separation by great +distances would be required to avoid this source of impurity. +Unfortunately the arrangements and necessities of large cultures make it +impossible to isolate the allied varieties from each other. + +From a theoretical point of view the origin of these impurities is a +highly important question. If the breeders' atavism is due to crosses, +and only to this cause, it has no bearing at all on the question of the +constancy of varieties. And the general belief, that varieties are +distinguished from true species by their repeated reversion and that +even such reversibility is the real distinction of a variety, would not +hold. + +For this reason I have taken much trouble in ascertaining the +circumstances which attend this form of atavism. I have visited a number +of the leading nurseries of Europe, tested their products in various +ways, and made some experiments on the unavoidable conditions of +hybridizing and on their effect on the ensuing generations. These +investigations have led me to the conclusion, that atavism, as it is +generally described, always or nearly always is due to hybridization, +and therefore it is to be considered as untrue or false atavism. + +True atavism, or reversion caused by an innate latent tendency, seems +to be very rare, [188] and limited to such cases as we have spoken of +under our last heading. And since the definition, given to this term by +its author, Duchesne, is generally accepted in scientific works, it +seems better not to use it in another sense, but rather to replace it in +such cases by another term. For this purpose I propose the word +vicinism, derived from the Latin vicinus or neighbor, as indicating the +sporting of a variety under the influence of others in its vicinity. +Used in this way, this term has the same bearing as the word atavism of +the breeders, but it has the advantage of indicating the true cause +thereof. + +It is well known that the term variability is commonly employed in the +broadest possible sense. No single phenomenon can be designated by this +name, unless some primary restriction be given. Atavism and vicinism are +both cases of variability, but in wholly different sense. For this +reason it may be as well, to insert here a short survey of the general +meanings to be conveyed by the term variation. It implies in the first +place the occurrence of a wide range of forms and types, irrespective of +their origin, and in the second place the process of the change in such +forms. In the first signification it is nearly identical with +polymorphy, or richness of types, especially so when these [189] types +are themselves quite stable, or when it is not at all intended to raise +the question of their stability. In scientific works it is commonly used +to designate the occurrence of subspecies or varieties, and the same is +the case in the ordinary use of the term when dealing with cultivated +plants. A species may consist of larger or smaller groups of such units, +and they may be absolutely constant, never sporting if hybridization is +precluded, and nevertheless it may be called highly variable. The +opium-poppy affords a good instance. It "varies" in height, in color of +foliage and flowers; the last are often double or laciniated; it may +have white or bluish seeds, the capsules may open themselves or remain +closed and so on. But every single variety is absolutely constant, and +never runs into another, when the flowers are artificially pollinated +and the visits of insects excluded. So it is with many other species. +They are at the same time wholly stable and very variable. + +The terms variation and variety are used frequently when speaking of +hybrids. By crossing forms, which are already variable in the sense just +mentioned, it is easy to multiply the number of the types, and even in +crossing pure forms the different characters may be combined in +different ways, the resulting combinations [190] yielding new, and very +often, valuable varieties. But it is manifest that this form of +variation is of quite another nature from the variations of pure races. +Many hybrid varieties are quite constant, and remain true to their type +if no further crosses are made; many others are artificially propagated +only in a vegetative way, and for this reason are always found true. +Hybrid varieties as a rule were formerly confused with pure varieties, +and in many instances our knowledge as to their origin is quite +insufficient for sharp distinctions. To every student of nature it is +obvious, that crossing and pure variability are wholly distinct groups +of phenomena, which should never be treated under the same head, or +under the same name. + +Leaving aside polymorphy, we may now discuss those cases of variability, +in which the changes themselves, and not only their final results play a +part. Of such changes two types exist. First, the ever-recurring +variability, never absent in any large group of individuals, and +determining the differences which are always to be seen between parents +and their children, or between the children themselves. This type is +commonly called "individual variability" and since this term also has +still other meanings, it has of late become customary to use instead the +term "fluctuating variability." [191] And to avoid the repetition of the +latter word it is called "fluctuation." In contrast to these +fluctuations are the so-called sports or single varieties, not rarely +denominated spontaneous variations, and for which I propose to use the +term "mutations." They are of very rare occurrence and are to be +considered as sudden and definite steps. + +Lastly, we have to consider those varieties, which vary in a much wider +range than the ordinary ones, and seem to fluctuate between two opposite +extremes, as for instance variegated leaves, cultivated varieties with +variegated or striped flowers, double flowers and some other anomalies. +They are eversporting and ever-returning from one type to the other. If +however, we take the group of these extremes and their intermediates as +a whole, this group remains constant during the succeeding generations. +Here we find once more an instance of the seemingly contradictory +combination of high variability and absolute constancy. It means that +the range of variability has quite definite limits, which in the common +course of things, are never transgressed. + +We may infer therefore that the word variability has such a wide range +of meanings that it ought never be used without explanation. [192] +Nothing indeed, is more variable than the signification of the term +variable itself. + +For this reason, we will furthermore designate all variations under the +influence of neighbors with the new and special term "vicinism." It +always indicates the result of crossing. + +Leaving this somewhat lengthy terminological discussion, we now come to +the description of the phenomenon itself. In visiting the plantations of +the seedsmen in summer and examining the large fields of garden-flowers +from which seed is to be gathered, it is very rare to find a plot quite +pure. On the contrary, occasional impurities are the rule. Every plot +shows anomalous individuals, red or white flowers among a field of blue, +normal among laciniated, single among double and so on. The most curious +instance is afforded by dwarf varieties, where in the midst of hundreds +and thousands of small individuals of the same height, some specimens +show twice their size. So for instance, among the dwarfs of the +larkspur, _Delphinium Ajacis_. + +Everywhere gardeners are occupied in destroying these "atavists," as +they call them. When in full bloom the plants are pulled up and thrown +aside. Sometimes the degree of impurity is so high, that great piles of +discarded plants of the same species lie about the [193] paths, as I +have seen at Erfurt in the ease of numerous varieties of the Indian +cress or _Tropaeolum_. + +Each variety is purified at the time when it shows its characters most +clearly. With vegetables, this is done long before flowering, but with +flowers only when in full bloom, and with fruits, usually after +fertilization has been accomplished. It needs no demonstration to show +that this difference in method must result in very diverging degrees of +purity. + +We will confine ourselves to a consideration of the flowers, and ask +what degree of purity may be expected as the result of the elimination +of the anomalous plants during the period of blooming. + +Now it is evident that the colors and forms of the flowers can only be +clearly distinguished, when they are fully displayed. Furthermore it is +impossible to destroy every single aberrant specimen as soon as it is +seen. On the contrary, the gardener must wait until all or nearly all +the individuals of the same variety have displayed their characters, as +only in this way can all diverging specimens be eliminated by a single +inspection. Unfortunately the insects do not wait for this selection. +They fertilize the flowers from the beginning, and the damage will have +been done [194] long before the day of inspection comes around. Crosses +are unavoidable and hybrid seeds will unavoidably come into the harvest. +Their number may be limited by an early eradication of the vicinists, or +by the elimination of the first ripe seeds before the beginning of the +regular harvest, or by other devices. But some degree of impurity will +remain under ordinary circumstances. + +It seems quite superfluous to give more details. In any case in which +the selection is not done before the blooming period, some impurities +must result. Even if it is done before that time, errors may occur, and +among hundreds and thousands of individuals a single anomalous one may +escape observation. + +The conclusion is, that flower seeds as they are offered in commerce, +are seldom found absolutely pure. Every gardener knows that he will have +to weed out aberrant plants in order to be sure of the purity of his +beds. I tested a large number of samples of seeds for purity, bought +directly from the best seed growers. Most of them were found to contain +admixtures and wholly pure samples were very rare. + +I will now give some illustrative examples. From seeds of a yellow +snapdragon, I got one red-flowered specimen among half a hundred [195] +yellow ones, and from the variety "Delila" of the same species two red +ones, a single white and two belonging to another variety called +"Firefly." _Calliopsis tinctoria_ has three varieties, the ordinary +type, a brown-flowered one and one with tubular rays. Seeds of each of +these three sorts ordinarily contain a few belonging to the others. +_Iberis umbellata rosea_ often gives some white and violet examples. The +"Swan" variety of the opium-poppy, a dwarfish double-flowered form of a +pure white, contained some single-flowered and some red-flowered plants, +when sown from commercial seed are said to be pure. But these were only +occasional admixtures, since after artificial fertilization of the +typical specimens the strain at once became absolutely pure, and +remained so for a series of generations, as long as the experiment was +continued. Seeds of trees often contain large quantities of impurities, +and the laciniated varieties of birch, elder and walnut have often been +observed to come true only in a small number of seedlings. + +In the case of new or young varieties, seed merchants often warn their +customers as to the probable degree of purity of the seeds offered, in +order to avoid complaints. For example the snow-white variety of the +double daisy, _Bellis perennis plena_, was offered at the start as +containing [196] as much as 20% of red-flowered specimens. + +Many fine varieties are recorded to come true from seed, as in the case +of the holly with yellow fruits, tested by Darwin. Others have been +found untrue to a relatively high degree, as is notorious in the case of +the purple beech. Seeds of the laciniated beech gave only 10% of +laciniated plants in experiments made by Strasburger; seeds of the +monophyllous acacia, _Robinia Pseud-Acacia monophylla_, were found to be +true in only 30% of the seedlings. Weeping ashes often revert to the +upright type, red May-thorns (_Crataegus_) sometimes revert nearly +entirely to the white species and the yellow cornel berry is recorded to +have reverted in the same way to the red berries of the _Cornus Mas_. + +Varieties have to be freed by selection from all such impurities, since +isolation is a means which is quite impracticable under ordinary +circumstances. Isolation is a scientific requirement that should never +be neglected in experiments, indeed it may be said to be the first and +most important requisite for all exact research in questions of +variability and inheritance. But in cultivating large fields of allied +varieties for commercial purposes, it is impossible to grow them at such +distances from each other [197] as to prevent cross-pollination by the +visits of bees. + +This purification must be done in nearly every generation. The oldest +varieties are to be subjected to it as well as the latest. There is no +regular amelioration, no slow progression in the direction of becoming +free from these admixtures. Continuous selection is indispensable to +maintain the races in the degree of purity which is required in +commerce, but it does not lead to any improvement. Nor does it go so far +as to become unnecessary in the future. This shows that there must be a +continuous source of impurities, which in itself is not neutralized by +selection, but of which selection can only eliminate the deteriorating +elements. + +The same selection is usually applied to new varieties, when they +occasionally arise. In this case it is called "fixing," as gardeners +generally believe that through selection the varieties are brought to +the required degree of purity. This belief seems to rest mainly on +observations made in practice, where, as we have seen, isolation is of +very rare application. Most varieties would no doubt be absolutely pure +from the first moment of their existence, if it were only possible to +have them purely fertilized. But in practice this is seldom to be +obtained. Ordinarily the breeder is content with such slow [198] +improvement as may be obtained with a minimum of cost, and this mostly +implies a culture in the same part of the nursery with older varieties +of the same species. Three, four or five years are required to purify +the novelty, and as this same length of time is also required to produce +sufficient quantities of seed for commercial purposes, there is no +strong desire to shorten the period of selection and fixation. I had +occasion to see this process going on with sundry novelties at Erfurt in +Germany. Among them a chamois-colored variety of the common stock, a +bluish _Clarkia elegans_ and a curiously colored opium-poppy may be +mentioned. In some cases the crossfertilization is so overwhelming, that +in the next generation the novelty seems entirely to have disappeared. + +The examples given may suffice to convey a general idea of the +phenomenon, ordinarily called atavism by gardeners, and considered +mostly to be the effect of some innate tendency to revert to the +ancestral form. It is on this conception that the almost universal +belief rests, that varieties are distinguished, as such, from species by +their inconstancy. Now I do not deny the phenomenon itself. The impurity +of seeds and cultures is so general and so manifest, and may so easily +be tested by every one [199] that it cannot reasonably be subjected to +any doubt. It must be conceded to be a fact, that varieties as a rule +revert to their species under the ordinary circumstances of commercial +culture. And I cannot see any reason why this fact should not be +considered as stating a principal difference between varieties and +species, since true species never sport into one another. + +My objection only refers to the explanation of the observed facts. +According to my view nearly all these ordinary reversions are due to +crosses, and it is for this reason that I proposed to call them by a +separate name, that of "vicinists." Varieties then, by means of such +spontaneous intercrossing sport into one another, while species either +do not cross, or when crossing produce hybrids that are otherwise +constituted and do not give the impression of atavistic reversion. + +I must not be content with proposing this new conception, but must give +the facts on which this assumption rests. These facts are the results of +simple experiments, which nevertheless are by no means easy to carry +out, as they require the utmost care to secure the absolute purity of +the seeds that are employed. This can only be guaranteed by previous +cultures of isolated plants or groups of plants, or by artificial +pollination. + +[200] Once sure of this preliminary condition, the experiment simply +consists in growing a variety at a given distance from its species and +allowing the insects to transfer the pollen. After harvesting the seed +thus subjected to the presumed cause of the impurities, it must be sown +in quantities, large enough to bring to light any slight anomaly, and to +be examined during the period of blooming. + +The wild seashore aster, _Aster Tripolium_, will serve as an example. It +has pale violet or bluish rays, but has given rise to a white variety, +which on testing, I have found pure from seed. Four specimens of this +white variety were cultivated at a distance of nearly 100 meters from a +large lot of plants of the bluish species. I left fertilization to the +bees, harvested the seeds of the four whites separately and had from +them the following year more than a thousand flowering plants. All of +them were of the purest white, with only one exception, which was a +plant with the bluish rays of the species, wholly reverting to its +general type. As the variety does not give such reversions when +cultivated in isolation, this sport was obviously due to some cross in +the former year. In the same way I tried the white Jacob's ladder, +_Polemonium coeruleum_ album in the neighborhood of the blue-flowered +species, the distance [202] in this case being only 40 meters. Of two +hundred seeds one became a blue atavist, or rather vicinist, while all +others remained true to the white type. The same was observed in the +white creeping thyme, or _Thymus Serpyllum album_, and the white +self-heal, _Brunella vulgaris alba_, gave even so much as 28% seedlings +with purple corollas out of some 400 specimens, after being cultivated +in close proximity to its parent-species. I have tried many other +species, but always with the same result. Such atavists only arise by +cultivation in the proximity of allied varieties, never in isolation. +They are not real atavists, but only vicinists. + +In order to show this yet more clearly, I made another experiment with +the white selfheal. I had a lot of the pinnate-leaved variety with +purple flowers and somewhat stouter stems, and cultivated single plants +of the whiteflowering sort at distances that varied from 2-16 meters. +The seeds of each plant were collected and sown separately, those of the +nearest gave up to 5 or 6 hybrids from the seeds of one parent, while +those of the farthest gave only one purple-flowered plant for each +parent. Evidently the chance of the pollen being carried by bees is much +greater on short than on longer distances. + +True hybrids between species may arise in quite the same way, and since +it is obviously impossible to attribute them to an innate tendency to +reversion, they afford an absolutely irrefutable proof of the assertion +that pollen is often brought by insects from one lot of plants to +another. In this way I obtained a hybrid between the common Jacob's +ladder and the allied species _Polemonium dissectum_. With a distance of +100 meters between them I had two hybrid seeds among a hundred of pure +ones. At a similar distance pollen was carried over from the wild +radish, _Raphanus Raphanistrum_, to the allied _Raphanus caudatus_, and +I observed the following year some very nice hybrids among my seedlings. +A hybrid-bean between _Phaseolus nanus_ and _P. multiflorus_, and some +hybrids between the yellow daisy, _Chrysanthemum segetum_ and the allied +_Chrysanthemum coronarium_ or ox-eye daisy which also arose +spontaneously in my garden between parents cultivated at recorded +distances, might further be noted. Further details of these experiments +need not be given. Suffice to say, that occasional crosses between +species do occur, and not even rarely, that they are easily recognized +as such and cannot be confused with cases of atavism, and that therefore +they give proof to the assumption that in the same way crosses +ordinarily occur also between varieties [203] of the same species, if +cultivated at small distances apart, say 40-50 meters or even more. +Vicinism therefore, may play a part in all such cultures, enough to +account for all the impurities observed in the nurseries or in +commercial seed-samples. + +Of course this whole discussion is limited to such species as are not +only as a rule visited by insects, but are dependent on these visits for +their fertilization. Most of our garden-flowers are included in this +category. If not then we may expect to find the cultures and seeds pure, +irrespective of the distances between allied varieties, as for instance +with peas, which are known to be self-fertilizing. Another instance is +given by the barley. One of the most curious anomalous varieties of this +cereal, is the "Nepaul-barley," with its small adventitious flowers on +the palets or inner scales. It is a very old, widely cultivated sort, +which always comes true from seed, and which has been tested in repeated +experiments in my garden. The spikelets of this curious plant are +oneflowered and provided with two linear glumes or outer scales. Of the +inner scales or palets, the outer one is three-lobed at the summit, +hence the varietal name of _Hordeum vulgare trifurcatum_. The central +lobe is oblong and hollow, covering a small supernumerary floret +inserted [204] at its base. The two lateral lobes are narrower, +sometimes linear, and are often prolonged into an awn, which is +generally turned away from the center of the spike. The central lobe +sometimes bears two florets at its base, although but one is usually +present and it may be incomplete. + +I might give one more instance from my own experience. A variety of the +evening-primrose with small linear petals was once found by one of my +sons growing wild near Amsterdam. It was represented by only one +individual, flowering among a great many of the ordinary type with broad +petals. But the evening-primroses open their anthers in the morning, +fertilize themselves during the day, and only display their beautiful +flowers in the evening, after the pollination has been accomplished. +They then allure evening moths, such as _Agrotis_ and _Plusia_, by their +bright color, their sweet honeysmell and their nectar. Since the +fertilization is accomplished many hours before opening, crosses are +effected only in rare instances, and the seeds commonly remain true to +the parent type. The seeds of this one plant, when sown separately in my +garden, produced exclusively flowers with the small linear petals of +their parent. Although I had a hundred individuals bearing many +thousands of flowers, there was not an instance of reversion. And such +would [205] immediately have been observed, had it occurred, because the +hybrids between the cruciate and the normal flowers are not +intermediate, but bear the broad petals of the _O. biennis_. + +We may now take up another phase of the question, that of the running +out of new varieties, shortly after their introduction into a new +country, or later. + +The most widely known instance of this is that of the American corn in +Baden, recorded by Metzger and quoted by Darwin as a remarkable instance +of the direct and prompt action of climate on a plant. It has since been +considered as a reversion to the old type. Such reversions invariably +occur, according to Wallace, in cases of new varieties, which have been +produced quickly. But as we now know, such reversions are due to +spontaneous crosses with the old form, and to the rule, that the hybrids +of such origin are not intermediate, but assume the features of the +older of the two parents. In the light of this experience, Metzger's +observation becomes a typical instance of vicinism. It relates to the +"Tuscarora" corn of St. Louis, a variety with broad and flat white +seeds. + +About the year 1840, this corn was introduced into Baden in Germany, and +cultivated by Metzger. In the first year it came true to type, and [206] +attained a height of 12 feet, but the season did not allow its seeds to +ripen normally. Only a few kernels were developed before the winter. +From this seed plants of a wholly different type came the next year, of +smaller stature, and with more brownish and rounded kernels. They also +flowered earlier and ripened a large number of seeds. The depression on +the outer side of the seed had almost disappeared, and the original +white had become darker. Some of the seeds had even become yellow and in +their rounded form they approached the common European maize. Obviously +they were hybrids, assuming the character of their pollen-parent, which +evidently was the ordinary corn, cultivated all around. The observation +of the next year showed this clearly, for in the third generation nearly +all resemblance to the original and very distinct American species was +lost. If we assume that only those seeds ripened which reverted to the +early-ripening European type, and that those that remained true to the +very late American variety could not reach maturity, the case seems to +be wholly comprehensible, without supposing any other factors to have +been at work than those of vicinism, which though unknown at the period +of Metzger's and Darwin's writings, seems now to be fully understood. No +innate tendency to run out and no changing influence of the climate are +required for an adequate explanation of the facts. + +In the observation quoted, what astonishes us most, is the great +rapidity of the change, and the short time necessary for the offspring +of the accidental crosses to completely supplant the introduced type. In +the lecture on the selection of elementary species, closely analogous +cases were described. One of them was the wild oat or _Avena fatua_ +which rapidly supplants the cultivated oats in bad years in parts of the +fields. Other instances were the experiments of Risler with the +"Galland" wheat and the observation of Rimpau on "Rivett's bearded" +wheat. + +Before leaving the question of vicinism and its bearing on the general +belief of the instability of varieties, which when tested with due care, +prove to be quite stable, it may be as well to consider the phenomena +from another point of view. Our present knowledge of the effects of +crosses between varieties enables us to formulate some general rules, +which may be used to calculate, and in some way to predict, the nature +of the impurities which necessarily attend the cultivation of allied +species in close vicinity. And this mode of cultivation being in almost +universal use in the larger nurseries, [208] we may, by this discussion, +arrive at a more scientific estimation of the phenomena of vicinism, +hitherto described. + +The simplest case that may be given, is when an ordinary retrograde +variety is cultivated with the species to which it belongs. For +instance, if dwarfs are cultivated next to the taller type, or a white +variety next to the red or blue-flowering species, or thornless forms in +neighboring beds with the armed species. Bees and Bumble-bees, +butterflies and moths are seen flying from flower to flower, collecting +the honey and carrying pollen. I frequently saw them cross the limits of +the neighboring beds. Loaded with the pollen of the variety they visit +the flowers of the different species and impregnate the stigma with it. +And returning to the variety they bring about similar crosses in the +flowers of the latter. Hybrid seeds will develop in both cases and +become mixed with the crop. We now have to ask the question, what sort +of plants will arise from these hybrid seeds. As a general rule we may +state, first, that the hybrids of either form of cross are practically +the same, secondly that they are not intermediate, but that the +character of one parent prevails to the almost absolute exclusion of the +other and in the third place that the older character dominates the +younger. + +[209] The hybrid offspring will therefore, in the main, have the +character of the species and be indistinguishable from it, or show only +such differences as escape ordinary observation. When occurring in the +seeds of the variety they betray themselves as soon as the differential +characters are displayed. Between the thousands of flowering plants of a +white variety the hybrids will instantly catch the eye by their red or +blue corollas. Quite the contrary effect results from the admixture of +hybrids with the seeds of the species itself. Here no difference will +show itself, even in the fullest bloom. The effect of the spontaneous +crosses will pass unobserved. The strain, if pure in the first year, +will seem to be still in the same condition. Or in other terms, the +unavoidable spontaneous crosses will disturb the purity of the variety +in the second year, while they do not seem to interfere at all with the +uniformity of the species. The direct effect of the visits of the +insects is evident in the first case, but passes unobserved in the +latter. + +From this it would seem, that spontaneous crosses are hurtful to +varieties, but are innocuous to true species. Certainly this would be +so, were there no selection. But it is easily seen, that through this +operation the effect becomes quite the opposite. For when the fields +[210] are inspected at the time of the fullest display of the varietal +characters, the obvious hybrids will be eliminated, but the hidden ones +will of necessity be spared, as they are concealed among the species by +the similarity of their type. Hence, the harvest of the variety may be +rendered pure or nearly so, while the harvest of the species will retain +the seeds of the hybrids. Moreover it will contain seeds originated by +the spontaneous but numerous crosses of the true plants with the +sparsely intermingled hybrids. + +This brings us to the question, as to what will be the visible +consequences of the occurrence of such invisible hybrids in the +following generation. In opposition to the direct effects just +described, we may call them indirect. To judge of their influence, we +must know how hybrid seeds of the first generation behave. + +In one of our lectures we will deal with the laws that show the +numerical relations known as the laws of Mendel. But for our present +purpose, these numerical relations are only of subordinate importance. +What interests us here is the fact that hybrids of varieties do not +remain constant in the second generation but usually split as it is +said, remaining hybrid only in part of their offspring, the other +portion returning to the parental types. This however, will show itself +only in those individuals [211] which reassume the character of the +varietal parent, all the others apparently remaining true to the type of +the species. Now it is easy to foresee what must happen in the second +generation if the first generation after the cross is supposed to be +kept free from new vicinistic influences, or from crosses with +neighboring varieties. + +We may limit ourselves in the first place to the seeds of the unobserved +hybrids. For the greater part they will repeat the character of their +parents and still remain concealed. But a small number will display the +varietal marks, as for example showing white flowers in a field of blue +ones. Hence, the indirect consequence of the spontaneous crosses will be +the same in the species, as was the direct effect in the variety, only +that it appears a year later. It will then be eliminated in the process +of selection. + +Obviously, this elimination conduces only to a partial purification. The +conspicuous plants will be destroyed, but a greater number of hybrids +will remain, still concealed by their resemblance to the general type +and will be spared to repeat the same process next year. So while the +variety may be freed every year from the impurities brought into it in +the preceeding summer, the admixtures of the species [212] will continue +during a number of years, and it may not be possible to get rid of them +at all. + +It is an often recurring assertion that white varieties of colored +species are the most stable of all horticultural races. They are often +said to be at least as constant as the species itself, and even to +surpass it in this quality. With our present state of knowledge, the +explanation of this general experience is easily given. For selection +removes the effect of spontaneous crosses from the variety in each year, +and renders it practically pure, while it is wholly inadequate to +produce the same effects on the species, because of the concealed +hybrids. + +The explanation given in this simple instance may be applied to the case +of different varieties of the same species, when growing together and +crossed naturally by insects. + +It would take too long to go into all the details that present +themselves here to the student of nature and of gardens. I will only +state, that since varieties differ principally from their species by the +lack of some sharp character, one variety may be characterized by the +lack of color of the flowers, another by the lack of pubescence, a third +by being dwarfed, and so on. Every character must be studied separately +in its effects on the offspring [213] of the crosses. And it is +therefore easily seen, that the hybrids of two varieties may resemble +neither of them, but revert to the species itself. This is necessarily +and commonly the case, since it is always the older or positive +characters that prevail in the hybrids and the younger or negative that +lie hidden. So for instance, a blue dwarf larkspur, crossed with a tall +white variety, must give a tall blue hybrid, reassuming in both +characters the essentials of the species. + +Keeping this rule in view, it will be easy to calculate what may be +expected from spontaneous crosses for a wide range of occurrences, and +thus to find an explanation of innumerable cases of apparent variability +and reversion in the principle of vicinism. Students have only to +recollect that specific characters prevail over varietal ones, and that +every character competes only with its own antagonist. Or to give a +sharper distinction: whiteness of flowers cannot be expected to be +interchanged with pubescence of leaves. + +In concluding I will point out another danger which in the principle of +vicinism may be avoided. If you see a plant in a garden with all the +characteristics of its species, how can you be sure that it is truly a +representative of the species, and not a hybrid? The prevailing [214] +characters are in either case the same. Perhaps on close inspection you +may find in some cases a slight difference, some character being not as +fully developed in the hybrid as in the species. But when such is not +the case, or where the opportunity for such a closer examination is +wanting, a hybrid may easily be taken for a specimen of the pure race. +Now take the seeds of your plant and sow them. If you had not supposed +it to be hybrid you will be astonished at finding among its progeny some +of a wholly different type. You will be led to conclude that you are +observing a sudden change in structure such as is usually called a +sport. + +Or in other words you may think that you are assisting at the +origination of a new variety. If you are familiar with the principle of +vicinism, you will refrain from such an inference and consider the +supposition of a hybrid origin. But in former times, when this principle +was still unknown and not even guessed at, it is evident that many +mistakes must have been made, and that many an instance, which until now +has been considered reliable proof of a so-called single variation, is +in fact only a case of vicinism. In reading the sparse literature on +sports, numerous cases will be found, which cannot stand this test. In +many instances crossing must be looked to as an explanation, [215] and +in other cases the evidence relied upon does not suffice to exclude this +assumption. Many an old argument has of late lost its force by this +test. + +Returning to our starting point we may now state that regular reversions +to a specific type characterize a form as a variety of that species. +These reversions, however, are not due to an innate tendency, but to +unobserved spontaneous crosses. + + + +[217] + +LECTURE VIII + +LATENT CHARACTERS + +No organism exhibits all of its qualities at any one time. Many of them +are generally dormant and await a period of activity. For some of them +this period comes regularly, while in others the awakening depends upon +external influences, and consequently occurs very irregularly. Those of +the first group correspond to the differences in age; the second +constitute the responses of the plant to stimuli including +wound-injuries. + +Some illustrative examples may be quoted in order to give a precise idea +of this general conception of dormant or latent characters. Seed leaves +are only developed in the seed and the seedling; afterwards, during the +entire lifetime of the plant, the faculty of producing them is not made +use of. Every new generation of seeds however, bears the same kind of +seed leaves, and hence it is manifest that it is the same quality, which +shows itself from time to time. + +The primary leaves, following the seed-leaves, are different in many +species, from the later ones, and the difference is extremely pronounced +in some cases of reduction. Often, when leaves are lacking in the adult +plant, being replaced by flattened stalks as in the case of the acacias, +or by thorns, or green stems and twigs as in the prickly broom or _Ulex +europaeus_, the first leaves of the young plant may be more highly +differentiated, being pinnate in the first case and bearing three +leaflets in the second instance. This curious behavior which is very +common, brings the plants, when young, nearer to their allies than in +the adult state, and manifestly implies that the more perfect state of +the leaves is latent throughout the life of the plant, with the +exception of the early juvenile period. + +_Eucalyptus Globulus_, the Australian gum tree, has opposite and broadly +sessile leaves during the first years of its life. Later these disappear +and are replaced by long sickle-shaped foliage organs, which seem to be +scattered irregularly along the branches. The juvenile characters +manifestly lie dormant during the adult period, and that this is so, may +be shown artificially by cutting off the whole crown of the tree, when +the stem responds by producing numerous new branches, which assume the +[218] shape proper to the young trees, bearing sessile and opposite +leaves. + +It seems quite unnecessary to give further instances. They are familiar +to every student. It is almost safe to say that every character has its +periods of activity and of inactivity, and numbers of flowers and fruits +can be mentioned as illustrations. One fact may be added to show that +nearly every part of the plant must have the power of producing all or +nearly all the characters of the individual to which it belongs. This +proof is given by the formation of adventitious buds. These, when once +formed, may grow out into twigs, with leaves and flowers and roots. They +may even be separated from the plants and used as cuttings to reproduce +the whole. Hence we may conclude that all tissues, which possess the +power of producing adventitious buds, must conceal in a latent state, +all the numerous characters required for the full development of the +whole individual. + +Adventitious buds may proceed from specialized cells, as on the margin +of the leaves of _Bryophyllum calycinum_; or from the cells of special +tissues, as in the epidermis of the begonias; or they may be provoked by +wounds in nearly every part of the plant, provided it be able to heal +the wound by swelling tissues or [219] callus. The best instance is +afforded by elms and by the horse-chestnut. If the whole tree is hewn +down the trunk tries to repair the injury by producing small +granulations of tissue between the wood and the bark, which gradually +coalesce while becoming larger. From this new ring of living matter +innumerable buds arise, that expand into leafy branches, showing clearly +that the old trunk possesses, in a latent state, all the qualities of +the whole crown. Indeed, such injured stumps may be used for the +production of copses and hedges. + +All the hitherto recorded cases of latency have this in common, that +they may become active during the life-time of any given individual +once, or oftener. This may be called the ordinary type of latency. + +Besides this there is another form of latent characters, in which this +awakening power is extremely limited, or wholly absent. It is the +systematic latency, which may be said to belong to species and varieties +in the same way as the ordinary latency belongs to individuals. As this +individual latency may show itself from time to time during the life of +a given plant, the first may only become active from time to time during +the whole existence of the variety or the species. It has no regular +period of activity, nor may it be incited by artificial stimulation. + +[220] It emerges from concealment only very rarely and only on its own +initiative. Such instances of atavism have been described in previous +lectures, and their existence has been proved beyond doubt. + +Systematic latency explains the innumerable instances in which species +are seen to lack definite characteristics which ordinarily do not fail, +either in plants at large, or in the group or family to which the plant +belongs. If we take for instance the broom-rape or _Orobanche_, or some +other pale parasite, we explain their occurrence in families of plants +with green leaves, by the loss of the leaves and of the green color. But +evidently this loss is not a true one, but only the latency of those +characters. And even this latency is not a complete one, as little +scales remind us of the leaves, and traces of chlorophyll still exist in +the tissues. Numerous other cases will present themselves to every +practical botanist. + +Taking for granted that characters, having once been acquired, may +become latent, and that this process is of universal occurrence +throughout the whole vegetable and animal kingdom, we may now come to a +more precise and clear conception of the existing differences between +species and varieties. + +For this purpose we must take a somewhat [221] broader view of the whole +evolution of the vegetable kingdom. It is manifest that highly developed +plants have a larger number of characters than the lower groups. These +must have been acquired in some way, during preceding times. Such +evolution must evidently be called a process of improvement, or a +progressive evolution. Contrasted to this is the loss, or the latency of +characters, and this may be designated retrogressive or retrograde +evolution. But there is still a third possibility. For a latent +character may reassume its activity, return to the active state, and +become once more an important part of the whole organization. This +process may be designated as degressive evolution; it obviously +completes the series of the general types of evolution. + +Advancement in general in living nature depends on progressive +evolution. In different parts of the vegetable kingdom, and even in +different families this progression takes place on different lines. By +this means it results in an ever increasing divergency between the +several groups. Every step is an advance, and many a step must have been +taken to produce flowering plants from the simplest unicellular algae. + +But related to, and very intimately connected with this advancement is +the retrogressive [222] evolution. It is equally universal, perhaps +never failing. No great changes have been attained, without acquiring +new qualities on one side, and reducing others to latency. Everywhere +such retrogressions may be seen. The polypetalous genera _Pyrola_, +_Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkable +instance of this. The whole evolution of the monocotyledons from the +lowest orders of dicotyledons implies the seeming loss of cambial growth +and many other qualities. In the order of aroids, from the calamus-root +or sweet flag, with its small but complete flowers, up to the reduced +duckweeds (_Lemna_), almost an unbroken line of intermediate steps may +be traced showing everywhere the concurrence of progressive and +retrogressive evolution. + +Degressive evolution is not so common by far, and is not so easy to +recognize, but no doubt it occurs very frequently. It is generally +called atavism, or better, systematic atavism, and the clearest cases +are those in which a quality which is latent in the greater part of a +family or group, becomes manifest in one of its members. Bracts in the +inflorescence of crucifers are ordinarily wanting, but may be seen in +some genera, _Erucastrum pollichii_ being perhaps the [223] most widely +known instance, although other cases might easily be cited. + +For our special purpose we may take up only the more simple cases that +may be available for experimental work. The great lines of evolution of +whole families and even of genera and of many larger species obviously +lie outside the limits of experimental observation. They are the outcome +of the history of the ancestors of the present types, and a repetition +of their history is far beyond human powers. We must limit ourselves to +the most recent steps, to the consideration of the smallest differences. +But it is obvious that these may be included under the same heads as the +larger and older ones. For the larger movements are manifestly to be +considered only as groups of smaller steps, going in the same direction. + +Hence we conclude, that even the smallest steps in the evolution of +plants which we are able to observe, may be divided into progressive, +retrogressive and degressive ones. The acquisition of a single new +quality is the most simple step in the progressive line, the becoming +latent and the reactivating of this same quality are the prototypes of +the two other classes. + +Having taken this theoretical point of view, it remains to inquire, how +it concurs with the [224] various facts, given in former lectures and +how it may be of use in our further discussions. + +It is obvious that the differences between elementary species and +varieties on the one hand, and between the positive and negative +varieties as distinguished above, are quite comparable with our +theoretical views. For we have seen that varieties can always be +considered as having originated by an apparent loss of some quality of +the species, or by the resumption of a quality which in allied species +is present and visible. In our exposition of the facts we have of course +limited ourselves to the observable features of the phenomena without +searching for a further explanation. For a more competent inquiry +however, and for an understanding of wider ranges of facts, it is +necessary to penetrate deeper into the true nature of the implied +causes. + +Therefore we must try to show that elementary species are distinguished +from each other by the acquisition of new qualities, and that varieties +are derived from their species either by the reduction of one or more +characteristics to the latent state, or by the energizing of dormant +characters. + +Here we meet with a great difficulty. Hitherto varieties and subspecies +have never been clearly defined, or when they have been, it was [225] +not by physiological, but only by morphological research. And the claims +of these two great lines of inquiry are obviously very diverging. +Morphological or comparative studies need a material standard, by which +it may be readily decided whether certain groups of animals and plants +are to be described or de-nominated as species, as subspecies or as +varieties. To get at the inner nature of the differences is in most +cases impossible, but a decision must be made. The physiological line of +inquiry has more time at its disposal; it calls for no haste. Its +experiments ordinarily cover years, and a conclusion is only to be +reached after long and often weary trials. There is no making a decision +on any matter until all doubtful points have been cleared up. Of course, +large groups of facts remain uncertain, awaiting a closer inquiry, and +the teacher is constrained to rely on the few known instances of +thoroughly investigated cases. These alone are safe guides, and it seems +far better to trust to them and to make use of them for the construction +of sharp conceptions, which may help us to point out the lines of +inquiry which are still open. + +Leaving aside all such divisions and definitions, as were stamped with +the name of provisional species and varieties by the great systematist, +[226] Alphonse De Candolle, we may now try to give the proofs of our +assertion, by using only those instances that have been thoroughly +tested in every way. + +We may at once proceed to the retrogressive or negative varieties. The +arguments for the assumption that elementary species owe their origin to +the acquisition of new qualities may well be left for later lectures +when we shall deal with the experimental proofs in this matter. + +There are three larger groups of facts, on which the assumption of +latent characters in ordinary varieties rests. These are true atavism, +incomplete loss of characters, and systematic affinity. Before dealing +with each of these separately, it may be as well to recall once more +that in former lectures we have treated the apparent losses only as +modifications in a negative way, without contemplating the underlying +causes. + +Let us recall the cases of bud-atavism given by the whitish variety of +the scarlet _Ribes_, by peaches and nectarines, and by conifers, +including _Cephalotaxus_ and _Cryptomeria_. These and many other +analogous facts go to prove the relation of the variety to the species. +Two assumptions are allowable. In one the variety differs from the +species by the total loss of the [227] distinctive character. In the +other this character is simply reduced to an inactive or dormant state. +The fact of its recurrence from time to time, accompanied by secondary +characters previously exhibited, is a manifest proof of the existence of +some relation between the lost and the resumed peculiarity. Evidently +this relation cannot be accounted for on the assumption of an absolute +disappearance; something must remain from which the old features may be +restored. + +This lengthy discussion may be closed by the citation of the cases, in +which plants not only show developmental features of a former state, but +also reproduce the special features they formerly had, but seemingly +have lost. Two good illustrative examples may be given. One is afforded +by the wheat-ear carnation, the other by the green dahlias, and both +have occurred of late in my own cultures. + +A very curious anomaly may from time to time be observed in large beds +of carnations. It bears no flowers, but instead of them small green +ears, which recall the ears of wheat. Thence the name of "Wheat-ear" +carnation. On closer inspection it is easily seen how they originate. +The normal flowers of the carnations are preceded by a small group of +bracts, [228] which are arranged in opposite pairs and therefore +constitute four rows. + +In this variety the flower is suppressed and this loss is attended by a +corresponding increase of the number of the pairs of bracts. This +malformation results in square spikes or somewhat elongated heads +consisting only of the greenish bracts. As there are no flowers, the +variety is quite sterile, and as it is not regarded by horticulturists +as an improvement on the ordinary bright carnations, it is seldom +multiplied by layering. Notwithstanding this, it appears from time to +time and has been seen in different countries and at different periods, +and, what is of great importance for us, in different strains of +carnations. Though sterile, and obviously dying out as often as it +springs into existence, it is nearly two centuries old. It was described +in the beginning of the 18th century by Volckamer, and afterwards by +Jaeger, De Candolle, Weber, Masters, Magnus and many other botanists. I +have had it twice, at different times and from different growers. + +So far as I have been able to ascertain reversions of this curious +carnation to normal flowers have not yet been recorded. Such a +modification occurred last summer in my garden on a plant which had not +been divided or layered, but on which the slender branches had [229] +been left on the stem. Some of them remained true to the varietal type +and bore only green spikes. Others reverted wholly or partially to the +production of normal flowers. Some branches bore these only, others had +spikes and flowers on neighboring twigs, and in still other instances +little spikes had been modified in such manner that a more or less well +developed flower was preceded by some part of an ear. + +The proof that this retrograde modification was due to the existence of +a character in the latent state was given by the color of the flowers. +If the reverted bud had only lost the power of producing spikes, they +would evidently simply have returned to the characteristics of the +ordinary species, and their color would have been a pale pink. Instead +of this, all flowers displayed corollas of a deep brown. They obviously +reverted to their special progenitor, the chance variety from which they +had sprung, and not to the common prototype of the species. Of course it +was not possible to ascertain from which variety the plant had really +originated, but the reproduction of any one clearly defined varietal +mark is in itself proof enough of their origin, and of the latency of +the dark brown flower-color in this special case. + +A still better proof is afforded by a new type of green dahlia. The +ordinary green dahlia [230] has large tufts of green bracts instead of +flowering heads, the scales of the receptacle having assumed the texture +and venation of leaves, and being in some measure as fleshy. But the +green heads retain the form of the ordinary flower-heads, and as they +have no real florets that may fade away, they remain unchanged on the +plants, and increase in number through the whole summer. The new types +of green dahlia however, with which I have now to deal, are +distinguished by the elongation of the axis of the head, which is +thereby changed into a long leafy stalk, attaining a length of several +inches. These stalks continue growing for a very long time, and for the +most part die without producing anything else than green fleshy scales. + +This long-headed green dahlia originated at Haarlem some years ago, in +the nursery of Messrs. Zocher & Co. It was seen to arise twice, from +different varieties. Both of these were double-flowered, one a deep +carmine with white tips on the rays, the other of a pale orange tint, +known by the name of "Surprise." As they did not bear any florets or +seeds, they were quite sterile. The strain arising from the carmine +variety was kindly given to me by Messrs. Zocher & Co., and was +propagated in my garden, while the other was kept in the nursery. In the +earlier cultures both remained true to [231] their types, never +producing true florets. No mark of the original difference was to be +seen between them. But last summer (1903) both reverted to their +prototypes, bearing relatively large numbers of ordinary double +flowerheads among the great mass of green stalks. Some intermediate +forms also occurred consisting of green-scaled stalks ending in small +heads with colored florets. + +Thus far we have an ordinary case of reversion. But the important side +of the phenomenon was, that each plant exactly "recollected" from which +parent it had sprung. All of those in my garden reverted to the carmine +florets with white tips, and all of those in the nursery to the pale +orange color and the other characteristics of the "Surprise" variety. + +It seems absolutely evident, that no simple loss can account for this +difference. Something of the character of the parent-varieties must have +remained in the plant. And whatever conception we may formulate of these +vestigial characters it is clear that the simplest and most obvious idea +is their preservation in a dormant or latent state. Assuming that the +distinguishing marks have only become inactive by virescence, it is +manifest that on returning each will show its own peculiarities, as +recorded above. Our second point was the incomplete loss of [232] the +distinguishing quality in some varieties. It is of general occurrence, +though often overlooked. Many white varieties of colored flowers give +striking instances, among them many of the most stable and most prized +garden-flowers. If you look at them separately or in little bouquets +they seem to be of irreproachable purity. But if you examine large beds +a pale hue will become visible. In many cases this tinge is so slight as +to be only noticeable in a certain illumination, or by looking in an +oblique direction across the bed; in others it is at once evident as +soon as it has been pointed out. It always reminds the observer of the +color of the species to which the variety belongs, being bluish in +violets and harebells, reddish in godetias and phloxes, in _Silene +Armeria_ and many others. It proves that the original color quality of +the species has not wholly, but only partly disappeared. It is dormant, +but not entirely obliterated; latent, but not totally concealed; +inactive, but only partially so. Our terminology is an awkward one; it +practically assumes, as it so often does in other cases, a conventional +understanding, not exactly corresponding to the simple meaning of the +words. But it would be cumbrous to speak always of partial inactivity, +incomplete latency or half awakening qualities. Even such words as +sub-latent, [233] which would about express the real state of things, +would have little chance of coming into general use. + +Such sub-latent colors are often seen on special parts in white +varieties of flowers. In many cases it is the outer side of the petals +which recalls the specific color, as in some white roses. In violets it +is often on the spur that the remains of the original pigment are to be +seen. In many instances it is on the tips of the petals or of the +segments of the corolla, and a large number of white or yellow flowers +betray their affinity to colored species by becoming red or bluish at +the edges or on the outer side. + +The reality of such very slight hues, and their relation to the original +pigment of the species may in some cases be proved by direct experiment. +If it is granted that latency is not an absolute quality, then it will +be readily accepted, that even latency must be subjected to the laws of +gradual variation or fluctuating variability. We will deal with these +laws in a later lecture but every one knows that greater deviations than +the ordinary may be attained by sowing very large numbers and by +selecting from among them the extreme individuals and sowing anew from +their seed. In this way the slightest tinge of any latent color may be +[234] strengthened, not indeed to the restoration of the tinge of the +species, but at least so far as to leave no doubt as to the identity of +the visible color of the species and the latent or sublatent one of the +variety. + +I made such an experiment with the peach leaved harebell or _Campanula +persicifolia_. The white variety of this species, which is often met +with in our gardens, shows a very pale bluish hue when cultivated in +large quantities, which however is subject to individual variations. I +selected some plants with a decided tinge, flowered them separately, +sowed their seeds, and repeated this during two generations. The result +was an increase of the color on the tips of the segments of the corolla +in a few individuals, most of them remaining as purely white as the +original strain. But in those few plants the color was very manifest, +individually variable in degree, but always of the same blue as in the +species itself. + +Many other instances could be given. Smooth varieties are seldom +absolutely so, and if scattering hairs are found on the leaves or only +on some more or less concealed parts, they correspond in their character +to those of the species. So it is with prickles, and even the thornless +thorn-apple has fruits with surfaces far from smooth. The thornless +horse-chestnut [235] has in some instances such evident protuberances on +the valves of its fruits, that it may seem doubtful whether it is a pure +and stable variety. + +Systematic latency may betray itself in different ways, either by normal +systematic marks, or by atavism. With the latter I shall deal at length +on another occasion, and therefore I will give here only one very clear +and beautiful example. It is afforded by the common red clover. +Obviously the clovers, with their three leaflets in each leaf, stand in +the midst of the great family of papilionaceous plants, the leaves of +which are generally pinnate. Systematic affinity suggests that the +"three leaved" forms must have been derived from pinnate ancestors, +evidently by the reduction of the number of the leaflets. In some +species of clover the middle of the three is more or less stalked, as is +ordinarily the case in pinnate leaves; in others it is as sessile as are +its neighbors. In a subsequent chapter I will describe a very fine +variety, which sometimes occurs in the wild state and may easily be +isolated and cultivated. It is an ordinary red clover with five leaflets +instead of three, and with this number varying between three and seven, +instead of being nearly wholly stable as in the common form. It produces +from time to time pinnate leaves, [236] very few indeed, and only +rarely, but then often two or three or even more on the same individual. +Intermediate stages are not wanting, but are of no consequence here. The +pinnate leaves obviously constitute a reversion to some prototype, to +some ancestor with ordinary papilionaceous leaves. They give proof of +the presence of the common character of the family, concealed here in a +latent state. Any other explanation of this curious anomaly would +evidently be artificial. On the other hand nothing is really known about +the ancestors of clover, and the whole conception rests only on the +prevailing views of the systematic relationships in this family. But, as +I have already said, further proof must be left for a subsequent +occasion. + +Many instances, noted in our former lectures, could be quoted here. The +systematic distribution of rayed and rayless species and varieties among +the daisy-group of the composites affords a long series of examples. +Accidental variations in both directions occur. The Canada fleabane or +_Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some others +may at times be seen with ray-florets, and according to Murr, they may +sometimes be wanting in _Aster Tripolium_, _Bellis perennis_, some +species of _Anthemis_, _Arnica montana_ and in a number [237] of other +well-known rayed species. Another instance may be quoted; it has been +pointed out by Grant Allen, and refers to the dead-nettle or Lamium +album. Systematically placed in a genus with red-flowering species, we +may regard its white color as due to the latency of the general red +pigment. + +But if the flower of this plant is carefully examined, it will be found +in most cases not to be purely white, but to have some dusky lines and +markings on its lower lip. Similar devices are observed on the lip of +the allied _Lamium maculatum_, and in a less degree on the somewhat +distant _Lamium purpureum_. With _Lamium maculatum_ or spotted +dead-nettle, the affinity is so close that even Bentham united the two +in a single species, considering the ordinary dead-nettle only as a +variety of the dappled purple type. For the support of this conception +of a specific or varietal retrograde change many other facts are +afforded by the distribution of the characteristic color and of the +several patterns of the lips of other labiates, and our general +understanding of the relationships of the species and genera in this +family may in a broad sense be based on the comparison of these +seemingly subordinate characteristics. + +The same holds good in many other cases, and systematists have often +become uncertain [238] as to the true value of some form, by its +relationship to the allied types in the way of retrogressive +modification. Color-differences are so showy, that they easily +overshadow other characters. The white and the blue thorn-apple, the +white and the red campion (_Lychnis vespertina_ and _diurna_) and many +other illustrative cases could be given, in which two forms are +specifically separated by some authors, but combined by others on the +ground of the retrograde nature of some differentiating mark. + +Hitherto we have dealt with negative characters and tried to prove that +the conception of latency of the opposite positive characteristics is a +more natural explanation of the phenomenon than the idea of a complete +loss. We have now to consider the positive varieties, and to show that +it is quite improbable that here the species have struck out for +themselves a wholly new character. In some instances such may have been +the case, but then I should prefer to treat these rather as elementary +species. But in the main we will have to assume the latency of the +character in the species and its reassumption by the variety when +originating, as the most probable explanation. + +Great stress is laid upon this conception by the fact, that positive +varieties are so excessively rare when compared with the common +occurrence [239] of negative ones. Indeed, if we put aside the radiate +and the color-varieties of flowers and foliage, hardly any cases can be +cited. We have dealt with this question in a former lecture, and may now +limit ourselves to the positive color-varieties. + +The latency of the faculty of producing the red pigment in leaves must +obviously be accepted for nearly the whole vegetable kingdom. Oaks and +elms, the beautiful climbing species of Ampelopsis, many conifers, as +for instance _Cryptomeria japonica_, some brambles, the Guelder-rose +(_Viburnum Opulus_) and many other trees and shrubs assume a more or +less bright red color in the fall. During summer this tendency must have +been dormant, and that this is so, is shown by the young leaves of oaks +and others, which, when unfolding in the spring show a similar but paler +hue. Moreover, there is a way of awakening the concealed powers at any +time. We have only to inflict small wounds on the leaves, or to cut +through the nerves or to injure them by a slight bruising, and the +leaves frequently respond with an intense reddening of the living +tissues around and especially above the wounds. _Azolla caroliniana_, a +minute mosslike floating plant allied to the ferns, responds to light +and cold with a reddish tinge, and to shade or warmth with a pure green. +The foliage [240] of many other plants behaves likewise, as also do +apples and peaches on the insolated sides of the fruits. It is quite +impossible to state these groups of facts in a more simple way than by +the statement that the tendency to become red is almost generally +present, though latent in leaves and stems, and that it comes into +activity whenever a stimulus provokes it. + +Now it must be granted that the energizing of such a propensity under +ordinary circumstances is quite another thing from the origination of a +positive variety by the evolution of the same character. In the variety +the activity has become independent of outer influences or dependent +upon them in a far lesser degree. The power of producing the red +pigments is shown to be latent by the facts given above, and we see that +in the variety it is no longer latent but is in perfect and lasting +activity throughout the whole life of the plant. + +Red varieties of white flowers are much more rare. Here the latency of +the red pigment may be deduced partly from general arguments like those +just given, partly from the special systematic relations in the given +cases. Hildebrand has clearly worked out this mode of proof. He showed +by the critical examination of a large number of instances that the +occurrence of the red-flowered varieties is contingent upon the [241] +existence of red species in the same genus, or in some rare cases, in +nearly allied genera. Colors that are not systematically present in the +group to which a white species belongs are only produced in its +varieties in extremely rare cases. + +We may quote some special rules, indicated by Hildebrand. Blue species +are n the main very rare, and so are blue varieties of white species +also. Carnations, Asiatic or cultivated buttercups (_Ranunculus +asiaticus_), _Mirabilis_, poppies, _Gladiolus_, _Dahlia_, and some other +highly cultivated or very old garden-plants have not been able to +produce true blue flowers. But the garden-anemone (_Anemone coronaria_) +has allies with very fine blue flowers. The common stock has bluish +varieties and is allied to _Aubretia_ and _Hesperis_, and gooseberries +have a red form, recalling the ordinary currant. In nearly all other +instances of blue or red varieties every botanist will be able to point +out some allied red or blue species, as an indication of the probable +source of the varietal character. + +Dark spots on the lower parts of the petals of some plants afford +another instance, as in poppies and in the allied _Glaucium_, where they +sometimes occur as varietal and in other cases as specific marks. + +The yellow fails in many highly developed [242] flowers, which are not +liable to produce yellow variations, as in _Salvia_, _Aster_, +_Centaurea_, _Vinca_, _Polygala_ and many others. Even the rare pale +yellowish species of some of these genera have no tendency in this +direction. The hyacinths are the most remarkable, if not the sole known +instance of a species having red and blue and white and yellow +varieties, but here the yellow is not the bright golden color of the +buttercups. + +The existence of varietal colors in allied species obviously points to a +common cause, and this cause can be no other than the latency of the +pigment in the species that do not show it. + +The conception of latency of characters as the common source of the +origination of varieties, either in the positive or in the negative way, +leads to some rules on variability, which are known under the names +given to them by Darwin. They are the rules of repeated, homologous, +parallel and analogous variability. Each of them is quite general, and +may be recognized in instances from the most widely distant families. +Each of them is quite evident and easily understood on the principle of +latency. + +By the term of repeated variability is meant the well-known phenomenon, +that the same variety has sprung at different times and in different +[243] countries from the same species. The repetition obviously +indicates a common internal cause. The white varieties of blue- and +red-flowered plants occur in the wild state so often, and in most of the +instances in so few individuals that a common pedigree is absolutely +improbable. In horticulture this tendency is widely and vexatiously +known, since the repetition of an old variety does not bring any +advantage to the breeder. The old name of "conquests," given by the +breeders of hyacinths, tulips and other flower-bulbs to any novelty, in +disregard of the common occurrence of repetitions, is an indication of +the same experience in the repeated appearance of certain varieties. + +The rule of parallel variations demands that the same character +occasionally makes its appearance in the several varieties or races, +descended from the same species, and even in widely distinct species. +This is a rule, which is very important for the general conception of +the meaning of the term variety as contrasted with elementary species. +For the recurrence of the same deviation always impresses us as a +varietal mark. Laciniated leaves are perhaps the most beautiful +instance, since they occur in so many trees and shrubs, as the walnut +tree, the beech, the birch, the hazelnut, and even in [244] brambles and +some garden-varieties of the turnip (_Brassica_). + +In such cases of parallel variations the single instances obviously +follow the same rules and are therefore to be designated as analogous. +Pitchers or ascidia, formed by the union of the margins of a leaf, are +perhaps the best proof. They were classified by Morren under two heads, +according to their formation from one or more leaves. Monophyllous +pitchers obey the same law, viz.: that the upper side of the leaf has +become the inner side of the pitcher. Only one exception to this rule is +known to me. It is afforded by the pitchers of the banyan or holy +fig-tree, _Ficus religiosus_, but it does not seem to belong to the same +class as other pitchers, since as far as it has been possible to +ascertain the facts, these pitchers are not formed by a few leaves as in +all other cases, but by all the leaves of the tree. + +In some cases pitchers are only built up of part of the leaf-blade. Such +partial malformations obey a rule, that is common to them and to other +foliar enations, viz.: that the side of the leaf from which they emerge, +is always their outer side. The inner surface of these enations +corresponds to the opposite side of the leaf, both in color and in +anatomical structure. The last of the four rules above mentioned is +[245] that of the homologous variability. It asserts that the same +deviation may occur in different, but homologous parts of the same +plant. We have already dealt with some instances, as the occurrence of +the same pigment in the flowers and foliage, in the fruits and seeds of +the same plant, as also illustrated by the loss of the red or blue tinge +by flowers and berries. Other instances are afforded by the curious fact +that the division of the leaves into numerous and small segments is +repeated by the petals, as in the common celandine and some sorts of +brambles. + +It would take too long to make a closer examination of the numerous +cases which afford proof of these statements. Suffice it to say that +everywhere the results of close inspection point to the general rule, +that the failure of definite qualities both in species and in varieties +must, in a great number of cases, be considered as only apparent. Hidden +from view, occasionally reappearing, or only imperfectly concealed, the +same character must be assumed to be present though latent. + +In the case of negative or retrogressive varieties it is the transition +from the active into a dormant state to which is due the origin of the +variety. Positive varieties on the contrary owe their origin to the +presence of some character [246] in the species in the latent state, and +to the occasional re-energizing thereof. + +Specific or varietal latency is not the same thing as the ordinary +latency of characters that only await their period of activity, or the +external influence which will awake them. They are permanently latent, +and could well be designated by the word perlatent. They spring into +activity only by some sudden leap, and then at once become independent +of ordinary external stimulation. + + + +[247] + +LECTURE IX + +CROSSES OF SPECIES AND VARIETIES + +In the foregoing lectures I have tried to show that there is a real +difference between elementary species and varieties. The first are of +equal rank, and together constitute the collective or systematic +species. The latter are usually derived from real and still existing +types. Elementary species are in a sense independent of each other, +while varieties are of a derivative nature. + +Furthermore I have tried to show that the ways in which elementary or +minor species must have originated from their common ancestor must be +quite different from the mode of origin of the varieties. We have +assumed that the first come into existence by the production of +something new, by the acquirement of a character hitherto unnoticed in +the line of their ancestors. On the contrary, varieties, in most cases, +evidently owe their origin to the loss of an already existing character, +or in other less frequent cases, to the re-assumption of a quality [248] +formerly lost. Some may originate in a negative, others in a positive +manner, but in both cases nothing really new is acquired. + +This distinction holds good for all cases in which the relationship +between the forms in question is well known. It seems entirely +justifiable therefore to apply it also to cases in which the systematic +affinity is doubtful, as well as to instances in which it is impossible +to arrive at any taxonomic conclusions. The extreme application of the +principle would no doubt disturb the limits between many species and +varieties as now recognized. It is not to be forgotten however that all +taxonomic distinctions, which have not been confirmed by physiologic +tests are only provisional, a view acknowledged by the best +systematists. Of course the description of newly discovered forms can +not await the results of physiologic inquiries; but it is absolutely +impossible to reach definite conclusions on purely morphologic evidence. +This is well illustrated by the numerous discords of opinion of +different authors on the systematic worth of many forms. + +Assuming the above mentioned principle as established, and disregarding +doubtful cases as indicated, the term progressive evolution is used to +designate the method in which elementary species must have originated. +It is the [249] manner in which all advance in the animal and vegetable +kingdoms must have taken place, continuously adding new characters to +the already existing number. Contrasted with this method of growing +differentiation, are the retrogressive modifications, which simply +retrace a step, and the degressive changes in which a backward step is +retraced and old characters revived. No doubt both of these methods have +been operative on a large scale, but they are evidently not in the line +of general advancement. + +In all of these directions we see that the differentiating marks show +more or less clearly that they are built up of units. Allied forms are +separated from each other without intermediates. Transitions are wholly +wanting, although fallaciously apparent in some instances owing to the +wide range of fluctuating variability of the forms concerned, or to the +occurrence of hybrids and subvarieties. + +These physiologic units, which in the end must be the basis for the +distinction of the systematic units, may best be designated by the term +of "unit-characters." Their internal nature is as yet unknown to us, and +we will not now look into the theories, which have been propounded as to +the probable material basis underlying them. For our present purpose the +empirical evidence of the general occurrence of [250] sharp limits +between nearly related characters must suffice. As Bateson has put it, +species are discontinuous, and we must assume that their characters are +discontinuous also. + +Moreover there is as yet no reason for trying to make a complete +analysis of all the characters of a plant. No doubt, if attained, such +an analysis would give us a deep insight into the real internal +construction of the intricate properties of organisms in general. But +taxonomic studies in this direction are only in their infancy and do not +give us the material required for such an analysis. Quite on the +contrary, they compel us to confine our study to the most recently +acquired, or youngest characters, which constitute the differentiating +marks between nearly allied forms. + +Obviously this is especially the case in the realm of the hybrids, since +only nearly related forms are able to give hybrid offspring. In dealing +with this subject we must leave aside all questions concerning more +remote relationships. + +It is not my purpose to treat of the doctrine of hybridization at any +length. Experience is so rapidly increasing both in a practical and in a +purely scientific direction that it would take an entire volume to give +only a brief survey of the facts and of all the proposed theories. + +[251] For our present purposes we are to deal with hybrids only in so +far as they afford the means of a still better distinction between +elementary species and varieties. I will try to show that these two +contrasting groups behave in quite a different manner, when subjected to +crossing experiments, and that the hope is justified that some day +crosses may become the means of deciding in any given instance, what is +to be called a species, and what a variety, on physiologic grounds. It +is readily granted that the labor required for such experiments, is +perhaps too great for the results to be attained, but then it may be +possible to deduce rules from a small series of experiments, which may +lead us to a decision in wider ranges of cases. + +To reach such a point of view it is necessary to compare the evidence +given by hybrids, with the conclusions already attained by the +comparison of the differentiating characteristics of allied forms. + +On this ground we first have to inquire what may be expected respecting +the internal nature and the outcome of the process of crossing in the +various cases cited in our former discussion. + +We must always distinguish the qualities, which are the same in both +parents, from those that constitute the differentiating marks in every +single cross. In respect to the first [252] group the cross is not at +all distinguished from a normal fertilization, and ordinarily these +characters are simply left out of consideration. But it should never be +forgotten that they constitute the enormous majority, amounting to +hundreds and thousands, whereas the differentiating marks in each case +are only one or two or a few at most. The whole discussion is to be +limited to these last-named exceptions. We must consider first what +would be the nature of a cross when species are symmetrically combined, +and what must be the case when varieties are subjected to the same +treatment. In so doing, I intend to limit the discussion to the most +typical cases. We may take the crosses between elementary species of the +same or of very narrowly allied systematic species on the one side, and +on the other, limit treatment to the crossing of varieties with the +species, from which they are supposed to have sprung by a retrograde +modification. Crosses of different varieties of the same species with +one another obviously constitute a derivative case, and should only be +discussed secondarily. And crosses of varieties with positive or +depressive characters have as yet so rarely been made that we may well +disregard them. + +Elementary species differ from their nearest allies by progressive +changes, that is by the acquirement [253] of some new character. The +derivative species has one unit more than the parent. All other +qualities are the same as in the parent. Whenever such a derivative is +combined with its parent the result for these qualities will be exactly +as in a normal fertilization. In such ordinary cases it is obvious that +each character of the pollen-parent is combined with the same character +of the pistil-parent. There may be slight individual differences, but +each unit character will become opposed to, and united with, the same +unit-character in the other parent. In the offspring the units will thus +be paired, each pair consisting of two equivalent units. As to their +character the units of each single pair are the same, only they may +exhibit slight differences as to the degree of development of this +character. + +Now we may apply this conception to the sexual combination of two +different elementary species, assuming one to be the derivative of the +other. The differentiating mark is only present in one of the parents +and wanting in the other. While all other units are paired in the +hybrid, this one is not. It meets with no mate, and must therefore +remain unpaired. The hybrid of two such elementary species is in some +way incomplete and unnatural. In the ordinary course of things all +individuals derive [254] their qualities from both parents; for each +single mark they possess at least two units. Practically but not +absolutely equal, these two opponents always work together and give to +the offspring a likeness to both parents. No unpaired qualities occur in +normal offspring; these constitute the essential features of the hybrids +of species and are at the same time the cause of their wide deviations +from the ordinary rules. + +Turning now to the varieties, we likewise need discuss their +differentiating marks only. In the negative types, these consist of the +apparent loss of some quality which was active in the species. But it +was pointed out in our last lecture that such a change is an apparent +loss. On a closer inquiry we are led to the assumption of a latent or +dormant state. The presumably lost characters have not absolutely, or at +least not permanently disappeared. They show their presence by some +slight indication of the quality they represent, or by occasional +reversions. They are not wanting, but only latent. + +Basing our discussion concerning the process of crossing on this +conception, and still limiting the discussion to one differentiating +mark, we come to the inference, that this mark is present and active in +the species, and present but dormant in the variety. Thus it is present +in both, and as all other characters not differentiating [255] find +their mates in the cross, so these two will also meet one another. They +will unite just as well as though they were both active or both dormant. +For essentially they are the same, only differing in their degree of +activity. From this we can infer, that in the crossing of varieties, no +unpaired remainder is left, all units combining in pairs exactly as in +ordinary fertilization. + +Setting aside the contrast between activity and latency in this single +pair, the procedure in the inter-crossing of varieties is the same as in +ordinary normal fertilization. + +Summarizing this discussion we may conclude that in normal fertilization +and in the inter-crossing of varieties all characters are paired, while +in crosses between elementary species the differentiating marks are not +mated. + +In order to distinguish these two great types of fertilization we will +use the term bisexual for the one and unisexual for the other. The term +balanced crosses then conveys the idea of complete bisexuality, all +unit-characters combining in pairs. Unbalanced crosses are those in +which one or more units do not find their mates and therefore remain +unpaired. This distinction was proposed by Macfarlane when studying the +minute structure of plant-hybrids in comparison with that of their +parents (1892). + +[256] In the first place it shows that a species hybrid may inherit the +distinguishing marks of both parents. In this way it may become +intermediate between them, having some characters in common with the +pollen-parent and others with the pistil-parent. As far as these +characters do not interfere with each other, they may be fully developed +side by side, and in the main this is the way in which hybrid characters +are evolved. But in most cases our existing knowledge of the units is +far too slender to give a complete analysis, even of these +distinguishing marks alone. We recognize the parental marks more or less +clearly, but are not prepared for exact delimitations. Leaving these +theoretical considerations, we will pass to the description of some +illustrative examples. + +In the first place I will describe a hybrid between two species of +_Oenothera_, which I made some years ago. The parents were the common +evening-primrose or _Oenothera biennis_ and of its small-flowered +congener, _Oenothera muricata_. These two forms were distinguished by +Linnaeus as different species, but have been considered by subsequent +writers as elementary species or so-called systematic varieties of one +species designated with the name of the presumably older type, the _O. +biennis_. Varietal differences in a physiologic sense they [257] do not +possess, and for this reason afford a pure instance of unbalanced union, +though differing in more than one point. + +I have made reciprocal crosses, taking at one time the small-flowered +and at the other the common species as pistillate parent. These crosses +do not lead to the same hybrid as is ordinarily observed in analogous +cases; quite on the contrary, the two types are different in most +features, both resembling the pollen-parent far more than the +pistil-parent. The same curious result was reached in sundry other +reciprocal crosses between species of this genus. But I will limit +myself here to one of the two hybrids. + +In the summer of 1895 I castrated some flowers of _O. muricata_, and +pollinated them with _O. biennis_, surrounding the flowers with paper +bags so as to exclude the visits of insects. I sowed the seeds in 1896 +and the hybrids were biennial and flowered abundantly the next year and +were artificially fertilized with their own pollen, but gave only a very +small harvest. Many capsules failed, and the remaining contained only +some few ripe seeds. + +From these I had in the following year the second hybrid generation, and +in the same way I cultivated also the third and fourth. These were as +imperfectly fertile as the first, and in [258] some years did not give +any seed at all, so that the operation had to be repeated in order to +continue the experiment. Last summer (1903) I had a nice lot of some 25 +biennial specimens blooming abundantly. All in all I have grown some 500 +hybrids, and of these about 150 specimens flowered. + +These plants were all of the same type, resembling in most points the +pollen-parent, and in some others the pistil-parent of the original +cross. The most obvious characteristic marks are afforded by the +flowers, which in _O. muricata_ are not half so large as in _biennis_, +though borne by a calyx-tube of the same length. In this respect the +hybrid is like the _biennis_ bearing the larger flowers. These may at +times seem to deviate a little in the direction of the other parent, +being somewhat smaller and of a slightly paler color. But it is very +difficult to distinguish between them, and if _biennis_ and hybrid +flowers were separated from the plants and thrown together, it is very +doubtful whether one would succeed in separating them. + +The next point is offered by the foliage. The leaves of _O. biennis_ are +broad, those of _O. muricata_ narrow. The hybrid has the broad leaves of +_O. biennis_ during most of its life and at the time of flowering. Yet +small deviations in the [259] direction of the other parent are not +wanting, and in winter the leaves of the hybrid rosettes are often much +narrower than those of _O. biennis_, and easily distinguishable from +both parents. A third distinction consists in the density of the spike. +The distance between the insertion of the flowers of _O. biennis_ is +great when compared with that of _O. muricata_. Hence the flowers of the +latter species are more crowded and those of _O. biennis_ more +dispersed, the spikes of the first being densely crowned with flowers +and flower-buds while those of _O. biennis_ are more elongated and +slender. As a further consequence the _O. biennis_ opens on the same +evening only one, two or three flowers on the same spike, whereas _O. +muricata_ bears often eight or ten or more flowers at a time. In this +respect the hybrid is similar to the pistil-parent, and the crowding of +the broad flowers at the top of the spikes causes the hybrids to be much +more showy than either of the parent types. + +Other distinguishing marks are not recorded by the systematists, or are +not so sharply separated as to allow of the corresponding qualities of +the hybrids being compared with them. + +This hybrid remains true to the description given. In some years I +cultivated two generations [260] so as to be able to compare them with +one another, but did not find any difference. The most interesting point +however, is the likeness between the first generation, which obviously +must combine in its internal structure the units of both parents, and +the second and later generations which are only of a derivative nature. +Next to this stands the fact that in each generation all individuals are +alike. No reversion to the parental forms either in the whole type or in +the single characteristics has ever been observed, though the leaves of +some hundreds, and the spikes and flowers of some 150 individual plants +have been carefully examined. No segregation or splitting up takes +place. + +Here we have a clear, undoubted and relatively simple, case of a true +and pure species hybrid. No occurrence of possible varietal +characteristics obscures the result, and in this respect this hybrid +stands out much more clearly than all those between garden-plants, where +varietal marks nearly always play a most important part. + +From the breeder's point of view our hybrid _Oenothera_ would be a +distinct gain, were it not for the difficulty of its propagation. But to +enlarge the range of the varieties this simple and stable form would +need to be treated anew, by [261] crossing it with the parent-types. +Such experiments however, have miscarried owing to the too stable nature +of the unit-characters. + +This stability and this absence of the splitting shown by varietal marks +in the offspring of hybrids is one of the best proofs of unisexual +unions. It is often obscured by the accompanying varietal marks, or +overlooked for this reason. Only in rare cases it is to be met with in a +pure state and some examples are given of this below. + +Before doing so, I must call your attention to another feature of the +unbalanced unions. This is the diminution of the fertility, a phenomenon +universally known as occurring in hybridizations. It has two phases. +First, the diminished chance of the crosses themselves of giving full +crops of seed, as compared with the pure fertilization of either parent. +And, secondly, the fertility of the hybrids themselves. Seemingly, all +grades of diminished fertility occur and the oldest authors on hybrids +have pointed out that a very definite relation exists between the +differences of the parents and the degree of sterility, both of the +cross and of the hybrid offspring. In a broad sense these two factors +are proportionate to each other, the sterility being the greater, the +lesser the affinity between the parents. Many writers have [262] tried +to trace this rule in the single cases, but have met with nearly +unsurmountable difficulties, owing chiefly to our ignorance of the units +which form the differences between the parents in the observed cases. + +In the case of _Oenothera muricata x biennis_ the differentiating units +reduce the fertility to a low degree, threatening the offspring with +almost complete infertility and extinction. But then we do not know +whether these characters are really units, or perhaps only seemingly so +and are in reality composed of smaller entities which as yet we are not +able to segregate. And as long as we are devoid of empirical means of +deciding such questions, it seems useless to go farther into the details +of the question of the sterility. It should be stated here however, that +pure varietal crosses, when not accompanied by unbalanced characters, +have never showed any tendency to diminished fertility. Hence there can +be little doubt that the unpaired units are the cause of this decrease +in reproductive power. + +The genus _Oenothera_ is to a large degree devoid of varietal +characteristics, especially in the subgenus _Onagra_, to which +_biennis_, _muricata_, _lamarckiana_ and some others belong. On the +other hand it seems to be rich in elementary species, but an adequate +study of [263] them has as yet not been made. Unfortunately many of the +better systematists are in the habit of throwing all these interesting +forms together, and of omitting their descriptive study. I have made a +large number of crosses between such undescribed types and as a rule got +constant hybrid races. Only one or two exceptions could be quoted, as +for instance the _Oenothera brevistylis_, which in its crosses always +behaves as a pure retrogressive variety. Instead of giving an exhaustive +survey of hybrids, I simply cite my crosses between _lamarckiana_ and +_biennis_, as having nearly the aspect of the last named species, and +remaining true to this in the second generation without any sign of +reversion or of splitting. I have crossed another elementary species, +the _Oenothera hirtella_ with some of my new and with some older Linnean +species, and got several constant hybrid races. Among these the +offspring of a cross between _muricata_ and _hirtella_ is still in +cultivation. The cross was made in the summer of 1897 and last year +(1903) I grew the fourth generation of the hybrids. These had the +characters of the _muricata_ in their narrow leaves, but the elongated +spikes and relatively large flowers of the _hirtella_ parent, and +remained true to this type, showing only slight fluctuations and never +reverting or segregating [264] the mixed characters. Both parents bear +large capsules with an abundance of seed, but in the hybrids the +capsules remain narrow and weak, ripening not more than one-tenth the +usual quantity of seed. Both parents are easily cultivated in annual +generations and the same holds good for the hybrid. But whereas the +hybrid of muricata and biennis is a stout plant, this type is weak with +badly developed foliage, and very long strict spikes. Perhaps it was not +able to withstand the bad weather of the last few years. + +A goodly number of constant hybrids are described in literature, or +cultivated in fields and gardens. In such cases the essential question +is not whether they are now constant, but whether they have been so from +the beginning, or whether they prove to be constant whenever the +original cross is repeated. For constant hybrids may also be the issue +of incipient splittings, as we shall soon see. + +Among other examples we may begin with the hybrid alfalfa or hybrid +lucerne (_Medicago media_). It often originates spontaneously between +the common purple lucerne or alfalfa and its wild ally with yellow +flowers and procumbent stems, the _Medicago falcata_. This hybrid is +cultivated in some parts of Germany on a large scale, as it is more +productive than [265] the ordinary lucerne. It always comes true from +seed and may be seen in a wild state in parks and on lawns. It is one of +the oldest hybrids with a pure and known lineage. The original cross has +been repeated by Urban, who found the hybrid race to be constant from +the beginning. + +Another very notorious constant hybrid race is the _Aegilops +speltaeformis_. It has been cultivated in botanic gardens for more than +half a century, mostly in annual or biennial generations. It is +sufficiently fertile and always comes true. Numerous records have been +made of it, since formerly it was believed by Fabre and others to be a +spontaneous transition from some wild species of grass to the ordinary +wheat, not a cross. Godron, however, showed that it can be produced +artificially, and how it has probably sprung into existence wherever it +is found wild. The hybrid between _Aegilops ovata_, a small weed, and +the common wheat is of itself sterile, producing no good pollen. But it +may be fertilized by the pollen of wheat and then gives rise to a +secondary hybrid, which is no other than the _Aegilops speltaeformis_. +This remained constant in Godron's experiments during a number of +generations, and has been constant up to the present time. + +[266] Constant hybrids have been raised by Millardet between several +species of strawberries. He combined the old cultivated forms with newly +discovered types from American localities. They ordinarily showed only +the characteristics of one of their parents and did not exhibit any new +combination of qualities, but they came true to this type in the second +and later generations. + +In the genus _Anemone_, Janczewski obtained the same results. Some +characters of course may split, but others remain constant, and when +only such are present, hybrid races result with new combinations of +characters, which are as constant as the best species of the same genus. +The hybrids of Janczewski were quite fertile, and he points out that +there is no good reason why they should not be considered as good new +species. If they had not been produced artificially, but found in the +wild state, their origin would have been unknown, and there can be no +doubt that they would have been described by the best systematists as +species of the same value as their parents. Such is especially the case +with a hybrid between _Anemone magellanica_ and the common _Anemone +sylvestris_. + +Starting from similar considerations Kerner von Marilaun pointed out the +fact long ago that many so-called species, of rare occurrence, [267] +standing between two allied types, may be considered to have originated +by a cross. Surely a wide field for abuse is opened by such an +assertion, and it is quite a common habit to consider intermediate forms +as hybrids, on the grounds afforded by their external characters alone, +and without any exact knowledge of their real origin and often without +knowing anything as to their constancy from seed. All such apparent +explanations are now slowly becoming antiquated and obsolete, but the +cases adduced by Kerner seem to stand this test. + +Kerner designates a willow, _Salix ehrhartiana_ as a constant hybrid +between _Salix alba_ and _S. pentandra_. _Rhododendron intermedium_ is +an intermediate form between the hairy and the rusty species from the +Swiss Alps, _R. hirsutum_ and _R. ferrugineum_, the former growing on +chalky, and the other on silicious soils. Wherever both these types of +soil occur in the same valley and these two species approach one +another, the hybrid _R. intermedium_ is produced, and is often seen to +be propagating itself abundantly. As is indicated by the name, it +combines the essential characters of both parents. + +_Linaria italica_ is a hybrid toad-flax between _L. genistifolia_ and +_L. vulgaris_, a cross which I have repeated in my garden. _Drosera +obovata_ [268] is a hybrid sundew between _D. anglica_ and _D. +rotundifolia_. _Primula variabilis_ is a hybrid between the two common +primroses, _P. officinalis_ and _P. grandiflora_. The willow-herb +(_Epilobium_), the self-heal (_Brunella_) and the yellow pond-lilies +(Nuphar) afford other instances of constant wild hybrids. + +Macfarlane has discovered a natural hybrid between two species of sundew +in the swamps near Atco, N.J. The parents, _D. intermedia_ and _D. +filiformis_, were growing abundantly all around, but of the hybrid only +a group of eleven plants was found. A detailed comparison of the hybrid +with its parents demonstrated a minute blending of the anatomical +peculiarities of the parental species. + +Luther Burbank of Santa Rosa, California, has produced a great many +hybrid brambles, the qualities of which in many respects surpass those +of the wild species. Most of them are only propagated by cuttings and +layers, not being stable from seed. But some crosses between the +blackberry and the raspberry (_R. fruticosus_ and _R. idaeus_) which +bear good fruit and have become quite popular, are so fixed in their +type as to reproduce their composite characters from seed with as much +regularity as the species of _Rubus_ found in nature. Among them are the +"Phenomenal" and the [269] "Primus." The latter is a cross between the +Californian dewberry and the Siberian raspberry and is certainly to be +regarded as a good stable species, artificially produced. Bell Salter +crossed the willow-herbs _Epilobium tetragonum_ and _E. montanum_, and +secured intermediate hybrids which remained true to their type during +four successive generations. + +Other instances might be given. Many of them are to be found in +horticultural and botanical journals which describe their systematic and +anatomical details. The question of stability is generally dealt with in +an incidental manner, and in many cases it is difficult to reach +conclusions from the facts given. Especially disturbing is the +circumstance that from a horticultural point of view it is quite +sufficient that a new type should repeat itself in some of its offspring +to be called stable, and that for this reason absolute constancy is +rarely proved. + +The range of constant hybrids would be larger by far were it not for two +facts. The first is the absolute sterility of so many beautiful hybrids, +and the second is the common occurrence of retrogressive characters +among cultivated plants. To describe the importance of both these groups +of facts would take too much [270] time, and therefore it seems best to +give some illustrative examples instead. + +Among the species of _Ribes_ or currant, which are cultivated in our +gardens, the most beautiful are without doubt the Californian and the +Missouri currant, or _Ribes sanguineum_ and _R. aureum_. A third form, +often met with, is "Gordon's currant," which is considered to be a +hybrid between the two. It has some peculiarities of both parents. The +leaves have the general form of the Californian parent, but are as +smooth as the Missouri species. The racemes or flower-spikes are densely +flowered as in the red species, but the flowers themselves are of a +yellow tinge, with only a flesh-red hue on the outer side of the calyx. +It grows vigorously and is easily multiplied by cuttings, but it never +bears any fruit. Whether it would be constant, if fertile, is therefore +impossible to decide. _Berberis ilicifolia_ is considered as a hybrid +between the European barberry (_B. vulgaris_) and the cultivated shrub +_Mahonia aquifolia_. The latter has pinnate leaves, the former undivided +ones. The hybrid has undivided leaves which are more spiny than those of +the European parent, and which are not deciduous like them, but persist +during the winter, a peculiarity inherited from the _Mahonia_. As far as +I [271] have been able to ascertain, this hybrid never produces seed. + +Another instance of an absolutely sterile hybrid is the often quoted +_Cytisus adami_. It is a cross between the common laburnum (_Cytisus +Laburnum_) and another species of the same genus, _C. purpureus_, and +has some traits of both. But since the number of differentiating marks +is very great in this case, most of the organs have become intermediate. +It is absolutely sterile. But it has the curious peculiarity of +splitting in a vegetative way. It has been multiplied on a large scale +by grafting and was widely found in the parks and gardens of Europe +during the last century. Nearly all these specimens reverted from time +to time to the presumable parents. Not rarely a bud of Adam's laburnum +assumed all the qualities of the common laburnum, its larger leaves, +richer flowered racemes, large and brightly yellow flowers and its +complete fertility. Other buds on the same tree reverted to the purple +parent, with its solitary small flowers, its dense shrublike branches +and very small leaves. These too are fertile, though not producing their +seeds as abundantly as the _C. Laburnum_ reversions. Many a botanist has +sown the seeds of the latter and obtained only pure common _C. Laburnum_ +plants. I had a lot of nearly a hundred seedlings [272] myself, many of +which have already flowered, bearing the leaves and flowers of the +common species. Seeds of the purple reversions have also been sown, and +also yielded the parental type only. + +Why this most curious hybrid sports so regularly and why others always +remain true to their type is as yet an open question. + +But recalling our former consideration of this subject the supposition +seems allowable that the tendency to revert is not connected with the +type of the hybrid, but is apt to occur in some rare individuals of +every type. But since most of the sterile hybrids are only known to us +in a single individual and its vegetative offspring, this surmise offers +an explanation of the rare occurrence of sports. + +Finally, we must consider some of the so called hybrid races or strains +of garden-plants. _Dahlia_, _Gladiolus_, _Amaryllis_, _Fuchsia_, +_Pelargonium_ and many other common flowers afford the best known +instances. Immeasurable variability seems here to be the result of +crossing. But on a closer inspection the range of characters is not so +very much wider in these hybrid races than in the groups of parent +species which have contributed to the origin of the hybrids. Our +tuberous begonias owe their variability to at least seven original +parent species, [273] and to the almost incredible number of +combinations which are possible between their characters. The first of +these crosses was made in the nursery of Veitch and Sons near London by +Seden, and the first hybrid is accordingly known as _Begonia sedeni_ and +is still to be met with. It has been superseded by subsequent crosses +between the _sedeni_ itself and the _Veitchi_ and _rosiflora_, the +_davisii_, the _clarkii_ and others. Each of them contributed its +advantageous qualities, such as round flowers, rosy color, erect flower +stalks, elevation of the flowers above the foliage and others. New +crosses are being made continuously, partly between the already existing +hybrids and partly with newly introduced wild species. Only rarely is it +possible to get pure seeds, and I have not yet been able to ascertain +whether the hybrids would come true from seed. Specific and varietal +characters may occur together in many of the several forms, but nothing +is as yet accurately known as to their behavior in pure fertilizations. +Constancy and segregation are thrown together in such a manner that +extreme variability results, and numerous beautiful types may be had, +and others may be expected from further crosses. For a scientific +analysis, however, the large range of recorded facts and the written +history, which at first sight [274] seems to have no lacunae, are not +sufficient. Most of the questions remain open and need investigation. It +would be a capital idea to try to repeat the history of the begonias or +any other hybrid race, making all the described crosses and then +recording the results in a manner requisite for complete and careful +scientific investigations. + +Many large genera of hybrid garden-flowers owe their origin to species +rich in varieties or in elementary subspecies. Such is the case with the +gladiolus and the tulips. In other cases the original types have not +been obtained from the wild state but from the cultures of other +countries. + +The dahlias were cultivated in Mexico when first discovered by +Europeans, and the chrysanthemums have been introduced from the old +gardens of Japan. Both of them consisted of various types, which +afterwards have been increased chiefly by repeated intercrossing. + +The history of many hybrid races is obscure, or recorded by different +authorities in a different way. Some have derived their evidence from +one nursery, some from another, and the crosses evidently may have been +different in different places. The early history of the gladiolus is an +instance. The first crosses are recorded to have been made between +_Gladiolus_ [275] _psittacinus_ and _G. cardinalis_, and between their +hybrid, which is still known under the name of gandavensis_ and the +_purpureo-auratus_. But other authors give other lines of descent. So it +is with _Amaryllis_, which is said by De Graaff to owe its stripes to +_A. vittata_, its fine form to _A. brasiliensis_, the large petals to +_A. psittacina_, the giant flowers to _A. leopoldi_, and the piebald +patterns to _A. pardina_. But here, too, other authors give other +derivations. + +Summarizing the results of our inquiry we see in the first place how +very much remains to be done. Many old crosses must be repeated and +studied anew, taking care of the purity of the cross as well as of the +harvesting of the seeds. Many supposed facts will be shown to be of +doubtful validity. New facts have to be gathered, and in doing so the +distinction between specific and varietal marks must be taken strictly +into account. The first have originated as progressive mutations; they +give unbalanced crosses with a constant offspring, as far as experience +now goes. The second are chiefly due to retrograde modifications, and +will be the subject of the next lecture. + + +[276] + +LECTURE X + +MENDEL'S LAW OF BALANCED CROSSES + +In the scientific study of the result of crosses, the most essential +point is the distinction of the several characters of the parents in +their combination in the hybrids and their offspring. From a theoretical +point of view it would be best to choose parents which would differ only +in a single point. The behavior of the differentiating character might +then easily be seen. + +Unfortunately, such simple cases do not readily occur. Most species, and +even many elementary species are distinguished by more than one quality. +Varieties deviating only in one unit-character from the species, are +more common. But a closer inspection often reveals some secondary +characters which may be overlooked in comparative or descriptive +studies, but which reassume their importance in experimental crossings. + +In a former lecture we have dealt with the qualities which must be +considered as being due to the acquisition of new characters. If we +[277] compare the new form in this case with the type from which it has +originated, it may be seen that the new character does not find its +mate, or its opposite, and it will be unpaired in the hybrid. + +In the case of retrogressive changes the visible modification is due, at +least in the best known instances, to the reduction of an active quality +to a state of inactivity or latency. Now if we make a cross between a +species and its variety, the differentiating character will be due to +the same internal unit, with no other difference than that it is active +in the species and latent in the variety. In the hybrid these two +corresponding units will make a pair. But while all other pairs in the +same hybrid individuals consist of like antagonists, only this pair +consists of slightly unlike opponents. + +This conception of varietal crosses leads to three assertions, which +seem justifiable by actual experience. + +First, there is no reason for a diminution of the fertility, as all +characters are paired in the hybrid, and no disturbance whatever ensues +in its internal structure. Secondly, it is quite indifferent, how the +two types are combined, or which of them is chosen as pistillate and +which as staminate parent. The deviating pair will have the same +constitution in both cases, being [278] built up of one active and one +dormant unit. Thirdly this deviating pair will exhibit the active unit +which it contains, and the hybrid will show the aspect of the parent in +which the character was active and not that of the parent in which it +was dormant. Now the active quality was that of the species, and its +latent state was found in the variety. Hence the inference that hybrids +between a species and its retrograde variety will bear the aspect of the +species. This attribute may be fully developed, and then the hybrid will +not be distinguishable from the pure species in its outer appearance. Or +the character may be incompletely evolved, owing to the failure of +cooperation of the dormant unit. In this case the hybrid will be in some +sense intermediate between its parents, but these instances are more +rare than the alternate ones, though presumably they may play an +important part in the variability of many hybrid garden-flowers. + +All of these three rules are supported by a large amount of evidence. +The complete fertility of varietal hybrids is so universally +acknowledged that it is not worth while to give special instances. With +many prominent systematists it has become a test between species and +varieties, and from our present point of view this assumption is +correct. Only the test is of little use in practice, as fertility may be +diminished [279] in unbalanced unions in all possible degrees, according +to the amount of difference between the parents. If this amount is +slight, if for instance, only one unit-character causes the difference, +the injury to fertility may, be so small as to be practically nothing. +Hence we see that this test would not enable us to judge of the doubtful +cases, although it is quite sufficient as a proof in cases of wider +differences. + +Our second assertion related to the reciprocal crosses. This is the name +given to two sexual combinations between the same parents, but with +interchanged places as to which furnishes the pollen. In unbalanced +crosses of the genus _Oenothera_ the hybrids of such reciprocal unions +are often different, as we have previously shown. Sometimes both +resemble the pollen parent more, in other instances the pistil-parent. +In varietal crosses no such divergence is as yet known. It would be +quite superfluous to adduce single cases as proofs for this rule, which +was formerly conceived to hold good for hybrids in general. The work of +the older hybridists, such as Koelreuter and Gaertner affords numerous +instances. + +Our third rule is of a wholly different nature. Formerly the distinction +between elementary species and varieties was not insisted upon, and the +principle which stamps retrograde changes [280] as the true character of +varieties is a new one. Therefore it is necessary to cite a considerable +amount of evidence in order to prove the assertion that a hybrid bears +the active character of its parent-species and not the inactive +character of the variety chosen for the cross. + +We may put this assertion in a briefer form, stating that the active +character prevails in the hybrid over its dormant antagonist. Or as it +is equally often put, the one dominates and the other is recessive. In +this terminology the character of the species is dominant in the hybrid +while that of the variety is recessive. Hence it follows that in the +hybrid the latent or dormant unit is recessive, but it does not follow +that these three terms have the same meaning, as we shall see presently. +The term recessive only applies to the peculiar state into which the +latent character has come in the hybrid by its pairing with the +antagonistic active unit. + +In the first place it is of the highest importance to consider crosses +between varieties of recorded origin and the species from which they +have sprung. When dealing with mutations of celandine we shall see that +the laciniated form originated from the common celandine in a garden at +Heidelberg about the year 1590. Among my _Oenotheras_ one of the eldest +of the recent productions is the _O. brevistylis_ or short [281] styled +species which was seen for the first time in the year 1889. The third +example offered is a hairless variety of the evening campion, _Lychnis +vespertina_, found the same year, which hitherto had not been observed. + +For these three cases I have made the crosses of the variety with the +parent-species, and in each case the hybrid was like the species, and +not like the variety. Nor was it intermediate. Here it is proved that +the older character dominates the younger one. + +In most cases of wild, and of garden-varieties, the relation between +them and the parent-species rests upon comparative evidence. Often the +variety is known to be younger, in other cases it may be only of local +occurrence, but ordinarily the historic facts about its origin have +never been known or have long since been forgotten. + +The easiest and most widely known varietal crosses are those between +varieties with white flowers and the red- or blue-flowered species. Here +the color prevails in the hybrid over the lack of pigment, and as a rule +the hybrid is as deeply tinted as the species itself, and cannot be +distinguished from it, without an investigation of its hereditary +qualities. Instances may be cited of the white varieties of the +snapdragon, of the red clover, the long-spurred violet (_Viola_ [282] +_cornuta_) the sea-shore aster (_Aster Tripolium_), corn-rose +(_Agrostemma Githago_), the Sweet William (_Silene Armeria_), and many +garden flowers, as for instance, the _Clarkia pulchella_, the +_Polemonium coeruleum_, the _Veronica longifolia_, the gloxinias and +others. If the red hue is combined with a yellow ground-color in the +species, the variety will be yellow and the hybrid will have the red and +yellow mixture of the species as for instance, in the genus _Geum_. The +toad-flax has an orange-colored palate, and a variety occurs in which +the palate is of the same yellow tinge as the remaining parts of the +corolla. The hybrid between them is in all respects like the +parent-species. + +Other instances could be given. In berries the same rule prevails. The +black nightshade has a variety with yellow berries, and the black color +returns in the hybrid. Even the foliage of some garden-plants may afford +instances, as for instance, the purplish amaranth (_Amaranthus +caudatus_). It has a green variety, but the hybrid between the two has +the red foliage of the species. + +Special marks in leaves and in flowers follow the same rule. Some +varieties of the opium poppy have large black patches at the basal end +of the petals, while in others this pattern is entirely white. In +crossing two such varieties, [283] for instance, the dark "Mephisto" +with the white-hearted "Danebrog," the hybrid shows the active character +of the dark pattern. + +Hairy species crossed with their smooth varieties produce hairy hybrids, +as in some wheats, in the campion (_Lychnis_), in _Biscutella_ and +others. The same holds good for the crosses between spiny species and +their unarmed derivatives, as in the thorn-apple, the corn-crowfoot +(_Ranunculus arvensis_) and others. + +Lack of starch in seeds is observed in some varieties of corn and of +peas. When such derivatives are crossed with ordinary starch-producing +types, the starch prevails in the hybrid. + +It would take too much time to give further examples. But there is still +one point which should be insisted upon. It is not the systematic +relation of the two parents of a cross, that is decisive, but only the +occurrence of the same quality, in the one in an active, and in the +other in an inactive condition. Hence, whenever this relation occurs +between the parents of a cross, the active quality prevails in the +hybrid, even when the parents differ from each other in other respects +so as to be distinguished as systematic species. The white and red +campions give a red hybrid, the black and pale henbane (_Hyoscyamus +niger_ and _H. pallidus_) give a hybrid [284] with the purple veins and +center in the corolla of the former, the white and blue thornapple +produce a blue hybrid, and so on. Instances of this sort are common in +cultivated plants. + +Having given this long list of examples of the rule of the dominancy of +the active character over the opposite dormant unit, the question +naturally arises as to how the antagonistic units are combined in the +hybrid. This question is of paramount importance in the consideration of +the offspring of the hybrids. But before taking it up it is as well to +learn the real signification of recessiveness in the hybrids themselves. + +Recessive characters are shown by those rare cases, in which hybrids +revert to the varietal parent in the vegetative way. In other words by +bud-variations or sports, analogous to the splitting of Adam's laburnum +into its parents, by means of bud-variation already described. But here +the wide range of differentiating characters of the parents of this most +curious hybrid fail. The illustrative examples are extremely simple, and +are limited to the active and inactive condition of only one quality. + +An instance is given by the long-leaved veronica (_Veronica +longifolia_), which has bluish flowers in long spikes. The hybrid +between [285] this species and its white variety has a blue corolla. But +occasionally it produces some purely white flowers, showing its power of +separating the parental heritages, combined in its internal structures. +This reversion is not common, but in thousands of flowering spikes one +may expect to find at least one of them. Sometimes it is a whole stem +springing from the underground system and bearing only white flowers on +all its spikes. In other instances it is only a side branch which +reverts and forms white flowers on a stem, the other spikes of which +remain bluish. Sometimes a spike even differentiates longitudinally, +bearing on one side blue and on the other white corollas, and the white +stripe running over the spike may be seen to be long and large, or +narrow and short in various degrees. In such cases it is evident that +the heritages of the parents remain uninfluenced by each other during +the whole life of the hybrid, working side by side, but the active +element always prevails over its latent opponent which is ready to break +free whenever an opportunity is offered. + +It is now generally assumed that this incomplete mixture of the parental +qualities in a hybrid, this uncertain and limited combination is the +true cause of the many deviations, exhibited by varietal hybrids when +compared with their [286] parents. Partial departures are rare in the +hybrids themselves, but in their offspring the divergence becomes the +rule. + +Segregation seems to be a very difficult process in the vegetative way, +but it must be very easy in sexual reproduction, indeed so easy as to +show itself in nearly every single instance. + +Leaving this first generation, the original hybrids, we now come to a +discussion of their offspring. Hybrids should be fertilized either by +their own pollen, or by that of other individuals born from the same +cross. Only in this case can the offspring be considered as a means of +arriving at a decision as to the internal nature of the hybrids +themselves. Breeders generally prefer to fertilize hybrids with the +pollen of their parents. But this operation is to be considered as a new +cross, and consequently is wholly excluded from our present discussion. +Hence it follows that a clear insight into the heredity of hybrids may +be expected only from scientific experiments. Furthermore some of the +diversity observed as a result of ordinary crosses, may be due to the +instability of the parents themselves or at least of one of them, since +breeders ordinarily choose for their crosses some already very variable +strain. Combining such a strain with the desirable qualities of some +newly imported species, a new strain may [287] result, having the new +attribute in addition to all the variability of the old types. In +scientific experiments made for the purpose of investigating the general +laws of hybridity, such complex cases are therefore to be wholly +excluded. The hereditary purity of the parents must be considered as one +of the first conditions of success. + +Moreover the progeny must be numerous, since neither constancy, nor the +exact proportions in the case of instability, can be determined with a +small lot of plants. + +Finally, and in order to come to a definite choice of research material, +we should keep in mind that the chief object is to ascertain the +relation of the offspring to their parents. Now in nearly all cases the +seeds are separated from the fruits and from one another, before it +becomes possible to judge of their qualities. One may open a fruit and +count the seeds, but ordinarily nothing is noted as to their characters. +In this respect no other plant equals the corn or maize, as the kernels +remain together on the spike, and as it has more than one variety +characterized by the color, or constitution, or other qualities of the +grains. A corn-grain, however, is not a seed, but a fruit containing a +seed. Hence the outer parts pertain to the parent plant and only the +innermost ones to the [288] seedling and therefore to the following +generation. Fruit-characters thus do not offer the qualities we need, +only the qualities resulting from fertilizations are characteristic of +the new generation. Such attributes are afforded in some cases by the +color, in others by the chemical constitution. + +We will choose the latter, and take the sugarcorn in comparison with the +ordinary or starch producing forms for our starting point. Both sugar- +and starch-corns have smooth fruits when ripening. No difference is to +be seen in the young ripe spikes. Only the taste, or a direct chemical +analysis might reveal the dissimilarity. But as soon as the spikes are +dried, a diversity is apparent. The starchy grains remain smooth, but +the sugary kernels lose so much water that they become wrinkled. The +former becomes opaque, the latter more or less transparent. Every single +kernel may instantly be recognized as belonging to either of the types +in question, even if but a single grain of the opposite quality might be +met with on a spike. Kernels can be counted on the spike, and since +ordinary spikes may bear from 300-500 grains and often more, the +numerical relation of the different types may be deduced with great +accuracy. + +Coming now to our experiment, both starchy [289] and sugary varieties +are in this respect wholly constant, when cultivated separately. No +change is to be seen in the spikes. Furthermore it is very easy to make +the crosses. The best way is to cultivate both types in alternate rows +and to cut off the staminate panicles a few days before they open their +first flowers. If this operation is done on all the individuals of one +variety, sparing all the panicles of the other, it is manifest that all +the plants will become fertilized by the latter, and hence that the +castrated plants will only bear hybrid seeds. + +The experiment may be made in two ways; by castrating the sugary or the +starchy variety. In both cases the hybrid kernels are the same. As to +their composition they repeat the active character of the starchy +variety. The sugar is only accumulated as a result of an incapacity of +changing it into starch, and the lack of this capacity is to be +considered as a retrogressive varietal mark. The starch-producing unit +character, which is active in the ordinary sorts of corns, is therefore +latent in sugar-corn. + +In order to obtain the second generation, the hybrid grains are sown +under ordinary conditions, but sufficiently distant from any other +variety of corn to insure pure fertilization. The several individuals +may be left to pollinate [290] each other, or they may be artificially +pollinated with their own pollen. + +The outcome of the experiments is shown by the spikes, as soon as they +dry. Each spike bears two sorts of kernels irregularly dispersed over +its surface. In this point all the spikes are alike. On each of them one +may see on the first inspection that the majority of the kernels are +starch-containing seeds, while a minor part becomes wrinkled and +transparent according to the rule for sugary seeds. This fact shows at +once that the hybrid race is not stable, but has differentiated the +parental characters, bringing those of the varietal parent to perfect +purity and isolation. Whether the same holds good for the starchy +parent, it is impossible to judge from the inspection of the spikes, +since it has been seen in the first generation that the hybrid kernels +are not visibly distinguished from those of the pure starch-producing +grains. + +It is very easy to count the number of both sorts of grains in the spike +of such a hybrid. In doing so we find, that the proportion is nearly the +same on all the spikes, and only slight variations would be found in +hundreds of them. One-fourth of the seeds are wrinkled and three-fourths +are always smooth. The number may vary in single instances and be a +little more or a little less than 25%, ranging, for [291] instance, from +20 to 27%, but as a rule, the average is found nearly equal to 25%. + +The sugary kernels, when separated from the hybrid spikes and sown +separately, give rise to pure sugary race, in no degree inferior in +purity to the original variety. But the starchy kernels are of different +types, some of them being internally like the hybrids of the first +generation and others like the original parent. To decide between these +two possibilities, it is necessary to examine their progeny. + +For the study of this third hybrid generation we will now take another +example, the opium poppies. They usually have a dark center in the +flowers, the inferior parts of the four petals being stained a deep +purple, or often nearly black. Many varieties exhibit this mark as a +large black cross in the center of the flower. In other varieties the +pigment is wanting, the cross being of a pure white. Obviously it is +only reduced to a latent condition, as in so many other cases of loss of +color, since it reappears in a hybrid with the parent-species. + +For my crosses I have taken the dark-centered "Mephisto" and the +"Danebrog," or Danish flag, with a white cross on a red field. The +second year the hybrids were all true to the type of "Mephisto." From +the seeds of each artificially self-fertilized capsule, one-fourth +(22.5%) [292] in each instance reverted to the varietal mark of the +white cross, and three-fourths (77.5%) retained the dark heart. Once +more the flowers were self-pollinated and the visits of insects +excluded. The recessives now gave only recessives, and hence we may +conclude that the varietal marks had returned to stability. The dark +hearted or dominants behaved in two different ways. Some of them +remained true to their type, all their offspring being dark-hearted. +Evidently they had returned to the parent with the active mark, and had +reassumed this type as purely as the recessives had reached theirs. But +others kept true to the hybrid character of the former generation, +repeating in their progeny exactly the same mixture as their parents, +the hybrids of the first generation, had given. + +This third generation therefore gives evidence, that the second though +apparently showing only two types, really consists of three different +groups. Two of them have reassumed the stability of their original +grandparents, and the third has retained the instability of the hybrid +parents. + +The question now arises as to the numerical relation of these groups. +Our experiments gave the following results: [293] + + + Cross 1. Generation 2. Generation 3. Generation + + Mephisto 4- 100% Mephisto + | / + | / + | 77.5 % Dom. + | / \ + > --All Mephisto \ + | \ 9- all hybrids with 83-68% + | 22.5 % Rec. dominants and 17-32% + | recessives. 100% Danebrog. + Danebrog + +Examining these figures we find one-fourth of constant recessives, as +has already been said, further one-fourth of constant dominants, and the +rest or one half as unstable hybrids. Both of the pure groups have +therefore reappeared [293] in the same numbers. Calling A the specimens +with the pure active mark, L those with the latent mark, and H the +hybrids, these proportions may be expressed as follows: + + 1A+2H+1L. + +This simple law for the constitution of the second generation of +varietal hybrids with a single differentiating mark in their parents is +called the law of Mendel. Mendel published it in 1865, but his paper +remained nearly unknown to scientific hybridists. It is only of late +years that it has assumed a high place in scientific literature, and +attained the first rank as an investigation on fundamental questions of +heredity. [294] Read in the light of modern ideas on unit characters it +is now one of the most important works on heredity and has already +widespread and abiding influence on the philosophy of hybridism in +general. + +But from its very nature and from the choice of the material made by +Mendel, it is restricted to balanced or varietal crosses. It assumes +pairs of characters and calls the active unit of the pair dominant, and +the latent recessive, without further investigations of the question of +latency. It was worked out by Mendel for a large group of varieties of +peas, but it holds good, with only apparent exceptions, for a wide range +of cases of crosses of varietal characters. Recently many instances have +been tested, and even in many cases third and later generations have +been counted, and whenever the evidence was complete enough to be +trusted, Mendel's prophecy has been found to be right. + +According to this law of Mendel's the pairs of antagonistic characters +in the hybrid split up in their progeny, some individuals reverting to +the pure parental types, some crossing with each other anew, and so +giving rise to a new generation of hybrids. Mendel has given a very +suggestive and simple explanation of his formula. Putting this in the +terminology of to-day, and limiting it to the occurrence of only [295] +one differential unit in the parents, we may give it in the following +manner. In fertilization, the characters of both parents are not +uniformly mixed, but remain separated though most intimately combined in +the hybrid throughout life. They are so combined as to work together +nearly always, and to have nearly equal influence on all the processes +of the whole individual evolution. But when the time arrives to produce +progeny, or rather to produce the sexual cells through the combination +of which the offspring arises, the two parental characters leave each +other, and enter separately into the sexual cells. From this it may be +seen that one-half of the pollen-cells will have the quality of one +parent, and the other the quality of the other. And the same holds good +for [296] the egg-cells. Obviously the qualities lie latent in the +pollen and in the egg, but ready to be evolved after fertilization has +taken place. + +Granting these premises, we may now ask as to the results of the +fertilization of hybrids, when this is brought about by their own +pollen. We assume that numerous pollen grains fertilize numerous egg +cells. This assumption at once allows of applying the law of +probability, and to infer that of each kind of pollen grains one-half +will reach egg-cells with the same quality [297] and the other half +ovules with the opposite character. + +Calling P pollen and O ovules, and representing the active mark by P and +O, the latent qualities by P' and O', they would combine as follows: + + P + 0 giving uniform pairs with the active mark, + P + 0' giving unequal pairs, + P' + 0 giving unequal pairs, + P' + 0' giving uniform pairs with the latent mark. + +In this combination the four groups are obviously of the same size, each +containing one-fourth of the offspring. Manifestly they correspond +exactly to the direct results of the experiments, P + O representing the +individuals which reverted to the specific mark, P' + O' those who +reassumed the varietal quality and P + O' and P + O' those who +hybridized [298] for the second time. These considerations lead us to +the following form of Mendel's, + + P + O = 1/4 Active or 1A, + + P + O' + > = 1/2 Hybrid or 2 H, + P' + O + + P' + O' = 1/4 Latent or 1 L, + +Which is evidently the same as Mendel's empirical law given above. + +To give the proof of these assumptions Mendel has devised a very simple +crossing experiment, [299] which he has effected with his varieties of +peas. I have repeated it with the sugar-corn, which gives far better +material for demonstration. It starts from the inference that if +dissimilarity among the pollen grains is excluded, the diversity of the +ovules must at once became manifest and vice versa. In other terms, if a +hybrid of the first generation is not allowed to fertilize itself, but +is pollinated by one of its parents, the result will be in accordance +with the Mendelian formula. + +In order to see an effect on the spikes produced in this way, it is of +course necessary to fertilize them with the pollen of the variety, and +not with that of the specific type. The latter would give partly pure +starchy grains and partly hybrid kernels, but these would assume the +same type. But if we pollinate the hybrid with pollen of a pure +sugar-corn, we may predict the result as follows. + +If the spike of the hybrid contains dormant paternal marks in one-half +of its flowers and in the other half maternal latent qualities, the +sugar-corn pollen will combine with one-half of the ovules to give +hybrids, and with the other half so as to give pure sugar-grains. Hence +we see that it will be possible to count out directly the two groups of +ovules on inspecting the ripe and dry spikes. Experience teaches us +[298] that both are present, and in nearly equal numbers; one-half of +the grains remaining smooth, and the other half becoming wrinkled. + +The corresponding experiment could be made with plants of a pure +sugar-race by pollination with hybrid pollen. The spikes would show +exactly the same mixture as in the above case, but now this may be +considered as conclusive proof that half the pollen-grains represent the +quality of one parent and the other half the quality of the other. + +Another corollary of Mendel's law is the following. In each generation +two groups return to purity, and one-half remains hybrid. These last +will repeat the same phenomenon of splitting in their progeny, and it is +easily seen that the same rule will hold good for all succeeding +generations. According to Mendel's principle, in each year there is a +new hybridization, differing in no respect from the first and original +one. If the hybrids only are propagated, each year will show one-fourth +of the offspring returning to the specific character, one-fourth +assuming the type of the variety and one-half remaining hybrid. I have +tested this with a hybrid between the ordinary nightshade with black +berries, and its variety, _Solanum nigrum chlorocarpum_, with pale +yellow fruits. Eight generations of the hybrids were cultivated, [299] +disregarding always the reverting offspring. At the end I counted the +progeny of the sixth and seventh generations and found figures for their +three groups of descendants, which exactly correspond to Mendel's +formula. + +Until now we have limited ourselves to the consideration of single +differentiating units. This discussion gives a clear insight into the +fundamental phenomena of hybrid fertilization. It at once shows the +correctness of the assumption of unit-characters, and of their pairing +in the sexual combinations. + +But Mendel's law is not at all restricted to these simple cases. Quite +on the contrary, it explains the most intricate questions of +hybridization, providing they do not transgress the limits of +symmetrical unions. But in this realm nearly all results may be +calculated beforehand, on the ground of the principle of probability. +Only one more assumption need be discussed. The several pairs of +antagonistic characters must be independent from, and uninfluenced by, +one another. This premise seems to hold good in the vast majority of +cases, though rare exceptions seem to be not entirely wanting. Hence the +necessity of taking all predictions from Mendel's law only as +probabilities, which will prove true in most, but not necessarily in all +cases. [300] But here we will limit ourselves to normal cases. + +The first example to be considered is obviously the assumption that the +parents of a cross differ from each other in respect to two characters. +A good illustrative example is afforded by the thorn-apple. I have +crossed the blue flowered thorny form, usually known as _Datura Tatula_, +with the white thornless type, designated as _D. Stramonium inermis_. +Thorns and blue pigment are obviously active qualities, as they are +dominant in the hybrids. In the second generation both pairs of +characters are resolved into their constituents and paired anew +according to Mendel's law. After isolating my hybrids during the period +of flowering, I counted among their progeny: + + 128 individuals with blue flowers and thorns + 47 individuals with blue flowers and without thorns + 54 individuals with white flowers and thorns + 21 individuals with white flowers and without thorns + ---- + 250 + +The significance of these numbers may easily be seen, when we calculate +what was to be expected on the assumption that both characters follow +Mendel's law, and that both are independent from each other. Then we +would have three-fourths blue offspring and one-fourth individuals with +white flowers. Each of these [301] two groups would consist of +thorn-bearing and thornless plants, in the same numerical relation. +Thus, we come to the four groups observed in our experiment, and are +able to calculate their relative size in the following way: + + Proportion + Blue with thorns 3/4 X 3/4 = 9/16 = 56.25% 9 + Blue, unarmed 3/4 X 1/4 = 3/16 = 18.75% 3 + White with thorns 1/4 X 3/4 = 3/16 = 18.75% 3 + White, unarmed 1/4 X 1/4 = 1/16 = 6.25% 1 + +In order to compare this inference from Mendel's law and the assumption +of independency, with the results of our experiments, we must calculate +the figures of the latter in percentages. In this way we find: + + Found Calculated + Blue with thorns 128=51% 56.25% + Blue unarmed 47=19% 18.75% + White with thorns 54=22% 18.75% + White unarmed 21= 8% 6.25% + +The agreement of the experimental and the theoretical figures is as +close as might be expected. + +This experiment is to be considered only as an illustrative example of a +rule of wide application. The rule obviously will hold good in all such +cases as comply with the two conditions already premised, viz.: that +each character agrees with Mendel's law, and that both are wholly +independent of each other. It is clear that our figures show the +numerical composition [302] of the hybrid offspring for any single +instance, irrespective of the morphological nature of the qualities +involved. + +Mendel has proved the correctness of these deductions by his experiments +with peas, and by combining their color (yellow or green) with the +chemical composition (starch or sugar) and other pairs of characters. I +will now give two further illustrations afforded by crosses of the +ordinary campion. I used the red-flowered or day-campion, which is a +perennial herb, and a smooth variety of the white evening-campion, which +flowers as a rule in the first summer. The combination of flower-color +and pubescence gave the following composition for the second hybrid +generation: + + Number % Calculation + Hairy and red 70 44 56.25% + Hairy and white 23 14 18.75% + Smooth and red 46 23 18.75% + Smooth and white 19 12 6.25% + +For the combination of pubescence and the capacity of flowering in the +first year I found: + + Number % Calculated + Hairy, flowering 286 52 56.25% + Hairy, without stem 128 23 18.75% + Smooth, flowering 96 17 18.75% + Smooth, without stem 42 8 6.25% + +Many other cases have been tested by different writers and the general +result is the [303] applicability of Mendel's formula to all cases +complying with the given conditions. + +Intentionally I have chosen for the last example two pairs of +antagonisms, relating to the same pair of plants, and which may be +tested in one experiment and combined in one calculation. + +For the latter we need only assume the same conditions as mentioned +before, but now for three different qualities. It is easily seen that +the third quality would split each of our four groups into two smaller +ones in the proportion of 3/4 : 1/4. + +We would then get eight groups of the following composition: + + 9/16 X 3/4 = 27/64 or 42.2% + 9/16 X 1/4 = 9/64 " 14.1% + 3/16 X 3/4 = 9/64 " 14.1% + 3/16 X 1/4 = 3/64 " 4.7% + 3/16 X 3/4 = 9/64 " 14.1% + 3/16 X 1/4 = 3/64 " 4.7% + 1/16 X 3/4 = 3/64 " 4.7% + 1/16 X 1/4 = 1/64 " 1.6% + +The characters chosen for our experiment include the absence of stem and +flowers in the first year, and therefore would require a second year to +determine the flower-color on the perennial specimens. Instead of doing +so I have taken another character, shown by the teeth of the capsules +when opening. These curve outwards [304] in the red campion, but lack +this capacity in the evening-campion, diverging only until an upright +position is reached. The combination of hairs, colors and teeth gives +eight groups, and the counting of their respective numbers of +individuals gave the following: + + Teeth + Hairs Flowers of capsules Number % Calculated + + Hairy red curved 91 47 42.2% + Hairy red straight 15 7.5 14.1% + Hairy white curved 23 12 14.1% + Hairy white straight 17 8.5 4.7% + Smooth red curved 23 12 14.1% + Smooth red straight 9 4.5 4.7% + Smooth white curved 5 2.5 4.7% + Smooth white straight 12 6 1.6% + +The agreement is as comprehensive as might be expected from an +experiment with about 200 plants, and there can be no doubt that a +repetition on a larger scale would give still closer agreement. + +In the same way we might proceed to crosses with four or more +differentiating characters. But each new character will double the +number of the groups. Four characters will combine into 16 groups, five +into 32, six into 64, seven into 128, etc. Hence it is easily seen that +the size of the experiments must be made larger and larger in the same +ratio, if we intend to expect numbers equally trustworthy. For [305] +seven differentiating marks 16,384 individuals are required for a +complete series. And in this set the group with the seven attributes all +in a latent condition would contain only a single individual. + +Unfortunately the practical value of these calculations is not very +great. They indicate the size of the cultures required to get all the +possible combinations, and show that in ordinary cases many thousands of +individuals have to be cultivated, in order to exhaust the whole range +of possibilities. They further show that among all these thousands, only +very few are constant in all their characters; in fact, it may easily be +seen that with seven differentiating points among the 16,384 named +above, only one individual will have all the seven qualities in pure +active, and only one will have them all in a purely dormant condition. +Then there will be some with some attributes active and others latent, +but their numbers will also be very small. All others will split up in +the succeeding generation in regard to one or more of their apparently +active marks. And since only in very rare cases the stable hybrids can +be distinguished by external characters from the unstable ones, the +stability of each individual bearing a desired combination of characters +would have to be established by experiment [306] after pure +fertilization. Mendel's law teaches us to predict the difficulties, but +hardly shows any way to avoid them. It lays great stress on the old +prescript of isolation and pure fertilization, but it will have to be +worked out and applied to a large number of practical cases before it +will gain a preeminent influence in horticultural practice. + +Or, as Bailey states it, we are only beginning to find a pathway through +the bewildering maze of hybridization. + +This pathway is to be laid out with regard to the following +considerations. We are not to cross species or varieties, or even +accidental plants. We must cross unit-characters, and consider the +plants only as the bearers of these units. We may assume that these +units are represented in the hereditary substance of the cell-nucleus by +definite bodies of too small a size to be seen, but constituting +together the chromosomes. We may call these innermost representatives of +the unit-characters pangenes, in accordance with Darwin's hypothesis of +pangenesis, or give them any other name, or we may even wholly abstain +from such theoretical discussion, and limit ourselves to the conception +of the visible character-units. These units then may be present, or +lacking and in the first case active, or latent. + +[307] True elementary species differ from each other in a number of +unit-characters, which do not contrast. They have arisen by progressive +mutation. One species has one kind of unit, another species has another +kind. On combining these, there can be no interchange. Mendelism assumes +such an interchange between units of the same character, but in a +different condition. Activity and latency are such conditions, and +therefore Mendel's law obviously applies to them. They require pairs of +antagonistic qualities, and have no connection whatever with those +qualities, which do not find an opponent in the other parent. Now, only +pure varieties afford such pure conditions. When undergoing further +modifications, some of them may be in the progressive line and others in +the retrogressive. Progressive modifications give new units, which are +not in contrast with any other, retrograde changes turn active units +into the latent condition and so give rise to pairs. Ordinary species +generally originate in this way, and hence differ from each other partly +in specific, partly in varietal characters. As to the first, they give +in their hybrids stable peculiarities, while as to the latter, they +split up according to Mendel's law. + +Unpaired or unbalanced characters lie side by side with paired or +balanced qualities, and they [308] do so in nearly all the crosses made +for practical purposes, and in very many scientific experiments. Even +Mendel's peas were not pure in this respect, much less do the campions +noted above differ only in Mendelian characters. + +Comparative and systematic studies must be made to ascertain the true +nature of every unit in every single plant, and crossing experiments +must be based on these distinctions in order to decide what laws are +applicable in any case. + + +[309] +D. EVER-SPORTING VARIETIES + +LECTURE XI + +STRIPED FLOWERS + +Terminology is an awkward thing. It is as disagreeable to be compelled +to make new names, as to be constrained to use the old faulty ones. +Different readers may associate different ideas with the same terms, and +unfortunately this is the case with much of the terminology of the +science of heredity and variability. What are species and what are +varieties? How many different conceptions are conveyed by the terms +constancy and variability? We are compelled to use them, but we are not +at all sure that we are rightly understood when we do so. + +Gradually new terms arise and make their way. They have a more limited +applicability than the old ones, and are more narrowly circumscribed. +They are not to supplant the older terms, but permit their use in a more +general way. + +[310] One of these doubtful terms is the word _sport_. It often means +bud-variation, while in other cases it conveys the same idea as the old +botanical term of mutation. But then all sorts of seemingly sudden +variations are occasionally designated by the same term by one writer or +another, and even accidental anomalies, such as teratological ascidia, +are often said to arise by sports. + +If we compare all these different conceptions, we will find that their +most general feature is the suddenness and the rarity of the phenomenon. +They convey the idea of something unexpected, something not always or +not regularly occurring. But even this demarcation is not universal, and +there are processes that are regularly repeated and nevertheless are +called sports. These at least should be designated by another name. + +In order to avoid confusion as far as possible, with the least change in +existing terminology, I shall use the term "ever-sporting varieties" for +such forms as are regularly propagated by seed, and of pure and not +hybrid origin, but which sport in nearly every generation. The term is a +new one, but the facts are for the most part new, and require to be +considered in a new light. Its meaning will become clearer at once when +the illustrations afforded by [311] striped flowers are introduced. In +the following discussion it will be found most convenient to give a +summary of what is known concerning them, and follow this by a +consideration of the detailed evidence obtained experimentally, which +supports the usage cited. + +The striped variety of the larkspur of our gardens is known to produce +monochromatic flowers, in addition to striped ones. They may be borne by +the same racemes, or on different branches, or some seedlings from the +same parent-plant may bear monochromatic flowers while others may be +striped. Such deviations are usually called sports. But they occur +yearly and regularly and may be observed invariably when the cultures +are large enough. Such a variety I shall call "ever-sporting." + +The striped larkspur is one of the oldest garden varieties. It has kept +its capacity of sporting through centuries, and therefore may in some +sense be said to be quite stable. Its changes are limited to a rather +narrow circle, and this circle is as constant as the peculiarities of +any other constant species or variety. But within this circle it is +always changing from small stripes to broad streaks, and from them to +pure colors. Here the variability is a thing of absolute constancy, +while the constancy consists in eternal changes. Such apparent [312] +contradictions are unavoidable, when we apply the old term to such +unusual though not at all new cases. Combining the stability and the +qualities of sports in one word, we may evidently best express it by the +new term of eversporting variety. + +We will now discuss the exact nature of such varieties, and of the laws +of heredity which govern them. But before doing so, I might point out, +that this new type is a very common one. It embraces most of the +so-called variable types in horticulture, and besides these a wide range +of anomalies. + +Every ever-sporting variety has at least two different types, around and +between which it varies in numerous grades, but to which it is +absolutely limited. Variegated leaves fluctuate between green and white, +or green and yellow, and display these colors in nearly all possible +patterns. But there variability ends, and even the patterns are +ordinarily narrowly prescribed in the single varieties. Double flowers +afford a similar instance. On one side the single type, on the other the +nearly wholly double model are the extreme limits, between which the +variability is confined. So it is also with monstrosities. The race +consists of anomalous and normal individuals, and displays between them +all possible combinations of normal and monstrous [313] parts. But its +variability is restricted to this group. And large as the group may seem +on first inspection, it is in reality very narrow. Many monstrosities, +such as fasciated branches, pitchers, split leaves, peloric flowers, and +others constitute such ever-sporting varieties, repeating their +anomalies year by year and generation after generation, changing as much +as possible, but remaining absolutely true within their limits as long +as the variety exists. + +It must be a very curious combination of the unit-characters which +causes such a state of continuous variability. The pure quality of the +species must be combined with the peculiarity of the variety in such a +way, that the one excludes the other, or modifies it to some extent, +although both never fully display themselves in the same part of the +same plant. A corolla cannot be at once monochromatic and striped, nor +can the same part of a stem be twisted and straight. But neighboring +organs may show the opposite attributes side by side. + +In order to look closer into the real mechanism of this form of +variability, and of this constant tendency to occasional reversions, it +will be best to limit ourselves first to a single case, and to try to +gather all the evidence, which can be obtained by an examination of the +hereditary relations of its sundry constituents. + +[314] This may best be done by determining the degree of inheritance for +the various constituents of the race during a series of years. It is +only necessary to apply the two precautions of excluding all +cross-fertilization, and of gathering the seeds of each individual +separately. We do not need to ascertain whether the variety as such is +permanent; this is already clear from the simple fact of its antiquity +in so many cases. We wish to learn what part each individual, or each +group of individuals with similar characters, play in the common line of +inheritance. In other words, we must build up a genealogical tree, +embracing several generations and a complete set of the single cases +occurring within the variety, in order to allow of its being considered +as a part of the genealogy of the whole. It should convey to us an idea +of the hereditary relations during the life-time of the variety. + +It is manifest that the construction of such a genealogical tree +requires a number of separate experiments. These should be extended over +a series of years. Each should include a number of individuals large +enough to allow the determination of the proportion of the different +types among the offspring of a single plant. A species which is easily +fertilized by its own pollen, and which bears capsules with [315] large +quantities of seeds, obviously affords the best opportunities. As such, +I have chosen the common snapdragon of the gardens, _Antirrhinum majus_. +It has many striped varieties, some tall, others of middle height, or of +dwarfed stature. In some the ground-color of the flowers is yellow, in +others it is white, the yellow disappearing, with the exception of a +large mark in the throat. On these ground-colors the red pigment is seen +lying in streaks of pure carmine, with white intervals where the yellow +fails, but combined with yellow to make a fiery red, and with yellow +intervals when that color is present. This yellow color is quite +constant and does not vary in any marked degree, notwithstanding the +fact that it seems to make narrower and broader stripes, according to +the parts of the corolla left free by the red pigment. But it is easily +seen that this appearance is only a fallacious one. + +The variety of snapdragon chosen was of medium height and with the +yellow ground-color, and is known by horticulturists as _A. majus luteum +rubro-striatum_. As the yellow tinge showed itself to be invariable; I +may limit my description to the red stripes. + +Some flowers of this race are striped, others are not. On a hasty survey +there seem to be three types, pure yellow, pure red, and stripes [316] +with all their intermediate links of narrower and broader, fewer and +more numerous streaks. But on a close inspection one does not succeed in +finding pure yellow racemes. Little lines of red may be found on nearly +every flower. They are the extreme type on this side of the range of +variability. From them an almost endless range of patterns passes over +to the broadest stripes and even to whole sections of a pure red. But +then, between these and the wholly red flowers we observe a gap, which +may be narrower by the choice of numerous broad striped individuals, but +which is never wholly filled up. Hence we see that the red flowers are a +separate type within the striped variety. + +This red type springs yearly from the striped form, and yearly reverts +to it. This is what in the usual descriptions of this snapdragon, is +called its sporting. The breadth of the streaks is considered to be an +ordinary case of variability, but the red flowers appear suddenly, +without the expected links. Therefore they are to be considered as +sports. Similarly the red forms may suddenly produce striped ones, and +this too is to be taken as a sport, according to the usual conception of +the word. + +Such sports may occur in different ways. Either by seeds, or by buds, or +even within the single spikes. Both opposite reversions, [317] from +striped to red and from red to stripes, occur by seed, even by the +strictest exclusion of cross-fertilization. As far as my experiments go, +they are the rule, and parent-plants that do not give such reversions, +at least in some of their offspring, are very rare, if not wholly +wanting. Bud-variations and variations within the spike I have as yet +only observed on the striped individuals, and never on the red ones, +though I am confident that they might appear in larger series of +experiments. Both cases are more common on individuals with broad +stripes than on plants bearing only the narrower red lines, as might be +expected, but even on the almost purely yellow individuals they may be +seen from time to time. Bud-variations produce branches with spikes of +uniform red flowers. Every bud of the plant seems to have equal chances +to be transformed in this way. Some striped racemes bear a few red +flowers, which ordinarily are inserted on one side of the spike only. As +they often cover a sharply defined section of the raceme, this +circumstance has given rise to the term of sectional variability to +cover such cases. Sometimes the section is demarcated on the axis of the +flower-spike by a brownish or reddish color, sharply contrasting with +the green hue of the remaining parts. Sectional variation may be looked +at as a [318] special type of bud-variation, and from this point of view +we may simplify our inquiry and limit ourselves to the inheritance of +three types, the striped plants, the red plants and the red asexual +variants of the striped individuals. In each case the heredity should be +observed not only for one, but at least for two successive generations. + +Leaving these introductory remarks I now come at once to the +genealogical tree, as it may be deduced from my experiments: + + Year + 1896 95% Striped 84% Red + | | + 1895 Striped Individual Red Indiv. + \ / + 1895 98% Striped 71% Red + | | + 1894 Striped branches. Red branches. + \ / + 1894 98% Striped 76% Red + | | + 1893 90% Striped Indiv. 10% Red Indiv. + \ / + 1892 Striped Individual + +This experiment was begun in the year 1892 with one individual out of a +large lot of striped plants grown from seeds which I had purchased from +a firm in Erfurt. The capsules were gathered separately from this +individual and about 40 flowering plants were obtained from the seeds in +the following year. Most of them had neatly striped flowers, some +displayed broader stripes and spare flowers were seen with one [319] +half wholly red. Four individuals were found with only uniform red +flowers. These were isolated and artificially pollinated, and the same +was done with some of the best striped individuals. The seeds from every +parent were sown separately, so as to allow the determination of the +proportion of uniform red individuals in the progeny. + +Neither group was constant in its offspring. But as might be expected, +the type of the parent plant prevailed in both groups, and more strongly +so in the instances with the striped, than with the red ones. Or, in +other words seed-reversions were more numerous among the already +reverted reds than among the striped type itself. I counted 2% reversion +in the latter case, but 24% from the red parents. + +Among the striped plants from the striped parents, I found some that +produced bud variations. I succeeded in isolating these red flowering +branches in paper bags and in pollinating them with their own pollen, +and subjected the striped spikes of the same individuals to a similar +treatment. Three individuals gave a sufficient harvest from both types, +and these six lots of seeds were sown separately. The striped flowers +repeated their character in 98% of their offspring, the red twigs in +only 71%, the [320] remaining individuals sporting into the opposite +group. + +In the following year I continued the experiment with the seeds of the +offspring of the red bud-variations. The striped individuals gave 95%, +but in the red ones only 84% of the progeny remained true to the parent +type. + +From these figures it is manifest that the red and striped types differ +from one another not only in their visible attributes, but also in the +degree of their heredity. The striped individuals repeat their +peculiarity in 90-98% of their progeny, 2-10% sporting into the uniform +red color. On the other hand the red individuals are constant in 71-84% +of their offspring, while 16-29% go over to the striped type. Or, +briefly, both types are inherited to a high degree, but the striped type +is more strictly inherited than the red one. + +Moreover the figures show that the degree of inheritance is not +contingent upon the question as to how the sport may have arisen. +Bud-sports show the same degree of inheritance as seed-sports. Sexual +and asexual variability therefore seem to be one and the same process in +this instance. But the deeper meaning of this and other special features +of our genealogical tree are still awaiting further investigation. It +seems that much important evidence might [321] come from an extension of +this line of work. Perhaps it might even throw some light on the +intimate nature of the bud-variations of ever-sporting varieties in +general. Sectional variations remain to be tested as to the degree of +inheritance exhibited, and the different occurrences as to the breadth +of the streaks require similar treatment. + +In ordinary horticultural practice it is desirable to give some +guarantee as to what may be expected to come from the seeds of brightly +striped flowers. Neither the pure red type, nor the nearly yellow +racemes are the object of the culture, as both of them may be had pure +from their, own separate varieties. In order to insure proper striping, +both extremes are usually rejected and should be rooted out as soon as +the flowering period begins. Similarly the broad-striped ones should be +rejected, as they give a too large amount of uniform red flowers. +Clearly, but not broadly striped individuals always yield the most +reliable seed. + +Summing up once more the results of our pedigree-experiment, we may +assert that the striped variety of the snapdragon is wholly permanent, +including the two opposite types of uniform color and of stripes. It +must have been so since it first originated from the invariable uniform +[322] varieties, about the middle of the last century, in the nursery of +Messrs. Vilmorin, and probably it will remain so as long as popular +taste supports its cultivation. It has never been observed to transgress +its limits or to sport into varieties without reversions or sports. It +fluctuates from one extreme to the other yearly, always recurring in the +following year, or even in the same summer by single buds. Highly +variable within its limits, it is absolutely constant or permanent, when +considered as a definite group. + +Similar cases occur not rarely among cultivated plants. In the wild +state they seem to be wholly wanting. Neither are they met with as +occasional anomalies nor as distinct varieties. On the contrary, many +garden-flowers that are colored in the species, and besides this have a +white or yellow variety, have also striped sorts. The oldest instance is +probably the marvel of Peru, _Mirabilis Jalappa_, which already had more +than one striped variety at the time of its introduction from Peru into +the European gardens, about the beginning of the seventeenth century. +Stocks, liver-leaf (_Hepatica_), dame's violet (_Hesperis_), Sweet +William (_Dianthus barbatus_), and periwinkles (_Vinca minor_) seem to +be in the same condition, as their striped varieties were already quoted +[323] by the writers of the same century. Tulips, hyacinths, _Cyclamen_, +_Azalea_, _Camellia_, and even such types of garden-plants as the meadow +crane's-bill (_Geranium pratensev) have striped varieties. It is always +the red or blue color which occurs in stripes, the underlying ground +being white or yellow, according to the presence or absence of the +yellow in the original color mixture. + +All these varieties are known to be permanent, coming true during long +series of successive generations. But very little is known concerning +the more minute details of their hereditary qualities. They come from +seed, when this is taken from striped individuals, and thence revert +from time to time to the corresponding monochromatic type. But whether +they would do so when self-fertilized, and whether the reversionary +individuals are always bound to return towards the center of the group +or towards the opposite limit, remains to be investigated. Presumably +there is nowhere a real transgression of the limits, and never or only +very rarely and at long intervals of time a true production of another +race with other hereditary qualities. + +In order to satisfy myself on these points, I made some +pedigree-cultures with the striped forms of dame's violet (_Hesperis +matronalis_) [324] and of _Clarkia pulchella_. Both of them are +ever-sporting varieties. The experiments were conducted during five +generations with the violet, and during four with the striped Clarkia, +including the progeny of the striped and of the monochromatic red +offspring of a primitive striped plant. I need not give the figures here +for the numerical relations between the different types of each group, +and shall limit myself to the statement that they behaved in exactly the +same manner as the snapdragon. + +It is worth while to dwell a moment on the capacity of the individuals +with red flowers to reproduce the striped type among their offspring. +For it is manifest that this latter quality must have lain dormant in +them during their whole life. Darwin has already pointed out that when a +character of a grandparent, which is wanting in the progeny, reappears +in the second generation, this quality must always be assumed to have +been present though latent in the intermediate generation. To the many +instances given by him of such alternative inheritance, the +monochromatic reversionists of the striped varieties are to be added as +a new type. It is moreover, a very suggestive type, since the latency is +manifestly of quite another character than for instance in the case of +Mendelian hybrids, and probably more allied to those instances, [325] +where secondary sexual marks, which are as a rule only evolved by one +sex, are transferred to the offspring through the other. + +Stripes are by no means limited to flowers. They may affect the whole +foliage, or the fruits and the seeds, and even the roots. But all such +cases occur much more rarely than the striped flowers. An interesting +instance of striped roots is afforded by radishes. White and red +varieties of different shapes are cultivated. Besides them sometimes a +curious motley sort may be seen in the markets, which is white with red +spots, which are few and narrow in some samples, and more numerous and +broader in others. But what is very peculiar and striking is the +circumstance, that these stripes do not extend in a longitudinal, but in +a transverse direction. Obviously this must be the effect of the very +notable growth in thickness. Assuming that the colored regions were +small in the beginning, they must have been drawn out during the process +of thickening of the root, and changed into transverse lines. Rarely a +streak may have had its greatest extension in a transverse direction +from the beginning, in which case it would only be broadened and not +definitely changed in its direction. + +This variety being a very fine one, and more agreeable to the eye than +the uniform colors, is [326] being more largely cultivated in some +countries. It has one great drawback: it never comes wholly true from +seed. It may be grown in full isolation, and carefully selected, all red +or nearly monochromatic samples being rooted out long before blooming, +but nevertheless the seed will always produce some red roots. The most +careful selection, pursued through a number of years, has not been +sufficient to get rid of this regular occurrence of reversionary +individuals. Seed-growers receive many complaints from their clients on +this account, but they are not able to remove the difficulty. This +experience is in full agreement with the experimental evidence given by +the snapdragon, and it would certainly be very interesting to make a +complete pedigree-culture with the radishes to test definitely their +compliance with the rules observed for striped flowers. + +Horticulturists in such cases are in the habit of limiting themselves to +the sale of so-called mixed seeds. From these no client expects purity, +and the normal and hereditary diversity of types is here in some sense +concealed under the impurities included in the mixture from lack of +selection. Such cases invite scrutiny, and would, no doubt, with the +methods of isolation, artificial pollination, and the sowing of the +seeds separately from each parent, yield [327] results of great +scientific value. Any one who has a garden, and sufficient perseverance +to make pure cultures during a series of years might make important +contributions to scientific knowledge in this way. + +Choice might be made from among a wide range of different types. A +variety of corn called "Harlequin" shows stripes on its kernels, and one +ear may offer nearly white and nearly red seeds and all the possible +intermediate steps between them. From these seeds the next generation +will repeat the motley ears, but some specimens will produce ears of +uniform kernels of a dark purple, showing thus the ordinary way of +reversion. Some varieties of beans have spotted seeds, and among a lot +of them one may be sure to find some purely red ones. It remains to be +investigated what will be their offspring, and whether they are due to +partial or to individual variation. + +The cockscomb (_Celosia cristata_) has varieties of nearly all colors +from white and yellow to red and orange, and besides them some striped +varieties occur in our gardens, with the stripes going from the lower +parts of the stem up to the very crest of the comb. They are on sale as +constant varieties, but nothing has as yet been recorded concerning +their peculiar behavior in the inheritance of the stripes. [328] Striped +grapes, apples and other fruits might be mentioned in this connection. + +Before leaving the striped varieties, attention is called to an +interesting deduction, which probably gives an explanation of one of the +most widely known instances of ever-sporting garden plants. Striped +races always include two types. Both of them are fertile, and each of +them reproduces in its offspring both its own and the alternate type. It +is like a game of ball, in which the opposing parties always return the +ball. But now suppose that only one of the types were fertile and the +other for some reason wholly sterile, and assume the reversionary, or +primitive monochromatic individuals to be fertile, and the derivative +striped specimens to bloom without seed. If this were the case, +knowledge concerning the hereditary qualities would be greatly limited. +In fact the whole pedigree would be reduced to a monochromatic strain, +which would in each generation sport in some individuals into the +striped variety. But, being sterile, they would not be able to propagate +themselves. + +Such seems to be the case with the double flowered stocks. Their double +flowers produce neither stamens nor pistils, and as each individual is +either double or single in all its flowers, the doubles are wholly +destitute of seed. [329] Nevertheless, they are only reproduced by seed +from single flowers, being an annual or biennial species. + +Stocks are a large family, and include a wonderful variety of colors, +ranging from white and yellow to purple and red, and with some +variations toward blue. They exhibit also diversity in the habit of +growth. Some are annuals, including the ten-week and pyramidal forms; +others are intermediates and are suitable for pot-culture; and the +biennial sorts include the well-known "Brompton" and "Queen" varieties. +Some are large and others are small or dwarf. For their brightness, +durability and fragrance, they are deservedly popular. There are even +some striped varieties. Horticulturists and amateurs generally know that +seed can be obtained from single stocks only, and that the double +flowers never produce any. It is not difficult to choose single plants +that will produce a large percentage of double blossoms in the following +generation. But only a percentage, for the experiments of the most +skilled growers have never enabled them to save seed, which would result +entirely in double flowering plants. Each generation in its turn is a +motley assembly of singles and doubles. + +Before looking closer into the hereditary peculiarities of this old and +interesting ever-sporting [330] variety, it may be as well to give a +short description of the plants with double flowers. Generally speaking +there are two principal types of doubles. One is by the conversion of +stamens into petals, and the other is an anomaly, known under the name +of _petalomany_. + +The change of stamens into petals is a gradual modification. All +intermediate steps are easily to be found. In some flowers all stamens +may be enlarged, in others only part of them. Often the broadened +filaments bear one or two fertile anthers. The fertility is no doubt +diminished, but not wholly destroyed. Individual specimens may occur, +which cannot produce any seed, but then others of the same lot may be as +fertile as can be desired. As a whole, such double varieties are +regularly propagated by seed. + +Petalomany is the tendency of the axis of some flowers never to make any +stamens or pistils, not even in altered or rudimentary form. Instead of +these, they simply continue producing petals, going on with this +production without any other limit than the supply of available food. +Numerous petals fill the entire space within the outer rays, and in the +heart of the flower innumerable young ones are developed half-way, not +obtaining food enough to attain [331] full size. Absolute sterility is +the natural consequence of this state of things. + +Hence it is impossible to have races of petalomanous types. If the +abnormality happens to show itself in a species, which normally +propagates itself in an asexual way, the type may become a vegetative +variety, and be multiplied by bulbs, buds or cuttings, etc. Some +cultivated anemones and crowfoots (_Ranunculus_) are of this character, +and even the marsh-marigold (_Caltha palustris_) has a petalomanous +variety. I once found in a meadow such a form of the meadow-buttercup +(_Ranunculus_ acris_), and succeeded in keeping it in my garden for +several years, but it did not make seeds and finally died. Camellias are +known to have both types of double flowers. The petalomanous type is +highly regular in structure, so much so as to be too uniform in all its +parts to be pleasing, while the conversion of stamens into petals in the +alternative varieties gives to these flowers a more lively diversity of +structure. Lilies have a variety called _Lilium candidum flore pleno_, +in which the flowers seem to be converted into a long spike of bright, +white narrow bracts, crowded on an axis which never seems to cease their +production. + +It is manifestly impossible to decide how all such sterile double +flowers have originated. [332] Perhaps each of them originally had a +congruent single-flowered form, from which it was produced by seed in +the same way as the double stocks now are yearly. If this assumption is +right, the corresponding fertile line is now lost; it has perhaps died +out, or been masked. But it is not absolutely impossible that such +strains might one day be discovered for one or another of these now +sterile varieties. + +Returning to the stocks we are led to the conception that some varieties +are absolutely single, while others consist of both single-flowered and +double-flowered individuals. The single varieties are in respect to this +character true to the original wild type. They never give seed which +results in doubles, providing all intercrossing is excluded. The other +varieties are ever-sporting, in the sense of this term previously +assumed, but with the restriction that the sports are exclusively +one-sided, and never return, owing to their absolute sterility. + +The oldest double varieties of stocks have attained an age of a century +and more. During all this time they have had a continuous pedigree of +fertile and single-flowered individuals, throwing off in each generation +a definite number of doubles. This ratio is not at all dependent on +chance or accident, nor is it even variable to a remarkable degree. +Quite on the contrary [333] it is always the same, or nearly the same, +and it is to be considered as an inherent quality of the race. If left +to themselves, the single individuals always produce singles and doubles +in the same quantity; if cultivated after some special method, the +proportion may be slightly changed, bringing the proportion of doubles +up to 60% or even more. + +Ordinarily the single and double members of such a race are quite equal +in the remainder of their attributes, especially in the color of their +flowers. But this is not always the case. The colors of such a race may +repeat for themselves the peculiarities of the ever-sporting characters. +It often happens that one color is more or less strictly allied to the +doubles, and another to the singles. This sometimes makes it difficult +to keep the various colors true. There are certain sorts, which +invariably exhibit a difference in color between the single and the +double flowers. The sulphur-yellow varieties may be adduced as +illustrative examples, because in them the single flowers always come +white. Hence in saving seed, it is impossible so to select the plant, +that an occasional white does not also appear among the double flowers, +agreeing in this deviation with the general rule of the eversporting +varieties. + +I commend all the above instances to those [334] who wish to make +pedigree-cultures. The cooperation of many is needed to bring about any +notable advancement, since the best way to secure isolation is to +restrict one's self to the culture of one strain, so as to avoid the +intermixture of others. So many facts remain doubtful and open to +investigation, that almost any lot of purchased seed may become the +starting point for interesting researches. Among these the +sulphur-yellow varieties should be considered in the first place. + +In respect to the great questions of heredity, the stocks offer many +points of interest. Some of these features I will now try to describe, +in order to show what still remains to be done, and in what manner the +stocks may clear the way for the study of the ever-sporting varieties. + +The first point, is the question, which seeds become double-flowered and +which single-flowered plants? Beyond all doubt, the determination has +taken place before the ripening of the seed. But though the color of the +seed is often indicative of the color of the flowers, as in some red or +purple varieties, and though in balsams and some other instances the +most "highly doubled" flowers are to be obtained from the biggest and +plumpest seeds, no such rule seems to exist respecting the double +stocks. Now if one half of the seeds gives doubles, and [335] the other +half singles, the question arises, where are the singles and the doubles +to be found on the parent-plant? + +The answer is partly given by the following experiment. Starting from +the general rule of the great influence of nutrition on variability, it +may be assumed that those seeds will give most doubles, that are best +fed. Now it is manifest that the stem and larger branches are, in a +better condition than the smaller twigs, and that likewise the first +fruits have better chances than the ones formed later. Even in the same +pod the uppermost seeds will be in a comparatively disadvantageous +position. This conception leads to an experiment which is the basis of a +practical method much used in France in order to get a higher percentage +of seeds of double-flowering plants. + +This method consists in cutting off, in the first place the upper parts +of all the larger spikes, in the second place, the upper third part of +each pod, and lastly all the small and weak twigs. In doing so the +percentage is claimed to go up to 67-70%, and in some instances even +higher. This operation is to be performed as soon as the required number +of flowers have ceased blossoming. All the nutrient materials, destined +for the seeds, are now forced to flow into these relatively few embryos, +and it is clear that [336] they will be far better nourished than if no +operation were made. + +In order to control this experiment some breeders have made the +operation on the fruits when ripe, instead of on the young pods, and +have saved the seeds from the upper parts separately. This seed, +produced in abundance, was found to be very poor in double flowers, +containing only some 20-30%. On the contrary the percentage of doubles +in the seed of the lower parts was somewhat augmented, and the average +of both would have given the normal proportion of 50%. + +Opposed to the French method is the German practice of cultivating +stocks, as I have seen it used on a very large scale at Erfurt and at +other places. The stocks are grown in pots on small scaffolds, and not +put on or into the earth. The obvious aim of this practice is to keep +the earth in the pots dry, and accordingly they are only scantily +watered. In consequence they cannot develop as fully as they would have +done when planted directly in the beds, and they produce only small +racemes and no weak twigs, eliminating thereby without further operation +the weaker seeds as by the French method. The effect is increased by +planting from 6-10 separate plants in each pot. + +It would be very interesting to make comparative [337] trials of both +methods, in order to discover the true relation between the practice and +the results reached. Bath should also be compared with cultures on open +plots, which are said to give only 50% of doubles. This last method of +culture is practiced wherever it is desired to produce great quantities +of seeds at a low cost. Such trials would no doubt give an insight into +the relations of hereditary characters to the distribution of the food +within the plant. + +A second point is the proportional increase of the double-flowering +seeds with age. If seed is kept for two or three years, the greater part +of the grains will gradually die, and among the remainder there is found +on sowing, a higher percentage of double ones. Hence we may infer that +the single-flowered seeds are shorter lived than the doubles, and this +obviously points to a greater weakness of the first. It is quite evident +that there is some common cause for these facts and for the above cited +experience, that the first and best pods give more doubles. Much, +however, remains to be investigated before a satisfactory answer can be +made to these questions. + +A third point is the curious practice, called by the French "esimpler," +and which consists in pulling out the singles when very young. It seems +to be done at an age when the flower-buds [338] are not yet visible, or +at least are not far enough developed to show the real distinctive +marks. Children may be employed to choose and destroy the singles. There +are some slight differences in the fullness and roundness of the buds +and the pubescence of the young leaves. Moreover the buds of the doubles +are said to be sweeter to the taste than those of the singles. But as +yet I have not been able to ascertain, whether any scientific +investigation of this process has ever been made, though according to +some communications made to me by the late Mr. Cornu, the practice seems +to be very general in the environs of Paris. In summer large fields may +be seen, bearing exclusively double flowers, owing to the weeding out of +the singles long before flowering. + +Bud-variation is the last point to be taken up. It seems to be very rare +with stocks, but some instances have been recorded in literature. Darwin +mentions a double stock with a branch bearing single flowers, and other +cases are known to have occurred. But in no instance does the seed of +such a bud-variant seem to have been saved. Occasionally other +reversions also occur. From time to time specimens appear with more +luxurious growth and with divergent instead of erect pods. They are +called, in Erfurt, "generals" on account [339] of their stiff and erect +appearance, and they are marked by more divergent horns crowning the +pods. They are said to produce only a relatively small number of doubles +from their seeds, and even this small number might be due to +fertilization with pollen of their neighbors. I saw some of these +reversionary types; when inspecting the nurseries of Erfurt, but as they +are, as a rule, thrown out before ripening their seed, nothing is +exactly known about their real hereditary qualities. + +Much remains to be cleared up, but it seems that one of the best means +to find a way through the bewildering maze of the phenomena of +inheritance, is to make groups of related forms and to draw conclusions +from a comparison of the members of such groups. Such comparisons must +obviously give rise to questions, which in their turn will directly lead +to experimental investigation. + + +[340] + +LECTURE XII + +FIVE-LEAVED CLOVER + +Every one knows the "four-leaved" clover. It is occasionally found on +lawns, in pastures and by the roadsides. Specimens with five leaflets +may be found now and then in the same place, or on the same plant, but +these are rarer. I have often seen isolated plants with quaternate +leaves, but only rarely have I observed individuals with more than one +such leaf. + +The two cases are essentially dissimilar. They may appear to differ but +little morphologically, but from the point of view of heredity they are +quite different. Isolated quaternate leaves are of but little interest, +while the occurrence of many on the same individual indicates a distinct +variety. In making experiments upon this point it is necessary to +transplant the divergent individuals to a garden in order to furnish +them proper cultural conditions and to keep them under constant +observation. When a plant bearing a quaternate leaf is thus transplanted +however, it rarely repeats the [341] anomaly. But when plants with two +or more quaternate leaves on the same individual are chosen it indicates +that it belongs to a definite race, which under suitable conditions may +prove to become very rich in the anomalies in question. + +Obviously it is not always easy to decide definitely whether a given +individual belongs to such a race or not. Many trials may be necessary +to secure the special race. I had the good fortune to find two plants of +clover, bearing one quinate and several quaternate leaves, on an +excursion in the neighborhood of Loosdrecht in Holland. After +transplanting them into my garden, I cultivated them during three years +and observed a slowly increasing number of anomalous leaves. This number +in one summer amounted to 46 quaternate and 16 quinate leaves, and it +was evident that I had secured an instance of the rare "five-leaved" +race which I am about to describe. + +Before doing so it seems desirable to look somewhat closer into the +morphological features of the problem. Pinnate and palmate leaves often +vary in the number of their parts. This variability is generally of the +nature of a common fluctuation, the deviations grouping themselves +around an average type in the ordinary way. Ash leaves bear five pairs, +and [342] the mountain-ash (_Sorbus Aucuparia_) has six pairs of +leaflets in addition to the terminal one. But this number varies +slightly, the weaker leaves having less, the stronger more pairs than +the average. Such however, is not the case, with ternate leaves, which +seem to be quite constant. Four leaflets occur so very rarely that one +seems justified in regarding them rather as an anomaly than, as a +fluctuation. And this is confirmed by the almost universal absence of +two-bladed clover-leaves. + +Considering the deviation as an anomaly, we may look into its nature. +Such an inquiry shows that the supernumerary leaflets owe their origin +to a splitting of one or more of the normal ones. This splitting is not +terminal, as is often the case with other species, and as it may be seen +sometimes in the clover. It is for the most part lateral. One of the +lateral nerves grows out becoming a median nerve of the new leaflet. +Intermediate steps are not wanting, though rare, and they show a gradual +separation of some lateral part of a leaflet, until this division +reaches the base and divides the leaflet into two almost equal parts. If +this splitting occurs in one leaflet we get the "four-leaved" Clover, if +it occurs in two there will be five leaflets. And if, besides this, the +terminal leaflet produces a derivative on one or both of its sides, +[343] we obtain a crown of six or seven leaflets on one stalk. Such were +often met with in the race I had under cultivation, but as a rule it did +not exceed this limit. + +The same phenomenon of a lateral doubling of leaflets may of course be +met with in other instances. The common laburnum has a variety which +often produces quaternate and quinate leaves, and in strawberries I have +also seen instances of this abnormality. It occurs also in pinnate +leaves, and complete sets of all the intermediate links may often be +found on the false or bastard-acacia (_Robinia Pseud_Acacia_). + +Opposed to this increase of the number of leaflets, and still more rare +and more curious is the occurrence of "single-leaved" varieties among +trees and herbs with pinnate or ternate leaves. Only very few instances +have been described, and are cultivated in gardens. The ashes and the +bastard-acacia may be quoted among trees, and the "one-leaved" +strawberry among herbs. Here it seems that several leaflets have been +combined into one, since this one is, as a rule, much larger than the +terminal leaflet of an ordinary leaf of the same species. These +monophyllous varieties are interesting also on account of their +continuous but often incomplete reversion to the normal type. + +[344] Pinnate and palmate leaves are no doubt derivative types. They +must have originated from the ordinary simple leaf. The monophylly may +therefore be considered as a reversion to a more primitive state and the +monophyllous varieties may be called atavistic. + +On the other hand we have seen that these atavistic varieties may revert +to their nearest progenitors, and this leads to the curious conception +of positive and negative atavism. For if the change of compound leaves +into single ones is a retrograde or negative step, the conversion of +single or ternate leaves into pinnate and palmate ones must evidently be +considered in this case as positive atavism. + +This discussion seems to throw some light on the increase of leaflets in +the clover. The pea family, or the group of papilionaceous plants, has +pinnate leaves ordinarily, which, according to our premises, must be +considered as a derivative type. In the clovers and their allies this +type reverts halfway to the single form, producing only three leaflets +on each stalk. If now the clover increases its number of leaflets, this +may be considered as a reversion to its nearest progenitors, the +papilionaceous plants with pinnate leaves. Hence a halfway returning and +therefore positive atavism. And as I have already mentioned in a former +lecture, pinnate [345] leaves are also sometimes produced by my new race +of clover. + +Returning to the original plants of this race, it is evidently +impossible to decide whether they were really the beginning of a new +strain, and had originated themselves by some sudden change from the +common type, or whether they belonged to an old variety, which had +propagated itself perhaps during centuries, unobserved by man. But the +same difficulty generally arises when new varieties are discovered. Even +the behavior of the plants themselves or of their progeny does not +afford any means of deciding the question. The simplest way of stating +the matter therefore, is to say that I accidentally found two +individuals of the "five-leaved" race. By transplanting them into my +garden, I have isolated them and kept them free from cross-fertilization +with the ordinary type. Moreover, I have brought them under such +conditions as are necessary for the full development of their +characters. And last but not least, I have tried to improve this +character as far as possible by a very rigid and careful selection. + +The result of all this effort has been a rapid improvement of my strain. +I saved the seed of the original plants in 1889 and cultivated the +second generation in the following year. It [346] showed some increase +of the anomaly, but not to a very remarkable degree. In the flowering +period I selected four plants with the largest number of quaternate and +quinate leaves and destroyed all the others. I counted in the average 25 +anomalous organs on each of them. From their seed I raised the third +generation of my culture in the year 1891. + +This generation included some 300 plants, on which above 8,000 leaves +were counted. More than 1,000 were quaternate or quinate, the ternate +leaves being still in the majority. But the experiment clearly showed +that "four-leaved" clovers may be produced in any desired quantity, +provided that the seed of the variety is available. In the summer only +three, four and five leaflets on one stalk were seen, but towards the +fall, and after the selection of the best individuals, this number +increased and came up to six and seven in some rare instances. + +The selection in this year was by no means easy. Nearly all the +individuals produced at least some quaternate leaves, and thereby showed +the variety to be quite pure. I counted the abnormal organs on a large +group of the best plants, and selected 20 excellent specimens from them, +with more than one-third of all their leaves changed in the desired +manner. Having brought my race up to this point, I [347] was able to +introduce a new and far more easy mark, afforded by the seedlings, for +my selections. This mark has since remained constant, and has brought +about a rapid continuance of the improvement, without necessitating such +large cultures. + +This seedling in the various species of clover usually begins with a +first leaf above the cotyledons of a different structure from those that +follow. It has only one blade instead of three. But in my variety the +increase of the number of the leaflets may extend to these primary +organs, and make them binate or even ternate. Now it is obvious that an +individual, which begins with a divided primary leaf, will have a +greater tendency to produce a large number of supernumerary leaflets +than a plant which commences in the ordinary way. Or in other words, the +primary leaves afford a sure criterion for the selection, and this +selection may be made in the seed-pans. In consequence, no young +individual with an undivided primary leaf was planted out. Choosing the +20 or 30 best specimens in the seed-pan, no further selection was +required, and the whole lot could be left to cross-fertilization by +insects. + +The observation of this distinguishing mark in the young seedlings has +led to the discovery of another quality as a starting-paint for further +[348] selection. According to the general rule of pedigree-culture, the +seeds of each individual plant are always saved and sowed separately. +This is done even with such species as the clover, which are infertile +when self-pollinated, and which are incapable of artificial pollination +on the required scale, since each flower produces only one seed. My +clover was always left free to be pollinated by insects. Obviously this +must have led to a diminution of the differentiating characters of the +individual plants. But this does not go far enough to obliterate the +differences, and the selection made among the seedlings will always +throw out at least a large part of those that have suffered from the +cross. + +Leaving this discussion, we may inquire closer into the nature of the +new criterion afforded by the seedlings. Two methods present themselves. +First, the choice of the best seedlings. In the second place it becomes +possible to compare the parent-plants by counting the number of +deviating seedlings. This leads to the establishment of a percentage for +every single parent, and gives data for comparisons. Two or three +hundreds of seeds from a parent may easily be grown in one pan, and in +this way a sufficiently high degree of accuracy may be reached. Only +those parents that give [349] the highest percentage are chosen, and +among their progeny only the seedlings with trifoliolate primary leaves +are planted out. The whole procedure of the selection is by this means +confined to the glasshouse during the spring, and the beds need not be +large, nor do they require any special care during the summer. + +By this method I brought my strain within two years up to an average of +nearly 90% of the seedlings with a divided primary leaf. Around this +average the real numbers fluctuated between the maximum of 99% and the +minimum of 70% or thereabouts. This condition was reached by the sixth +generation in the year 1894, and has since proved to be the limit, the +group of figures remaining practically the same during all the +succeeding generations. + +Such selected plants are very rich in leaves with four, five and six +blades. Excluding the small leaves at the tops of the branches, and +those on the numerous weaker side-branches, these three groups include +the large majority of all the stronger leaves. In summer the range is +wider, and besides many trifoliolate leaves the curiously shaped +seven-bladed ones are not at all rare. In the fall and in the winter the +range of variability is narrowed, and at first sight the plants often +seem to bear only quinquefoliolate leaves. + +[350] I have cultivated a new generation of this race nearly every year +since 1894, using always the strictest selection. This has led to a +uniform type, but has not been adequate to produce any further +improvement. Obviously the extreme limit, under the conditions of +climate and soil, has been reached. This extreme type is always +dependent upon repeated selection. No constant variety, in the older +sense, has been obtained, nor was any indication afforded that such a +type might ever be produced. On the contrary, it is manifest that the +new form belongs to the group of ever-sporting varieties. It is never +quite free from the old atavistic type of the trifoliolate leaves, and +invariably, when external conditions become less favorable, this +atavistic form is apt to gain dominion over the more refined varietal +character. Reversions always occur, both partial and individual. + +Some instances of these reversions may now be given. They are not of +such a striking character as those of the snapdragon. Intermediate steps +are always occurring, both in the leaves themselves, and in the +percentages of deviating seedlings of the several parent plants. + +On normal plants of my variety the quinquefoliolate leaves usually +compose the majority, when there are no weak lateral branches, or when +they are left out of consideration. Next [351] to these come the fours +and the sixes, while the trifoliolate and seven-bladed types are nearly +equal in number. But out of a lot of plants, grown from seed of the same +parent, it is often possible to choose some in which one extreme +prevails, and others with a preponderating number of leaves with the +other extreme number of leaflets. If seed from these extremes are saved +separately, one strain, that with numerous seven-bladed leaves will +remain true to the type, but the other will diverge more or less, +producing leaves with a varying number of subdivisions. + +Very few generations of such opposite selection are required to reduce +the race to an utterly poor one. In three years I was able to nearly +obliterate the type of my variety. I chose the seedlings with an +undivided primary leaf, cultivated them and counted their offspring +separately after the sowing. I found some parents with only 2-3% of +seedlings with divided primary leaves. And by a repeated selection in +this retrograde direction I succeeded in getting a great number of +plants, which during the whole summer made only very few leaves with +more than three blades. But an absolute reversion could no more be +reached in this direction than in the normal one. Any sowing without +selection would be [352] liable to reduce the strain to an average +condition. + +The production of varietal and of atavistic leaves is dependent to a +high degree on external conditions. It agrees with the general rule, +that favorable circumstances strengthen the varietal peculiarities, +while unfavorable conditions increase the number of the parts with the +atavistic attribute. These influences may be seen to have their effect +on the single individuals, as well as on the generations growing from +their seed. I cannot cite here all the experimental material, but a +single illustrative example may be given. I divided a strong individual +into two parts, planted one in rich soil and the other in poor sand, and +had both pollinated by bees with the pollen of some normal individuals +of my variety growing between them. The seeds of both were saved and +sown separately, and the two lots of offspring cultivated close to each +other under the same external conditions. In the beginning no difference +was seen, but as soon as the young plants had unfolded three or four +leaves, the progeny of the better nourished half of the parent plant +showed a manifest advance. This difference increased rapidly and was +easily seen in the beds, even before the flowering period. + +This experience probably gives an explanation [353] why the +quinquefoliolate variety is so seldom met with in the wild state. For +even if it did occur more often, the plants would hardly find +circumstances favorable enough for the full development of their +varietal character. They must often be so poor in anomalous leaves as to +be overlooked, or to be taken for instances of the commonly occurring +quadrifoliolate leaves and therefore as not indicating the true variety. + +In the beginning of my discussion I have asserted the existence of two +different races of "four-leaved" clovers, a poor one and a rich one, and +have insisted on a sharp distinction between them. This distinction +partly depends on experiments with clover, but in great part on tests +with other plants. The previously mentioned circumstance, that clover +cannot be pollinated on a sufficiently large scale otherwise than by +insects, prevents trials in more than one direction at the same time and +in the same garden. For this reason I have chosen another species of +clover to be able to give proof or disproof of the assertion quoted. + +This species is the Italian, or crimson clover, which is sometimes also +called scarlet clover (_Trifolium incarnatum_). It is commonly used in +Europe as a crop on less fertile soils than are required by the red +clover. It is annual [354] and erect and more or less hairy, and has +stouter leaves than other kinds of clover. It has oblong or cylindrical +heads with bright crimson flowers, and may be considered as one of the +most showy types. As an annual it has some manifest advantages over the +perennial species, especially in giving its harvest of hay at other +seasons of the year. + +I found some stray quaternate leaves of this plant some years ago, and +tried to win from them, through culture and selection, a race that would +be as rich in these anomalies as the red clover. But the utmost care and +the most rigid selection, and all the attention I could afford, failed +to produce any result. It is now ten years since I commenced this +experiment, and more than once I have been willing to give it up. Last +year (1903) I cultivated some hundreds of selected plants, but though +they yielded a few more instances of the desired anomaly than in the +beginning, no trace of a truly rich race could be discovered. The +experimental evidence of this failure shows at least that stray +"four-leaves" may occur, which do not indicate the existence of a true +"four-" or "five-leaved" variety. + +This conception seems destined to become of great value in the +appreciation of anomalies, as they are usually found, either in the wild +state [355] or in gardens. And before describing the details of my +unsuccessful pedigree-culture, it may be as well to give some more +instances of what occurs in nature. + +Stray anomalies are of course rare, but not so rare that they might not +be found in large numbers when perseveringly sought for. Pitcher-like +leaves may be found on many trees and shrubs and herbs, but ordinarily +one or only two of them are seen in the course of many years on the same +plant, or in the same strain. In some few instances they occur annually +or nearly so, as in some individuals of the European lime-tree (_Tilia +parvifolia_) and of the common magnolia (_Magnolia obovata_). Many of +our older cultivated plants are very rich in anomalies of all kinds, and +_Cyclamen_, _Fuchsia_, _Pelargonium_ and some others are notorious +sources of teratologic phenomena. Deviations in flowers may often be +seen, consisting of changes in the normal number of the several organs, +or alterations in their shape and color. Leaves may have two tips, +instead of one, the mid-vein being split near the apex, and the fissure +extending more or less towards the base. Rays of the umbels of +umbelliferous plants may grow together and become united in groups of +two or more, and in the same way the fruits of [356] the composites may +be united into groups. Many other instances could easily be given. + +If we select some of these anomalies for breeding-experiments, our +results will not agree throughout, but will tend to group themselves +under two heads. In some cases the isolation of the deviating +individuals will at once show the existence of a distinct variety, which +is capable of producing the anomaly in any desired number of instances; +only dependent on a favorable treatment and a judicious selection. In +other cases no treatment and no selection are adequate to give a similar +result, and the anomaly remains refractory despite all our endeavors to +breed it. The cockscomb and the peloric fox-glove are widely known +instances of permanent anomalies, and others will be dealt with in +future lectures. On the other hand I have often tried in vain to win an +anomalous race from an accidental deviation, or to isolate a teratologic +variety out of more common aberrations. Two illustrative examples may be +quoted. In our next lecture we shall deal with a curious phenomenon in +poppies, consisting in the change of the stamens into pistils and giving +rise to a bright crown of secondary capsules around the central one. +Similar anomalies may be occasionally met with in other species of the +same genus. But they are rare, and may show [357] the conversion of only +a single stamen in the described manner. I observed this anomaly in a +poppy called _Papaver commutatum_, and subjected it during several years +to a rigid selection of the richest individuals. No amelioration was to +be gained and the culture had to be given up. In the same way I found on +the bulbous buttercup (_Ranunculus bulbosus_) a strain varying largely +in the number of the petals, amounting often to 6-8, and in some flowers +even yet to higher figures. During five succeeding years I cultivated +five generations, often in large numbers, selecting always those which +had the highest number of petals, throwing out the remainder and saving +the seed only from the very best plants. I got a strain of selected +plants with an average number of nine petals in every flower, and found +among 4,000 flowers four having 20 petals or more, coming up even to 31 +in one instance. But such rare instances had no influence whatever on +the selection, since they were not indicative of individual qualities, +but occurred quite accidentally on flowers of plants having only the +average number of petals. Now double flowers are widely known to occur +in other species of the buttercups, both in the cultivated varieties and +in some wild forms. For this reason it might be expected that through a +continuous selection of [358] the individuals with the largest numbers a +tendency to become double would be evolved. Such, however, was not the +case. No propensity to vary in any definite direction could be observed. +Quite on the contrary, an average condition was quickly reached, and +then remained constant, strongly counteracting all selection. + +Such experiences clearly show that the same anomaly may occur in +different species, and no doubt in strains of the same species from +different localities, according to at least two different standards. The +one is to be called the poor, and the other the rich variety. The first +always produces relatively few instances of the deviation, the last is +apt to give as many of them as desired. The first is only half-way a +variety, and therefore would deserve the name of a half-race; the second +is not yet a full constant variety, but always fluctuates to and fro +between the varietal and the specific mark, ever-sporting in both +directions. It holds a middle position between a half-race and a +variety, and therefore might be called a "middle-race." But the term +ever-sporting variety seems more adequate to convey a right idea of the +nature of this curious type of inheritance. + +From this discussion it will be seen that the behavior of the crimson +clover is not to be considered [359] as an exception, but as a widely +occurring type of phenomenon, occurring perhaps in all sorts of +teratologic deviations, and in wide ranges of species and genera. Hence +it may be considered worth while to give some more details of this +extended experiment. + +Ten years ago (1894-5) I bought and sowed about a pound of seed of the +crimson clover. Among many thousands of normal seedlings I found two +with three and one with four cotyledons. Trusting to the empirical rules +of correlation, I transplanted these three individuals in order to +isolate them in the flowering period. + +One of them produced during the ensuing summer one four-bladed and one +five-bladed leaf. The seeds were saved separately and sown the following +spring and the expected result could soon be seen. Among some 250 +individual plants I counted 22 with one or two deviations, and 10 with +from three to nine four- or five-bladed leaves. Proportions nearly +similar have been observed repeatedly. Better nourished individuals have +produced more deviating leaves on one plant, partly owing to the larger +number of stems and branches, and poor or average specimens have mostly +been without any aberration or with only one or two abnormal leaves. No +further improvement could be attained. Quadrifoliolate leaves were +always rare, never [360] attaining a number that would put its stamp on +a whole bed. I have endeavored to get some six- and seven-bladed crimson +clover leaves, but in vain; selection, culture of many hundreds of +individuals, manure, and the best possible treatment has not been +adequate to produce them. Of course I am quite convinced that a +repetition of my experiment on a far larger scale would yield the +desired types, but then only in such rare instances that they would have +no influence whatever on the average, or on the improvement of the race. +The eighth generation in the year 1903 has not been noticeably better +than the second and third generations after the first selection. + +In comparing this statement with the results gained in the experiment +with the red clover, the difference is at once striking. In one case a +rich variety was isolated, and, by better treatment and sharp methods of +selection, was brought up in a few years to its highest pitch of +development. In the other case a very weak race was shown to exist, and +no amount of work and perseverance was adequate to improve it to any +noticeable degree. + +I wish to point out that the decision of what is to be expected from +deviating specimens may become manifest within one or two generations. +Even the generation grown from the seeds of [361] the first observed +aberrant-individuals, if gathered after sufficient isolation during the +period of blossoming, may show which type of inheritance is present, +whether it is an unpromising half-race, or a richly endowed sporting +variety. I have kept such strains repeatedly after the first isolation, +and a special case, that of cotyledoneous aberrations, will be dealt +with later. The first generation always gave a final decision, provided +that a suitable method of cultivation for the species under observation +was found at the beginning. This however, is a condition, which it is +not at all easy to comply with, when new sorts are introduced into a +garden. Especially so when they had been collected in the wild state. +Often one or two years, sometimes more, are necessary to find the proper +method of sowing, manuring, transplanting and, other cultural methods +satisfactory to the plants. Many wild species require more care and more +manure in gardens than the finest garden flowers. And a large number are +known to be dependent on very particular conditions of soil. + +One of the most curious features of anomalies, which has been learned +from accumulated instances, is the fact that they obey definite laws as +to their occurrence on the different parts of the plant. Obviously such +laws are [362] not apparent as long as each plant produces only one or +two, or, at most, a few instances of the same deviation. On the +contrary, any existing regularity must betray itself, as soon as a +larger number of instances is produced. A rule of periodicity becomes +most clearly manifest in such cases. + +This rule is shown by no other race in a more undoubted and evident +manner than by the "five-leaved" clover. Evidently the several degrees +of deviation, going from three to seven leaflets, may be regarded as +responses to different degrees of variation, and their distribution over +the stems and branches, or over the whole plant, may be considered as +the manifestation of the ever-changing internal tendency to vary. + +Considered from this point of view, my plants always showed a definite +periodicity in this distribution, which is the same for the whole plant. +Each of them, and each of the larger branches, begin with atavistic +leaves or with slight deviations. These are succeeded by greater +deviations, but only the strongest axes show as many as seven leaflets +on a stalk. This ordinarily does not occur before the height of +development is reached, and often only towards its close. Then the +deviation diminishes rapidly, returning often to atavistic leaves at the +summit of the stem or branch. I give the numbers of the [363] leaves of +a branch, in their order from the base to the top. They were as follows: + + 3. 4. 5. 6. 7. 5. 5. 4. + +But this is a selected case, and such regular examples of the expected +periodicity are rarely found. Often one or more of the various steps are +lacking, or even leaves with smaller numbers may be interspersed among +those with larger numbers of leaflets. But while the regularity of the +periodicity is in some degree diminished by such occurrences, yet the +rule always holds good, when taken broadly. It may be expressed by +stating that the bases and apices have on the average fewer leaflets on +each leaf than the middle parts of the stem and branches, and that the +number of leaflets gradually increases from the base toward a maximum, +which is reached in organs on the middle or upper part of the axis, and +then diminishes from this toward the apex. + +This periodicity is not limited to the stems and branches, considered +singly, but also holds good in a comparison made between the branches of +a single stem, in regard to their relative places on that stem. So it is +also for the whole plant. The first stems, produced by the subterranean +axis, ordinarily show only a low maximum deviation: the next succeeding +being [364] more divergent and the last ones returning to less +differentiated forms. + +It is evident that on a given stem the group of deviating leaves will be +extended upward and downward, with the increase of the number of these +organs. This shows that a stem, or even a plant, promises a higher +degree of differentiation if it commences with its aberration earlier. +Hence it becomes possible to discern the most promising individuals in +early youth, and this conclusion leads to a very easy and reliable +method of selection, which may be expressed simply as follows: the +seedlings which commence earliest with the production of four- and +five-foliolate leaves are the best and should be selected for the +continuance of the race. And it is easily seen that this rule agrees +with that given above, and which was followed in my pedigree-culture. + +Furthermore it is seen that there is a complete agreement between the +law of periodicity and the responses of the deviations to nourishment +and other conditions of life. Weak plants only produce low degrees of +deviation, the stronger the individual becomes, the higher it reaches in +the scale of differentiation, and the more often it develops leaves with +five or more blades. Whether weakness or strength are derived from outer +causes, or from the internal [365] succession of the periods of life, is +evidently of no consequence, and in this way the law of periodicity may +be regarded as a special instance of the more general law of response to +external conditions. + +The validity of this law of periodicity is of course not limited to our +"five-leaved" clover. Quite on the contrary it is universal in +eversporting varieties. Moreover it may be ascertained and studied in +connection with the most widely different morphologic abnormalities, and +therefore affords easily accessible material for statistical inquiry. I +will now give some further instances, but wish to insist first upon the +necessity of an inquiry on a far larger scale, as the evidence as yet is +very scanty. + +The great celandine (_Chelidonium majus_) has a very curious double +variety. Its flowers are simpler and much more variable than in ordinary +garden-varieties. The process of doubling consists mainly in a change of +stamens into petals. This change is dependent on the season. On each +stem the earliest flowers are single. These are succeeded by blossoms +with one or two converted stamens, and towards the summer this number +increases gradually, attaining 10-11 and in some instances even more +altered filaments. Each year the same succession may be seen repeating +itself on the stems of [366] the old roots. Double tuberous begonias are +ordinarily absolutely sterile throughout the summer, but towards autumn +the new flowers become less and less altered, producing some normal +stamens and pistils among the majority of metamorphosed organs. From +these flowers the seeds are saved. Sometimes similar flowers occur at +the beginning of the flowering-period. Double garden-camomiles +(_Chrysanthemum inodorum plenissimum_) and many other double varieties +of garden-plants among the great family of the composites are very +sensitive to external agencies, and their flower-heads are fuller the +more favorable the external conditions. Towards the autumn many of them +produce fewer and fewer converted heads and often only these are fertile +and yield seeds. + +Ascidia afford another instance of this periodicity, though ordinarily +they are by far too rare to show any regularity in their distribution. +However, it is easy to observe that on lime-trees they prefer the lower +parts of each twig, while on magnolias the terminal leaves of the +branches are often pitcher-bearing. Ascidia of the white clover have +been found in numbers, in my own experiment-garden, but always in the +springtime. The thickleaved saxifrage (_Saxifraga crassifolia_) is often +very productive of ascidia, especially in [367] the latter part of the +season, and as these organs may be developed to very different degrees, +they afford fine material for the study of the law of periodicity. On a +garden-cytisus (_Cytisus candicans attleyanus_) I once had the good +fortune to observe a branch with ascidia, which ordinarily are very rare +in this species. It had produced seven ascidia in all, each formed by +the conversion of one leaflet on the trifoliolate leaves. The first six +leaves were destitute of this malformation and were quite normal. Then +followed a group of five leaves, constituting the maximum of the period. +The first bore one small pitcher-like blade, the second and third, each +one highly modified organ, the fourth, two ascidia, and the last, one +leaflet with slightly connate margins. The whole upper part of the +branch was normal, with the exception of the seventeenth leaf, which +showed a slight change in the same direction. All in all, the tendency +to produce ascidia increased from the beginning to the tenth leaf, and +decreased from this upward. + +The European Venus' looking-glass was observed in my garden to produce +some quaternate and some quinate flowers on the same specimens. The +quinate were placed at the end of the branches, those with four petals +and sepals lower down. The peloric fox-glove shows the [368] highest +degree of metamorphy in the terminal flowers of the stem itself, the +weaker branches having but little tendency towards the formation of the +anomaly. The European pine or _Pinus sylvestris_ ordinarily has two +needles in each sheath, but trifoliolate sheaths occur on the stems and +stronger branches, where they prefer, as a rule, the upper parts of the +single annual shoots. _Camellia japonica_ is often striped in the fall +and during the winter, but when flowering in the spring it returns to +the monochromatic type. + +Peloric flowers are terminal in some cases, but occur in the lower parts +of the flower-spikes in others. Some varieties of gladiolus commence on +each spike with more or less double flowers, which, higher up, are +replaced by single ones. A wide range of bulbs and perennial +garden-plants develop their varietal characters only partly when grown +from seed and flowering for the first time. The annual +garden-forget-me-not of the Azores (_Myosotis azorica_) has a variety +with curiously enlarged flowers, often producing 20 or more +corolla-segments in one flower. But this number gradually diminishes as +the season advances. It would be quite superfluous to give further proof +of the general validity of the law of periodicity in ever-sporting +varieties. + + +[369] + +LECTURE XIII + +PISTILLODY IN POPPIES + +One of the most curious anomalies that may be met with in ornamental +garden-plants is the conversion of stamens into pistils. It is neither +common nor rare, but in most cases the change is so slight comparatively +that it is ordinarily overlooked. In the opium-poppy, on the contrary, +it is very showy, and heightens the ornamental effect of the young +fruits after the fading of the flowers. Here the central capsule is +surrounded by a large crown of metamorphosed stamens. + +This peculiarity has attracted the attention both of horticulturists and +of botanists. As a rule not all the stamens are changed in this way but +only those of the innermost rows. The outer stamens remain normal and +fertile, and the flowers, when pollinated with their own pollen, bear as +rich a harvest of seeds as other opium-poppies. The change affects both +the filament and the anther, the former of which is dilated into a +sheath. Within this sheath perfect [370] and more or less numerous +ovules may be produced. The anthers become rudimentary and in their +place broad leafy flaps are developed, which protrude laterally from the +tip and constitute the stigmas. Ordinarily these altered organs are +sterile, but in some instances a very small quantity of seed is +produced, and when testing their viability I succeeded in raising a few +plants from them. + +The same anomaly occurs in other plants. The common wall-flower +(_Cheiranthus Cheiri_) and the houseleek (_Sempervivum tectorum_) are +the best known instances. Both have repeatedly been described by various +investigators. In compiling the literature of this subject it is very +interesting to observe the two contrasting views respecting the nature +of this anomaly. Some writers, and among them Masters in his "Vegetable +Teratology" consider the deviations to be merely accidental. According +to them some species are more subject to this anomaly than others, and +the houseleek is said to be very prone to this change. Goeppert, +Hofmeister and others occasionally found the pistilloid poppies in +fields or gardens, and sowed their seeds in order to ascertain whether +the accidental peculiarity was inheritable or not. On the other hand De +Candolle in his "Prodromus" mentions the pistilloid wall-flowers as a +distinct [371] variety, under the name of _Cheiranthus Cheiri +gynantherus_, and the analogous form of the opium-poppy is not at all an +accidental anomaly, but an old true horticultural variety, which can be +bought everywhere under the names of _Papaver somniferum monstruosum_ or +_polycephalum_. Since it is an annual plant, only the seeds are for +sale, and this at once gives a sufficient proof of its heredity. In all +cases, where it was met with accidentally by botanists, it is to be +assumed that stray seeds had been casually mixed with those of other +varieties, or that the habit had been transmitted by a spontaneous +cross. + +Wherever opportunity led to experiments on heredity, distinct races were +found to be in possession of this quality, while others were not. It is +of no use to cultivate large numbers of wall-flowers in the hope of one +day seeing the anomaly arise; the only means is to secure the strain +from those who have got it. With poppies the various varieties are so +often intercrossed by bees, that the appearance of an accidental change +may sometimes be produced, and in the houseleek the pistilloid warily +seems to be the ordinary one, the normal strain being very rare or +perhaps wholly wanting. + +Our three illustrative examples are good and permanent races, producing +their peculiar qualities [372] regularly and abundantly. In this respect +they are however very variable and dependent on external circumstances. +Such a regularity is not met with in other instances. Often +pedigree-experiments lead to poor races, betraying their tendency to +deviate only from time to time and in rare cases. Such instances +constitute what we have called in a former lecture, "half races," and +their occurrence indicates that the casual observation of an anomaly is +not in itself adequate to give an opinion as to the chance of repetition +in sowing experiments. A large number of species seem to belong to this +case, and their names may be found in the above mentioned work by +Masters and elsewhere. But no effort has yet been made to separate +thoroughly the pistilloid half-races from the corresponding +ever-sporting varieties. Some plants are recorded as being more liable +to this peculiarity than others. + +Stamens are sometimes replaced by open carpels with naked ovules arising +from their edges and even from their whole inner surfaces. This may be +seen in distinct strains of the cultivated bulbous Begonia, and more +rarely in primroses. Here the apex of the carpellary leaf is sometimes +drawn out into a long style, terminated by a flattened spatulate stigma. + +The pistillody of the stamens is frequently [373] combined with another +deviation in the poppies. This is the growing together of some of the +altered stamens so as to constitute smaller or larger connate groups. +Often two are united, sometimes three, four or more. Flowers with +numerous altered stamens are seldom wholly free from this most +undesirable secondary anomaly. I call it undesirable with respect to +experiments on the variability of the character. For it may easily be +seen that while it is feasible to count the stamens even when converted +into pistils, it is not possible when groups of them are more or less +intimately united into single bodies. This combination makes all +enumeration difficult and inaccurate and often wholly unreliable. In +such cases the observation is limited to a computation of the degree of +the change, rather than to a strict numerical inquiry. Happily the +responses to the experimental influences are so marked and distinct that +even this method of describing them has proved to be wholly sufficient. + +In extreme instances I have seen all the changed stamens of a flower of +the opium-poppy united into a single body, so as to form a close sheath +all around the central ovary. Lesser sheaths, surrounding one-half or +one-third of the capsule are of course less rarely met with. Leaving +this description of the outer appearance [374] of our anomaly, we may +now consider it from the double point of view of inheritance and +variability. + +The fact of inheritance is shown by the experience of many authors, and +by the circumstance already quoted, that the variety has been propagated +from seed for more than half a century, and may be obtained from various +seed merchants. In respect to the variability, the variety belongs to +the ever-sporting group, constituting a type which is more closely +related to the "five-leaved" clover than to the striped flowers or even +the double stocks. + +It fluctuates around an average type with half filled crowns, going as +far as possible in both directions, but never transgressing either +limit. It is even doubtful whether the presumable limits are, under +ordinary circumstances, ever reached. Obviously one extreme would be the +conversion of all the stamens, and the other the absolute deficiency of +any marked tendency to such a change. Both may occur, and will probably +be met with from time to time. But they must be extremely rare, since in +my own extensive experiments, which were strictly controlled, I never +was able to find a single instance of either of them. Some of the outer +stamens have always remained unchanged, yielding enough pollen for the +artificial pollination of [375] the central ovary, and on the other hand +some rudiments of hardened filaments were always left, even if they were +reduced to small protuberances on the thalamus of the flower. Between +these extremes all grades occur. From single, partially or wholly +changed stamens upwards to 150 and over, all steps may be seen. It is a +true fluctuating variability. There is an average of between 50 and 100, +constituting a nearly filled crown around the central capsule. Around +this average the smaller deviations are most numerous and the larger +ones more rare. The inspection of any bed of the variety suffices to +show that, taken broadly, the ordinary laws of fluctuating variability +are applicable. No counting of the single individuals is required to +dispel all doubts on this point. + +Moreover all intermediate steps respecting the conversion of the single +stamens may nearly always be seen. Rarely all are changed into normal +secondary ovaries with a stigma and with a cavity filled with ovules. +Often the stigma is incomplete or even almost wanting, in other +instances the ovules are lacking or the cavity itself is only partially +developed. Not rarely some stamens are reduced and converted into thin +hard stalks, without any appearance of an ovary at their tip. But then +the demarcation [376] between them and the thalamus fails, so that they +cannot be thrown off when the flower fades away, but remain as small +stumps around the base of the more fully converted filaments. This fact +would frequently render the enumeration of the altered organs quite +unreliable. + +For these reasons I have chosen a group of arbitrary stages in order to +express the degree of deviation for a given lot of plants. The limits +were chosen so as to be sufficiently trustworthy and easy to ascertain. +In each group the members could be counted, and a series of figures was +reached by this means which allowed of a further comparison of the +competing sets of plants. + +It should be stated that in such experiments and especially in the case +of such a showy criterion as the pistilloid heads afford after the time +of flowering is over, the inspection of the controlling beds at once +indicates the result of the experiment. Even a hasty survey is in most +cases sufficient to get a definite conclusion. Where this is not the +case, the counting of the individuals of the various groups often does +not add to the evidence, and the result remains uncertain. On the other +hand, the impression made by the groups of plants on the experimenter +and on his casual visitors, cannot well be conveyed to the readers of +his account by [377] other means than by figures. For this reason the +result of the experiments is expressed in this way. + +I made six groups. The first includes the cases where the whole circle +is reduced to small rudiments. The second shows 1-10 secondary capsules. +The two following constitute half a crown around the central fruit, the +third going up to this limit, the fourth going from this limit to a +nearly filled circle. Wholly filled circles of secondary capsules +without gaps give the two last degrees, the fifth requiring only +continuity of the circle, the sixth displaying a large and bright crown +all around the central head. The fifth group ordinarily includes from +90-100 altered stamens, while the sixth has from 100-150 of these +deviating parts. + +In ordinary cultures the third and fourth group, with their interrupted +crowns, predominate. Large crowns are rare and flowers which at first +sight seem to be wholly normal, occur only under circumstances +definitely known to be unfavorable to growth, and to the development of +the anomaly. + +Having reached by this means a very simple and easy method of stating +the facts shown by equal lots under contrasting influences, we will now +make use of it to inquire into the relation [378] of this exceptionally +high degree of variability to the inner and outer conditions of life. + +As a rule, all experiments show the existence of such a relation. +Unfavorable conditions reduce the numbers of altered stamens, favorable +circumstances raise it to its highest point. This holds true for lots +including hundreds of specimens, but also for the sundry heads of one +bed, and often for one single plant. + +We may compare the terminal flower with those of the lateral branches on +a plant, and when no special influences disturb the experiment, the +terminal head ordinarily bears the richest crown. If the first has more +than 100 metamorphosed parts, the latter have often less than 50 on the +same plant. In poor soil, terminal heads are often reduced to 10-20 +monstrous organs, and in such cases I found the lateral flowers of the +same plants ordinarily with less than 10 altered stamens. In some cases +I allowed the branches of the third and fourth degree, in other words, +the side twigs of the first branches of my selected plants to grow out +and produce flowers in the fall. They were ordinarily weak, sometimes +very small, having only 5-9 stigmas on their central fruit. Secondary +capsules were not seen on such flowers, even when the experiment was +repeated on a [379] somewhat larger scale and during a series of years. + +Among the same lot of plants individual differences almost always occur. +They are partly due to inequalities already existing in the seeds, and +partly to the diversity of the various parts of the same bed. Some of +the plants become stout and have large terminal heads. Others remain +very weak, with a slender stem, small leaves and undersized flowers. The +height and thickness of the stem, the growth of the foliage and of the +axillary buds are the most obvious measures of the individual strength +of the plant. The development of the terminal flower and the size of its +ovary manifestly depends largely on this individual strength, as may be +seen at once by the inspection of any bed of opium-poppies. Now this +size of the head can easily be measured, either by its height or +circumference, or by its weight. Moreover we can arrange them into a +series according to their size. If we do this with the polycephalous +variety, the relation between individual strength and degree of +metamorphosis at once becomes manifest. The largest heads have the +brightest crowns, and the number of supernumerary carpels diminishes in +nearly exact proportion to the size of the fruits. Fruits with less than +50 altered stamens weighed on an average 5 grams, [380] those with +50-100 such organs 7 grams and those with a bright crown 10 grams, the +appendices being removed before the weighing. Corresponding results have +been reached by the comparison of the height of the capsules with their +abnormal surroundings. The degree of development of the monstrosity is +shown by this observation to be directly dependent on, and in a sense +proportionate to the individual strength of the plant. + +The differences between the specimens grown from a single lot of seeds, +for instance from the seeds of one self-fertilized capsule are, as I +have said, partly due to the divergences which are always present in a +bed, even if the utmost care has been taken to make it as uniform as +possible. These local differences are ordinarily underrated and +overlooked, and it is often considered to be sufficient to cultivate +small lots of plants under apparently similar conditions on neighboring +beds, to be justified in imputing all the observed deviations of the +plants to hereditary inequalities. This of course is true for large +lots, whenever the averages only are compared. In smaller experiments +the external conditions of the single individuals should always be +considered carefully. Lots of one or two square meters suffice for such +comparisons, but smaller lots are always subject to chances and [381] +possibilities, which should never be left out of consideration. + +Therefore I will now point out some circumstances, which are ordinarily +different on various parts of one and the same bed. + +In the first place comes the inequality of the seeds themselves. Some of +them will germinate earlier and others later. Those that display their +cotyledons on a sunny day will be able to begin at once with the +production of organic food. Others appear in bad weather, and will thus +be retarded in their development. These effects are of a cumulative +nature as the young plants must profit by every hour of sunshine, +according to the size of the cotyledons. Any inequality between two +young seedlings is apt to be increased by this cumulative effect. + +The same holds good for the soil of the bed. It is simply impossible to +mix the manure so equally that all individuals receive the same amount +of it from the very beginning. I am in the habit of using manures in a +dry and pulverized condition, of giving definite quantities to each +square meter, and of taking the utmost care to get equal distribution +and mixture with the soil, always being present myself during this most +important operation. Nevertheless it is impossible to make the +nourishment exactly equal for all the plants of even a small bed. + +[382] Any inequality from this cause will increase the difference in the +size of the young leaves, augment the inequality of their production of +organic matter and for this reason go on in an ever increasing rate. + +Rain and spraying, or on the other hand dryness of the soil, have still +greater consequences. The slightest unevenness of the surface will cause +some spots to dry rapidly and others to retain moisture during hours and +even sometimes during days. + +Seeds, germinating in such little moist depressions grow regularly and +rapidly, while those on the dryer elevations may be retarded for hours +and days, before fully unfurling their seed-leaves. After heavy rains +these differences may be observed to increase continually, and in some +instances I found that plants were produced only on the wet spots, while +the dry places remained perfectly bare. From this the wet spots seem to +be the most favorable, but on the other hand, seeds may come to +germinate there too numerously and so closely that the young plants will +be crowded together and find neither space nor light enough, for a free +and perfect development. The advantage may change to disadvantage in +this way unless the superfluous individuals are weeded out in due time. + +[383] From all these and other reasons some plants will be favored by +the external conditions from the beginning, while others will be +retarded, and the effects will gradually increase until at last they +become sufficient to account for a considerable amount of individual +variability. There is no doubt that the difference in the strength of +the plant and in the size of the capsules, going from 5-10 grams for a +single fruit, are for the most part due to these unavoidable +circumstances. I have tried all conceivable means to find remedies for +these difficulties, but only by sowing my seeds in pans in a glass-house +have I been able to reach more constant and equal conditions. But +unfortunately such a method requires the planting out of the young +seedlings in the beginning of the summer, and this operation is not +without danger for opium-poppies, and especially not without important +influence on the monstrosity of the pistilloid variety. Consequently my +sowings of this plant have nearly always been made in the beds. + +In order to show how great the influence of all these little things may +become, we only have to make two sowings on neighboring beds and under +conditions which have carefully been made as equal as possible. If we +use for these controlling experiments seeds from one and the same +capsule, it will soon become evident that [384] no exact similarity +between the two lots may be expected. Such differences as may be seen in +these cases are therefore never to be considered of value when comparing +two lots of seeds of different origin, or under varying conditions. No +amount of accuracy in the estimation of the results of a trial, or in +the counting out of the several degrees of the anomaly, is adequate to +overcome the inaccuracy resulting from these differences. + +It is certainly of great importance to have a correct conception in +regard to the influence of the surrounding conditions on the growth of a +plant and on the development of the attribute we are to deal with. No +less important is the question of the sensibility of the plants to these +factors. Obviously this sensibility must not be expected to remain the +same during the entire life-period, and periods of stronger and of +weaker responses may be discerned. + +In the first place it is evident that external or inner influences are +able to change the direction of the development of an organ only so long +as this development is not yet fully finished. In the young flower-bud +of the pistilloid poppy there must evidently be some moment in which it +is definitely decided whether the young stamens will grow out normally +or become metamorphosed into secondary pistils. From this [385] moment +no further change of external conditions is able to produce a +corresponding change in the degree of the anomaly. The individual +strength of the whole plant may still be affected in a more or less +manifest degree, but the number of converted stamens of the flower has +been definitely fixed. The sensitive period has terminated. + +In order to determine the exact moment of this termination of the period +of sensibility, I have followed the development of the flower buds +during the first weeks of the life of the young plants. The terminal +flower may already be seen in young plants only seven weeks old, with a +stem not exceeding 5-6 cm. in height and a flower-bud with a diameter of +nearly 1 mm., in which the stamens and secondary pistils are already +discernible, but still in the condition of small rounded protuberances +on the thalamus. Though it is not possible at that time to observe any +difference between the future normal and converted stamens, it does not +seem doubtful that the development is so far advanced, that in the inner +tissues the decision has already definitely been taken. In the next few +days this decision rapidly becomes visible, and the different parts of +the normal stamens and the metamorphosed carpels soon become apparent. +From this observation it [386] can be inferred that the sensitive period +of the anomaly is limited for the terminal flower-head, to the first few +weeks of the life of the young plants. The secondary heads manifestly +leave this period at a somewhat later stage. + +In order to prove the accuracy of this conclusion I have tried to injure +the anomalies after the expiration of the first six or seven weeks. I +deprived them of their leaves, and damaged them in different ways. I +succeeded in making them very weak and slender, without being able to +diminish the number of the supernumerary carpels. The proportionality of +the size of the central fruit and the development of the surrounding +crown can often be modified or even destroyed by this means, and the +apparent exceptions from this rule, which are often observed, may find +their explanation in this way. + +In the second place I have tried to change the development of the +anomaly during the period of sensibility, and even in the last part of +it. This experiment succeeded fully when carried out within the fifth or +sixth week after the beginning of the germination. As means of injury I +transplanted the young plants. To this end I sowed my seeds in pans in +unmanured soil, planted them out in little pots with richly prepared +earth, grew them in these during a few weeks and afterwards transferred +them to the [387] beds, taking care that the pats were removed, but the +balls of earth not broken. + +In consequence of this treatment the plants became very large and +strong, with luxuriant foliage and relatively numerous large flowers and +fruits. But almost without exception they were poor in anomalous +stamens, at least so on the terminal heads. On a lot of some 70 plants +more than 50 had less than half a crown of secondary capsules, while +from the same packet of seed the control-plants gave in an equal number +more than half of filled crowns on all plants with the exception of five +weak specimens. + +It is curious to compare such artificially injured plants with the +ordinary cultures. Strong stems and heavy fruits, which otherwise are +always indicative of showy crowns, now bear fruits wholly or nearly +destitute of any anomalous change. The commonly prevailing rule seems to +be reversed, showing thereby the possibility of abolishing the +correlation between individual strength and anomaly by an artificial +encroachment upon the normal conditions. + +Aside from these considerations the experiments clearly give proof of +the existence of a period of sensibility limited to the first weeks of +the life of the plant for the terminal flower. This knowledge enables us +to explain many apparent [388] parent abnormalities, which may occur in +the experiments. + +We now may take a broader view of the period of sensibility. Evidently +the response to external influences will be greater the younger the +organ. Sensibility will gradually diminish, and the phenomena observed +in the last part of this period may be considered as the last remainder +of a reaction which previously must have been much stronger and much +readier, providing that it would be possible to isolate them from, and +contrast them with, the other responses of the same plant. + +With the light thus cast upon the question, we may conclude that the +sensitive period commences not only at the beginning of the germination, +but must also be considered to include the life of the seed itself. From +the moment of fertilization and the formation of the young embryo the +development must be subjected to the influence of external agencies +which determine the direction it will take and the degree of development +it will finally be able to acquire. Probably the time of growth of the +embryo and of the ripening of the seed correspond exactly to the period +of highest sensibility. This period is only interrupted during the +resting stage of the seed, to be repeated in germination. Afterwards the +sensibility [389] slowly and gradually decreases, to end with the +definite decision of all further growth sometime before the outer form +of the organ becomes visible under the microscope. The last period of +life includes only an expansion of the tissues, which may still have +some influence on their final size, but not on their form. This has been +definitely arrested before the end of the sensitive period, and +ordinarily before the commencement of that rapid development, which is +usually designated by the name of growth, as contrasted with evolution. + +Within the seed the evolution of the young plant manifestly depends upon +the qualities and life-conditions of the parent-plant. The stronger this +is, and the more favorable circumstances it is placed under, the more +food will be available for the seed, and the healthier will be the +development of the embryo. Only well-nourished plants give +well-nourished seeds, and the qualities of each plant are for this +reason at least, partly dependent on the properties of its parents and +even of its grandparents. + +From these considerations the inference is forced upon us that the +apparently hereditary differences, which are observed to exist among the +seeds of a species or a variety and even of a single strain or a single +parent-plant, may for a large part, and perhaps wholly, be the result +[390] of the life-conditions of their parents and grandparents. Within +the race all ssvariability would in this way be reduced to the effects +of external circumstances. Among these nourishment is no doubt the most +momentous, and this to such a degree that older writers designated the +external conditions by the term nourishment. According to Knight +nutrition reigns supreme in the whole realm of variability, the kind of +food and the method of nourishment coming into consideration only in a +secondary way. The amount of useful nutrition is the all-important +factor. + +If this is so, and if nutrition decides the degree of deviation of any +given character, the widest deviating individuals are the best nourished +ones. The best nourished not only during the period of sensibility of +the attribute under consideration, but also in the broadest sense of the +word. + +This discussion casts a curious light upon the whole question of +selection. Not of course upon the choice of elementary species or +varieties out of the original motley assembly which nature and old +cultures offer us, but upon the selection of the best individuals for +isolation and for the improvement of the race. These are, according to +my views, only the best nourished ones. Their external conditions have +been the [391] most favorable, not only from the beginning of their own +life in the field, but also during their embryonic stages, and even +during the preparation of these latter in the life of their parents and +perhaps even their grandparents. Selection then, would only be the +choice of the best nourished individuals. + +In connection with the foregoing arguments I have tried to separate the +choicest of the poppies with the largest crown of pistilloid stamens, +from the most vigorous individuals. As we have already seen, these two +attributes are as a rule proportional to one another. Exceptions occur, +but they may be explained by some later changes in the external +circumstances, as I have also pointed out. As a rule, these exceptions +are large fruits with comparatively too few converted stamens; they are +exactly the contrary from what is required for a selection. Or plants, +which from the beginning were robust, may have become crowded together +by further growth, and for these reasons become weaker than their +congeners, though retaining the full development of the staminodal +crown, which was fixed during the sensitive period and before the +crowding. I have searched my beds yearly for several years in vain to +find individuals which might recommend themselves for selection without +having the stamp of permanent, [392] or at least temporarily better, +nourishment. No starting-point for such an independent selection has +ever been met with. + +Summing up the consequences of this somewhat extended discussion, we may +state it as a rule that a general proportion between the individual +strength and the degree of development of the anomaly exists. And from +this point of view it is easy to see that all external causes which are +known to affect the one, must be expected to influence the other also. + +It will therefore hardly be necessary to give a full description of all +my experiments on the relations of the monstrosity to external +conditions. A hasty survey will suffice. + +This survey is not only intended to convey an idea of the relations of +pistilloid poppies to their environment, but may serve as an example of +the principle involved. According to my experience with a large range of +other anomalies, the same rule prevails everywhere. And this rule is so +simple that exact knowledge of one instance may be considered as +sufficient to enable us to calculate from analogy what is to be expected +from a given treatment of any other anomaly. Our appreciation of +observed facts and the conditions to be chosen for intended cultures are +largely dependent on such calculations. What I am now going to describe +[393] is to be considered therefore as an experimental basis for such +expectations. + +First of all comes the question how many individuals are to be grown in +a given place. When sowing plants for experimental purposes it is always +best to sow in rows, and to give as few seeds to each row as possible, +so as to insure all necessary space to the young plants. On the other +hand the seeds do not all germinate, and after sowing too thinly, gaps +may appear in the rows. This would cause not only a loss of space, but +an inequality between the plants in later life, as those nearest the +gaps would have more space and more light, and a larger area for their +roots than those growing in the unbroken rows. Hence the necessity of +using large quantities of seed and of weeding out a majority of young +plants on the spots where the greatest numbers germinate. + +Crowded cultures as a rule, will give weak plants with thin stems, +mostly unbranched and bearing only small capsules. According to the +rule, these will produce imperfect crowns of secondary pistils. The +result of any culture will thus be dependent to a high degree on the +number of individuals per square meter. I have sown two similar and +neighboring beds with the thoroughly mixed seeds of parent-plants of the +same strain and culture, using as much [394] as 2.5 cu. cm. per square +meter. On one of the beds I left all the germinating plants untouched +and nearly 500 of them flowered, but among them 360 were almost without +pistillody, and only 10 had full crowns. In the other bed I weeded away +more than half of the young plants, leaving only some 150 individuals +and got 32 with a full crown, nearly 100 with half crowns and only 25 +apparently without monstrosity. + +These figures are very striking. From the same quantity of seed, in +equal spaces, by similar exposure and treatment I got 10 fully developed +instances in one and 32 in the other case. The weeding out of +supernumerary individuals had not only increased the percentage of +bright crowns, but also their absolute number per square meter. So the +greatest number of anomalies upon a given space may be obtained by +taking care that not too many plants are grown upon it: any increase of +the number beyond a certain limit will diminish the probability of +obtaining these structures. The most successful cultures may be made +after the maximum number of individuals per unit of area has been +determined. A control-experiment was made under the same conditions and +with the same seed, but allowing much less for the same space. I sowed +only 1 cu. cm. on my bed of 2 square meters, and thereby avoided [395] +nearly all weeding out. I got 120 plants, and among them 30 with full +crowns of converted stamens, practically the same number as after the +weeding out in the first experiment. This shows that smaller quantities +of seed give an equal chance for a greater number of large crowns, and +should therefore always be preferred, as it saves both seed and labor. + +Weeding out is a somewhat dangerous operation in a comparative trial. +Any one who has done it often, knows that there is a strong propensity +to root out the weaker plants and to spare the stronger ones. Obviously +this is the best way for ordinary purposes, but for comparisons +evidently one should not discriminate. This rule is very difficult in +practice, and for this reason one should never sow more than is +absolutely required to meet all requirements. + +Our second point is the manuring of the soil. This is always of the +highest importance, both for normal and for anomalous attributes. The +conversion of the stamens into pistils is in a large measure dependent +upon the conditions of the soil. I made a trial with some 800 flowering +plants, using one sample of seed, but sowing one-third on richly manured +soil, one-third on an unprepared bed of my garden, and one-third on +nearly pure sand. In all other respects the three groups were treated in +the same way. Of [396] the manured plants one-half gave full crowns, of +the non-manured only one-fifth, and on the sandy soil a still smaller +proportion. Other trials led to the same results. I have often made use +of steamed and ground horn, which is a manure very rich in nitrogenous +substances. One-eighth of a kilo per square meter is an ample amount. +And its effect was to increase the number of full crowns to an +exceptional degree. + +In the controlling trial and under ordinary circumstances this figure +reached some 50%, but with ground horn it came up as high as 90%. We may +state this result by the very striking assertion that the number of +large crowns in a given culture may be nearly doubled by rich manure. + +All other external conditions act in a similar manner. The best +treatment is required to attain the best result. A sunny exposure is one +of the most essential requisites, and in some attempts to cultivate my +poppies in the shade, I found the pistillody strongly reduced, not a +single full crown being found in the whole lot. Often the weather may be +hurtful, especially during the earlier stages of the plants. I protected +my beds during several trials by covering them with glass for a few +weeks, until the young plants reached the glass covering. I got a normal +number of full crowns, some 55%, at a time [397] when the weather was so +bad as to reduce the number in the control experiments to 10%. + +It would be quite superfluous to give more details or to describe +additional experiments. Suffice to say, that the results all point in +the same direction, and that pistillody of the poppies always clearly +responds to the treatment, especially to external conditions during the +first few weeks, that is, during the period of sensitiveness. The +healthier and the stronger the plants the more fully they will develop +their anomaly. + +In conclusion something is to be said about the choice of the seed. +Obviously it is possible to compare seeds of different origin by sowing +and treating them in the same way, giving attention to all the points +above mentioned. In doing so the first question will be, whether there +is a difference between the seeds of strong plants with a bright crown +around the head and those of weaker individuals with lesser development +of the anomaly. It is evident that such a difference must be expected, +since the nutrition of the seed takes place during the period of the +greatest sensitiveness. + +But the experiments will show whether this effect holds good against the +influences which tend to change the direction of the development of the +anomaly during the time of germination. [398] The result of my attempt +has shown that the choice of the seeds has a manifest influence upon the +ultimate development of the monstrosity, but that this influence is not +strong enough to overwhelm all other factors. + +The choice of the fullest or smallest crowns may be repeated during +succeeding generations, and each time compared with a culture under +average conditions. By this means we come to true selection-experiments, +and these result in a notable and rapid change of the whole strain. By +selecting the brightest crowns I have come up in three years from 40 to +90 and ultimately to 120 converted stamens in the best flower of my +culture, and in selecting the smallest crowns I was able in three years +to exclude nearly all good crowns, and to make cultures in which heads +with less than half-filled crowns constituted the majority. But such +selected strains always remain very sensitive to treatment, and by +changing the conditions the effect may be wholly lost in a single year, +or even turned in the contrary direction. In other words, the anomaly is +more dependent on external conditions during the germinating period than +on the choice of the seeds, providing these belong to the pistilloid +variety and have not deteriorated by some crossing with other sorts. + +At the beginning of this lecture I stated that [399] no selection is +adequate to produce either a pure strain of brightly crowned +flower-heads without atavism, or to conduce to an absolute and permanent +loss of the anomaly. During a series of years I have tested my plants in +both directions, but without the least effect. Limits are soon reached +on both sides, and to transgress these seems quite impossible. + +Taking these limits as the marks of the variety, and considering all +fluctuations between them as responses to external influences working +during the life of the individual or governing the ripening of the +seeds, we get a clear picture of a permanent ever-sporting type. The +limits are absolutely permanent during the whole existence of this +already old variety. They never change. But they include so wide a range +of variability, that the extremes may be said to sport into one another, +so much the more so as one of the extremes is to be considered +morphologically as the type of the variation, while the other extreme +can hardly be distinguished from the normal form of the species. + + + +[400] + +LECTURE XIV + +MONSTROSITIES + +I have previously dealt with the question of the hereditary tendencies +that cause monstrosities. These tendencies are not always identical for +the same anomaly. Two different types may generally, be distinguished. +One of them constitutes a poor variety, the other a rich one. But this +latter is abundant and the first one is poor in instances of exactly the +same conformation. Therefore the difference only lies in the frequency +of the anomaly, and not in its visible features. In discovering an +instance of any anomaly it is therefore impossible to tell whether it +belongs to a poor or to a rich race. This important question can only be +answered by direct sowing-experiments to determine the degree of +heredity. + +Monstrosities are often considered as accidents, and rightfully so, at +least as long as they are considered from a morphological point of view. +Physiology of course excludes all accidentality. And in our present ease +it shows [401] that some internal hereditary quality is present, though +often latent, and that the observed anomalies are to be regarded as +responses of this innate tendency to external conditions. Our two types +differ in the frequency of these responses. Rare in the poor race, they +are numerous in the rich variety. The external conditions being the same +for both, the hereditary factor must be different. The tendency is weak +in the one and strong in the other. In both cases, according to my +experience, it may be weakened or strengthened by selection and by +treatment. Often to a very remarkable degree, but not so far as to +transgress the limits between the two races. Such transgression may +apparently be met with from time to time, but then the next generation +generally shows the fallacy of the conclusion, as it returns more or +less directly to the type from which the strain had been derived. +Monstrosities should always be studied by physiologists from this point +of view. Poor and rich strains of the same anomaly seem at first sight +to be so nearly allied that it might be thought to be very easy to +change the one into the other. Nevertheless such changes are not on +record, and although I have made several attempts in this line, I never +succeeded in passing the limit. I am quite convinced that sometime [402] +a method will be discovered of arbitrarily producing such conversions, +and perhaps the easiest way to attain artificial mutations may lie +concealed here. But as yet not the slightest indication of this +possibility is to be found, save the fallacious conclusions drawn from +too superficial observations. + +Unfortunately the poor strains are not very interesting. Their chance of +producing beautiful instances of the anomaly for which they are +cultivated is too small. Exceptions to this rule are only afforded by +those curious and rare anomalies, which command general attention, and +of which, therefore, instances are always welcome. In such cases they +are searched for with perseverance, and the fact of their rarity +impresses itself strongly on our mind. + +Twisted stems are selected as a first example. This monstrosity, called +_biastrepsis_, consists of strongly marked torsions as are seen in many +species with decussate leaves, though as a rule it is very rare. Two +instances are the most generally known, those of the wild valerian +(_Valeriana officinalis_) and those of cultivated and wild sorts of +teasels (_Dipsacus fullonum_, _D. sylvestris_, and others). Both of +these I have cultivated during upwards of fifteen years, but with +contradictory results. The valerian is a perennial herb, multiplying +itself yearly by [403] slender rootstocks or runners producing at their +tips new rosettes of leaves and in the center of these the flowering +stem. My original plant has since been propagated in this manner, and +during several years I preserved large beds with hundreds of stems, in +others I was compelled to keep my culture within more restricted limits. +This plant has produced twisted stems of the curious shape, with a +nearly straight flag of leaves on one side, described by De Candolle and +other observers, nearly every year. But only one or two instances of +abnormal stems occurred in each year, and no treatment has been found +that proved adequate to increase this number in any appreciable manner. +I have sown the seeds of this plant repeatedly, either from normal or +from twisted stems, but without better results. It was highly desirable +to be able to offer instances of this rare and interesting peculiarity +to other universities and museums, but no improvement of the race could +be reached and I have been constrained to give it up. My twisted +valerian is a poor race, and hardly anything can be done with it. +Perhaps, in other countries the corresponding rich race may be hidden +somewhere, but I have never had the good fortune of finding it. + +This good fortune however, I did have with the wild teasel or _Dipsacus +sylvestris_. [404] Stems of this and of allied species are often met +with and have been described by several writers, but they were always +considered as accidents and nobody had ever tried to cultivate them. In +the summer of 1885 I saw among a lot of normal wild teasels, two nicely +twisted stems in the botanical garden of Amsterdam. I at once proposed +to ascertain whether they would yield a hereditary race and had all the +normal individuals thrown away before the flowering time. My two plants +flowered in this isolated condition and were richly pollinated by +insects. Of course, at that time, I knew nothing of the dependency of +monstrosities on external conditions, and made the mistake of sowing the +seeds and cultivating the next generation in too great numbers on a +small space. But nevertheless the anomaly was repeated, and the aberrant +individuals were once more isolated before flowering. The third +generation repeated the second, but produced sixty twisted stems on some +1,600 individuals. The result was very striking and quite sufficient for +all further researches, but the normal condition of the race was not +reached. This was the case only after I had discovered the bad effects +of growing too many plants in a limited space. In the fourth generation +I restricted my whole culture to about 100 individuals, and by this +simple [405] means at once got up to 34% of twisted stems. This +proportion has since remained practically the same. I have selected and +isolated my plants during five succeeding generations, but without any +further result, the percentage of twisted stems fluctuating between 30 +and about 45 according to the size of the cultures and the favorableness +or unfavorableness of the weather. + +It is very interesting to note that all depends on the question whether +one has the good fortune of finding a rich race or not, as this +pedigree-culture shows. Afterwards everything depends on treatment and +very little on selection. As soon as the treatment becomes adequate, the +full strength of the race at once displays itself, but afterwards no +selection is able to improve it to any appreciable amount. Of course, in +the long run, the responses will be the same as those of the pistilloid +poppies on the average, and some influence of selection will show itself +on closer scrutiny. + +Compared with the polycephalous poppies my race of twisted teasels is +much richer in atavists. They are never absent, and always constitute a +large part of each generation and each bed, comprising somewhat more +than half of the individuals. Intermediate stages between them and the +wholly twisted stems are not wanting, [406] and a whole series of steps +may easily be observed from sufficiently large cultures. But they are +always relatively rare, and any lot of plants conveys the idea of a +dimorphous race, the small twisted stems contrasting strongly with the +tall straight ones. + +A sharper contrast between good representatives of a race and their +atavists is perhaps to be seen in no other instance. All the details +contribute to the differentiation in appearance. The whole stature of +the plants is affected by the varietal mark. The atavists are not, as in +the case of the poppies, obviously allied with the type by a full range +of intermediate steps, but quite distant from it by their rarity. There +seems to be a gap in the same way as between the striped flowers of the +snapdragon and their uniform red atavists, while with the poppies the +atavists may be viewed as being only the extremes of a series of +variations fluctuating around some average type. + +From this reason it is as interesting to appreciate the hereditary +position of the atavists of twisted varieties as it was for the +red-flowered descendants of the striped flowers. In order to ascertain +this relation it is only necessary to isolate some of them during the +blooming-period. I made this experiment in the summer of 1900 with the +eighth generation of my race, and contrived [407] to isolate three +groups of plants by the use of parchment bags, covering them +alternately, so the flowers of only one group were accessible to +insects, at a time. I made three groups, because the atavists show two +different types. Some specimens have decussate stems, others bear all +their leaves in whorls of three, but in respect to the hereditary +tendency of the twisting character this difference does not seem to be +of any importance. + +In this way I got three lots of seeds and sowed enough of them to have +three groups of plants each containing about 150-200 well developed +stems. Among these I counted the twisted individuals, and found nearly +the same numbers for all three. The twisted parents gave as many as 41% +twisted children, but the decussate atavists gave even somewhat more, +viz., 44%, while the ternate specimens gave 37%. Obviously the +divergences between these figures are too slight to be dwelt upon, but +the fact that the atavists are as true or nearly as true inheritors of +the twisted race as the best selected individuals is clearly proved by +this experience. + +It is evident that here we have a double race, including two types, +which may be combined in different degrees. These combinations determine +a wide range of changes in the stature of the plants, and it seems +hardly right to use the [408] same term for such changes as for common +variations. It is more a contention of opposite characters than a true +phenomenon of simple variability. Or perhaps we might say that it is the +effect of the cooperation of a very variable mark, the twisting, with a +scarcely varying attribute of the normal structure of the stem. Between +the two types an endless diversity prevails, but outwardly there are +limits which are never transgressed. The double race is as permanent, +and in this sense as constant, as any ordinary simple variety, both in +external form, and in its intimate hereditary qualities. + +I have succeeded in discovering some other rich races of twisted plants. +One of them is the Sweet William (_Dianthus barbatus_), which yielded, +after isolation, in the second generation, 25% of individuals with +twisted stems, and as each individual produces often 10 and more stems, +I had a harvest of more than half a thousand of instances of this +curious, and ordinarily very rare anomaly. My other race is a twisted +variety of _Viscaria oculata_, which is still in cultivation, as it has +the very consistent quality of being an annual. It yielded last summer +(1903) as high a percentage as 65 of twisted individuals, many of them +repeating the monstrosity on several branches. After some occasional +observations _Gypsophila paniculata_ [409] seems to promise similar +results. On the other hand I have sowed in vain the seeds of twisted +specimens of the soapwort and the cleavewort (_Saponaria officinalis_ +and _Galium Aparine_). These and some others seem to belong to the same +group as the valerian and to constitute only poor or so-called +half-races. + +Next to the torsions come the fasciated stems. This is one of the most +common of all malformations, and consists, in its ordinary form, of a +flat ribbon-like expansion of the stems or branches. Below they are +cylindrical, but they gradually lose this form and assume a flattened +condition. Sometimes the rate of growth is unequal on different portions +or on the opposite sides of the ribbon, and curvatures are produced and +these often give to the fasciation a form that might be compared with a +shepherd's crook. It is a common thing for fasciated branches and stems +to divide at the summit into a number of subdivisions, and ordinarily +this splitting occurs in the lower part, sometimes dividing the entire +fasciated portion. In biennial species the rosette of the root-leaves of +the first year may become changed by the monstrosity, the heart +stretching in a transverse direction so as to become linear. In the next +year this line becomes the base from which the stem grows. In such cases +the fasciated stems [410] are broadened and flattened from the very +beginning, and often retain the incipient breadth throughout their +further development. Species of primroses (_Primula japonica_ and +others), of buttercups (_Ranunculus bulbosus_), the rough hawksbeard +(_Crepis biennis_), the Aster _Tripolium_, and many others could be +given as instances. + +Some of these are so rare as to be considered as poor races, and in +cultural trials do not produce the anomaly except in a very few +instances. Heads of rye are found in a cleft condition from time to +time, single at their base and double at the top, but this anomaly is +only exceptionally repeated from seed. Flattened stems of _Rubia +tinctorum_ are not unfrequently met with on the fields, but they seem to +have as little hereditary tendency as the split rye (_Secale Cereale_). +Many other instances could be given. Both in the native localities and +in pedigree-cultures such ribboned stems are only seen from time to +time, in successive years, in annual and biennial as well as in +perennial species. The purple pedicularis (_Pedicularis palustris_) in +the wild state, and the sunflower among cultivated plants, may be cited +instead of giving a long list of analogous instances. + +On the other hand rich races of flattened stems are not entirely +lacking. They easily betray [411] themselves by the frequency of the +anomaly, and therefore may be found, and tried in the garden. Under +adequate cultivation they are here as rich in aberrant individuals as +the twisted races quoted above, producing in good years from 30-40% and +often more instances. I have cultivated such rich races of the dandelion +(_Taraxacum officinale_), of _Thrincia hirta_, of the dame's violet +(_Hesperis matronalis_), of the hawkweed (_Picris hieracioides_), of the +rough hawksbeard (_Crepis biennis_), and others. + +Respecting the hereditary tendencies these rich varieties with flattened +stems may be put in the same category with the twisted races. Two points +however, seem to be of especial interest and to deserve a separate +treatment. + +The common cockscomb or _Celosia cristata_, one of the oldest and most +widely cultivated fasciated varieties may be used to illustrate the +first point. In beds it is often to be seen in quite uniform lots of +large and beautiful crests, but this uniformity is only secured by +careful culture and selection of the best individuals. In experimental +trials such selection must be avoided, and in doing so a wide range of +variability at once shows itself. Tall, branched stems with fan-shaped +tops arise, constituting a series of steps towards complete atavism. +This last [412] however, is not to be reached easily. It often requires +several successive generations grown from seed collected from the most +atavistic specimens. And even such selected strains are always reverting +to the crested type. There is no transgression, no springing over into a +purely atavistic form, such as may be supposed to have once been the +ancestor of the present cockscomb. The variety includes crests and +atavists, and may be perpetuated from both. Obviously every gardener +would select the seeds of the brightest crests, but with care the full +crests may be recovered, even from the worst reversionists in two or +three generations. It is a double race of quite the same constitution as +the twisted teasels. + +My second point is a direct proof of this assertion, but made with a +fasciated variety of a wild species. I took for my experiment the rough +hawksbeard. In the summer of 1895 I isolated some atavists of the fifth +generation of my race, which, by ordinary selection, gave in the average +from 20-40% of fasciated stems. My isolated atavists bore abundant +fruit, and from these I had the next year a set of some 350 plants, out +of which about 20% had broadened and linear rosettes. This proportion +corresponds with the degree of inheritance which is shown in many years +by the largest and strongest [413] fasciated stems. It strengthens our +conclusion as to the innermost constitution of the double races or +ever-sporting varieties. + +Twisted stems and fasciations are very striking monstrosities. But they +are not very good for further investigation. They require too much space +and too much care. The calculation of a single percentage requires the +counting of some hundreds of individuals, taking many square meters for +their cultivation, and this, as my best races are biennial, during two +years. For this reason the countings must always be very limited, and +selection is restrained to the most perfect specimens. + +Now the question arises, whether this mark is the best upon which to +found selection. This seems to be quite doubtful. In the experiments on +the heredity of the atavists, we have seen that they are, at least +often, in no manner inferior to even the best inheritors of the race. +This suggests the idea that it is not at all certain that the visible +characters of a given individual are a trustworthy measure of its value +as to the transmission of the same character to the offspring. In other +words, we are confronted with the existence of two widely different +groups of characters in estimating the hereditary tendency. One is the +visible quality of the individuals and the other is the direct +observation [414] of the degree in which the attribute is transmitted. +These are by no means parallel, and seem in some sense to be nearly +independent of each other. The fact that the worst atavists may have the +highest percentage of varietal units seems to leave no room for another +explanation. + +Developing this line of thought, we gradually arrive at the conclusion +that the visible attribute of a varying individual is perhaps the most +untrustworthy and the most unreliable character for selection, even if +it seems in many cases practically to be the only available one. The +direct determination of the degree of heredity itself is obviously +preferable by far. This degree is expressed by the proportion of its +inheritors among the offspring, and this figure therefore should be +elevated to the highest rank, as a measure of the hereditary qualities. +Henceforward we will designate it by the name of hereditary percentage. + +In scientific experiments this figure must be determined for every plant +of a pedigree-culture singly, and the selection should be founded +exclusively or at least mainly on it. It is easily seen that this method +requires large numbers of individuals to be grown and counted. Some two +or three hundred progeny of one plant are needed to give the decisive +figure for this one [415] individual, and selection requires the +comparison of at least fifty or more individuals. This brings the total +amount of specimens to be counted up to some tens of thousands. In +practice, where important interests depend upon the experiments, such +numbers are usually employed and often exceeded, but for the culture of +monstrosities, other methods are to be sought in order to avoid these +difficulties. + +The idea suggests itself here that the younger the plants are, when +showing their distinguishing marks, the more of them may be grown on a +small space. Hence the best way is to choose such attributes, as may +already be seen in the young seedlings, in the very first few weeks of +their lives. Fortunately the seed-leaves themselves afford such +distinctive marks, and by this means the plants may be counted in the +pans, requiring no culture at all in the garden. Only the selected +individuals need be grown to ripen their seeds, and the whole selection +may be made in the spring, in the glasshouse. Instead of being very +troublesome, the determination of the hereditary percentages becomes a +definite reduction of the size of the experiments. Moreover it may +easily be effected by any one who cares for experimental studies, but +has not the means required for cultures on a larger scale. And lastly, +there are [416] a number of questions about heredity, periodicity, +dependency on nourishment and other life conditions, and even about +hybridizing, which may be answered by this new method. + +Seed-leaves show many deviations from the ordinary shape, especially in +dicotyledonous plants. A very common aberration is the multiplication of +their number, and three seed-leaves in a whorl are not rarely met with. +The whorl may even consist of four, and in rare cases of five or more +cotyledons. Cleft cotyledons are also to be met with, and the fissure +may extend varying distances from the tips. Often all these deviations +may be seen among the seedlings of one lot, and then it is obvious that +together they constitute a scale of cleavages, the ternate and +quaternate whorls being only cases where the cleaving has reached its +greatest development. All in all it is manifest that here we are met by +one type of monstrosity, but that this type allows of a wide range of +fluctuating variability. For brevity's sake all these cleft and ternate, +double cleft and quaternate cotyledons and even the higher grades are +combined under one common name and indicated as tricotyls. + +A second aberration of young seed-plants is exactly opposite to this. It +consists of the union of the two seed-leaves into a single organ. This +ordinarily betrays its origin by [417] having two separate apices, but +not always. Such seedlings are called syncotyledonous or syncotyls. +Other monstrosities have been observed from time to time, but need not +be mentioned here. + +It is evident that the determination of the hereditary percentage is +very easy in tricotylous or syncotylous cultures. The parent plants must +be carefully isolated while blooming. Many species pollinate themselves +in the absence of bees; from these the insects are to be excluded. +Others have the stamens and stigmas widely separated and have to be +pollinated artificially. Still others do not lend themselves to such +operations, but have to be left free to the visits of bees and of +humble-bees, this being the only means of securing seed from every +plant. At the time of the harvest the seeds should be gathered +separately from each plant, and this precaution should also be observed +in studies of the hereditary percentage at large, and in all scientific +pedigree-cultures. Every lot of seeds is to be sown in a separate pan, +and care must be taken to sow such quantities the three to four hundred +seedlings will arise from each. As soon as they display their +cotyledons, they are counted, and the number is the criterion of the +parent-plant. Only parent-plants with the highest percentages are +selected, and out of [418] their seedlings some fifty or a hundred of +the best ones are chosen to furnish the seeds for the next generation. + +This description of the method shows that the selection is a double one. +The first feature is the hereditary percentage. But then not all the +seedlings of the selected parents can be planted out, and a choice has +to be made. This second selection may favor the finest tricotyls, or the +strongest individuals, or rely on some other character, but is +unavoidable. + +We now come to the description of the cultures. Starting points are the +stray tricotyls which are occasionally found in ordinary sowings. In +order to increase the chance of finding them, thousands of seeds of the +same species must be inspected, and the range of species must be widened +as much as possible. + +Material for beginning such experiments is easily obtained, and almost +any large sample of seeds will be found suitable. Some tricotyls will be +found among every thousand seedlings in many species, while in others +ten or a hundred times, as many plants must be examined to secure them, +but species with absolutely pure dicotylous seeds are very rare. + +The second phase of the experiment, however, is not so promising. Some +species are rich, and others are poor in this anomaly. This difference +[419] often indicates what can be expected from further culture. Stray +tricotyls point to poor species or half-races, while more frequent +deviations suggest rich or double-races. In both cases however, the +trial must be made, and this requires the isolation of the aberrant +individuals and the determination of their hereditary percentage. + +In some instances the degree of their inheritance is only a very small +one. The isolated tricotyls yield 1 or 2% of inheritors, in some cases +even less, or upwards up to 3 or 4%. If the experiment is repeated, no +amelioration is observed, and this result remains the same during a +series of successive generations. In the case of _Polygonum +convolvulus_, the Black bindweed, I have tried as many as six +generations without ever obtaining more than 3%. With other species I +have limited myself to four successive years with the same negative +result, as with spinage, the Moldavian dragon-head, (_Dracocephalum +moldavicum_), and two species of corn catch-fly (_Silene conica_ and _S. +conoidea_). + +Such poor races hardly afford a desirable material for further +inquiries. Happily the rich races, though rare, may be discovered also +from time to time. They seem to be more common among cultivated plants +and horticultural as well as agricultural species may be used. Hemp +[420] and mercury (_Mercurialis annua_) among the first, snapdragon, +poppies, _Phacelia_, _Helichrysum_, and _Clarkia_ among garden-flowers +may be given as instances of species containing the rich tricotylous +double races. + +It is very interesting to note how strong the difference is between such +cases and those which only yield poor races. The rich type at once +betrays itself. No repeated selection is required. The stray tricotyls +themselves, that are sought out from among the original samples, give +hereditary percentages of a much higher type after isolation than those +quoted above. They come up to 10-20% and in some cases even to 40%. As +may be expected, individual differences occur, and it must even be +supposed that some of the original tricotyls may not be pure, but +hybrids between tricotylous and dicotylous parents. These are at once +eliminated by selection, and if only the tricotyls which have the +highest percentages are chosen for the continuance of the new race, the +second generation comes up with equal numbers of dicotyls and tricotyls +among the seedlings. The figures have been observed to range from 51-58% +in the majority of the cases, and average 55%, rarely diverging somewhat +more from this average. + +Here we have the true type of an ever-sporting variety. Every year it +produces in the [421] same way heirs and atavists. Every plant, if +fertilized with its own pollen, gives rise to both types. The parent +itself may be tricotylous or dicotylous, or show any amount of +multiplication and cleavage in its seed-leaves, but it always gives the +entire range among its progeny of the variation. One may even select the +atavists, pollinate them purely and repeat this in a succeeding +generation without any chance of changing the result. On an average the +atavists may give lower hereditary figures, but the difference will be +only slight. + +Such tricotylous double races offer highly interesting material for +inquiries into questions of heredity, as they have such a wide range of +variability. There is little danger in asserting that they go upwards to +nearly 100%, and downwards to 0%, diverging symmetrically on both sides +of their average (50-55%). These limits they obviously cannot +transgress, and are not even able to reach them. Samples of seed +consisting only of tricotyls are very rare, and when they are met with +the presumption is that they are too few to betray the rare aberrants +they might otherwise contain. Experimental evidence can only be reached +by the culture of a succeeding generation, and this always discloses the +hidden qualities, showing that the double [422] type was only +temporarily lost, but bound to return as soon as new trials are made. + +This wide range of variability between definite limits is coupled with a +high degree of sensibility and adequateness to the most diverging +experiments. Our tricotylous double races are perhaps more sensitive to +selection than any other variety, and equally dependent on outer +circumstances. Here, however, I will limit myself to a discussion of the +former point. + +In the second generation after the isolation of stray tricotylous +seedlings the average condition of the race is usually reached, but only +by some of the strongest individuals, and if we continue the race, +sowing or planting only from their offspring, the next generation will +show the ordinary type of variability, going upwards in some and +downwards in other instances. With the _Phacelia_ and the mercury and +some others I had the good luck in this one generation to reach as high +as nearly 90% of tricotylous seedlings, a figure indicating that the +normal dicotylous type had already become rare in the race. In other +cases 80% or nearly 80% was easily attained. Any further divergence from +the average would have required very much larger sowings, the effect of +selection between a limited number of parents being only to retain the +high degree once [423] reached; so for instance with the mercury, I had +three succeeding generations of selection after reaching the average of +55%, but their extremes gave no increasing advance, remaining at 86, 92 +and 91%. + +If we compare these results with the effects of selection in twisted and +fasciated races, we observe a marked contrast. Here they reached their +height at 30-40%, and no number of generations had the power of making +any further improvement. The tricotyls come up in two generations to a +proportion of about 54%, which shows itself to correspond to the average +type. And as soon as this is reached, only one generation is required to +obtain a very considerable improvement, going up to 80 or even 90%. + +It is evident that the cause of this difference does not lie in the +nature of the monstrosity, but is due to the criterion upon which the +selection is made. Selection of the apparently best individuals is one +method, and it gives admirable results. Selection on the ground of the +hereditary percentages is another method and gives results which are far +more advantageous than the former. + +In the lecture on the pistillody of the poppies we limited ourselves to +the selection of the finest individuals and showed that there is always +a manifest correlation between the individual [424] strength of the +plant and the degree of development of its anomaly. The same holds good +with other monstrosities, and badly nourished specimens of rich races +with twisted or fasciated stems always tend to reversion. This +reversion, however, is not necessarily correlated with the hereditary +percentage and therefore does not always indicate a lessening of the +degree of inheritance. This shows that even in those cases an +improvement may be expected, if only the means can be found to subject +the twisted and the fasciated races to the same sharp test as the +tricotylous varieties. + +Much remains to be done, and the principle of the selection of parents +according to the average constitution of their progeny seems to be one +of the most promising in the whole realm of variability. + +Besides tricotylous, the syncotylous seedlings may be used in the same +way. They are more rarely met with, and in most instances seem to belong +only to the unpromising half-races. The black bindweed (_Polygonum +Convolvulus_), the jointed charlock (Raphanus Raphanistrum), the +glaucous evening-primrose (_Oenothera glauca_) and many other plants +seem to contain such half-races. On the other hand I found a plant of +_Centranthus macrosiphon_ yielding as much as 55% of syncotylous +children [425] and thereby evidently betraying the nature of a rich or +double race. Likewise the mercury was rich in such deviations. But the +best of all was the Russian sunflower, and this was chosen for closer +experiments. + +In the year of 1888 I had the good luck to isolate some syncotylous +seedlings and of finding among them one with 19% of inheritors among its +seeds. The following generation at once surpassed the ordinary average +and came up in three individuals to 76, 81 and even 89%. My race was at +once isolated and ameliorated by selection. I have tried to improve it +further and selected the parents with the highest percentages during +seven more generations; but without any remarkable result. I got figures +of 90% and above, coming even in one instance up to the apparent purity +of 100%. These, however, always remained extremes, the averages +fluctuating yearly between 80-90% or thereabouts, and the other extremes +going nearly every year downwards to 50%, the value which would be +attained, if no selection were made. + +Contra-selection is as easily made as normal selection. According to our +present principle it means the choice of the parents with the smallest +hereditary percentage. One might easily imagine that by this means the +dicotylous seedlings could be rendered pure. This, however, [426] is not +at all the case. It is easy to return from so highly selected figures as +for instance 95% to the average about of 50%, as regression to +mediocrity is always an easy matter. But to transgress this average on +the lower side seems to be as difficult as it is on the upper side. I +continued the experiment during four succeeding generations, but was not +able to go lower than about 10%, and could not even exclude the high +figures from my strain. Parents with 65-75% of syncotylous seedlings +returned in each generation, notwithstanding the most careful +contra-selection. The attribute is inherent in the race, and is not to +be eliminated by so simple a means as selection, nor even by a selection +on the ground of hereditary percentages. + +We have dealt with torsions and fasciations and with seedling variations +at some length, in order to point out the phases needing investigation +according to recent views. It would be quite superfluous to consider +other anomalies in a similar manner, as they all obey the same laws. A +hasty survey may suffice to show what prospects they offer to the +student of nature. + +First of all come the variegated leaves. They are perhaps the most +variable of all variations. They are evidently dependent on external +circumstances, and by adequate nutrition the leaves may even become +absolutely white or [427] yellowish, with only scarcely perceptible +traces of green along the veins. Some are very old cultivated varieties, +as the wintercress, or _Barbarea vulgaris_. They continuously sport into +green, or return from this normal color, both by seeds and by buds. +Sports of this kind are very often seen on shrubs or low trees, and they +may remain there and develop during a long series of years. Bud-sports +of variegated holly, elms, chestnuts, beeches and others might be cited. +One-sided variegation on leaves or twigs with the opposite side wholly +green are by no means rare. It is very curious to note that variegation +is perhaps the most universally known anomaly, while its hereditary +tendencies are least known. + +Cristate and plumose ferns are another instance. Half races or rare +accidental cleavages seem to be as common with ferns as cultivated +double races, which are very rich in beautiful crests. But much depends +on cultivation. It seems that the spores of crested leaves are more apt +to reproduce the variety than those of normal leaves, or even of normal +parts of the same leaves. But the experiments on which this assertion is +made are old and should be repeated. Other cases of cleft leaves should +also be tested. Ascidia are far more common than is usually believed. +Rare instances point [428] to poor races, but the magnolias and +lime-trees are often so productive of ascidia as to suggest the idea of +ever-sporting varieties. I have seen many hundred ascidia on one +lime-tree, and far above a hundred on the magnolia. They differ widely +in size and shape, including in some cases two leaves instead of one, or +are composed of only half a leaf or of even still a smaller part of the +summit. Rich ascidia-bearing varieties seem to offer notable +opportunities for scientific pedigree-cultures. + +Union of the neighboring fruits and flowers on flower-heads, of the rays +of the umbellifers or of the successive flowers of the racemes of +cabbages and allied genera, seem to be rare. The same holds good for the +adhesion of foliar to axial organs, of branches to stems and other cases +of union. Many of these cases return regularly in each generation, or +may at least be seen from time to time in the same strains. +Proliferation of the inflorescence is very common and changes in the +position of staminate and pistillate flowers are not rare. We find +starting points for new investigations in almost any teratological +structure. Half-races and double-races are to be distinguished and +isolated in all cases, and their hereditary qualities, the periodicity +of the recurrence of the anomaly, the dependency on external +circumstances [429] and many other questions have to be answered. + +Here is a wide field for garden experiments easily made, which might +ultimately yield much valuable information on many questions of heredity +of universal interest. + + +[430] + +LECTURE XV + +DOUBLE ADAPTATIONS + +The chief object of all experimentation is to obtain explanations of +natural phenomena. Experiments are a repetition of things occurring in +nature with the conditions so guarded and so closely followed that it is +possible to make a clear analysis of facts and their causes, it being +rightfully assumed that the laws are the same in both cases. + +Experiments on heredity and the experience of the breeder find their +analogy in the succession of generations in the wild state. The +stability of elementary species and of retrograde varieties is quite the +same under both conditions. Progression and retrogression are narrowly +linked everywhere, and the same laws govern the abundance of forms in +cultivated and in wild plants. + +Elementary species and retrograde varieties are easily recognizable. +Ever-sporting varieties on the contrary are far less obvious, and in +many cases their hereditary relations have [431] had to be studied anew. +A clear analogy between them and corresponding types of wild plants has +yet to be pointed out. There can be no doubt that such analogy exists; +the conception that they should be limited to cultivated plants is not +probable. Striped flowers and variegated leaves, changes of stamens into +carpels or into petals may be extremely rare in the wild state, but the +"five-leaved" clover and a large number of monstrosities cannot be said +to be typical of the cultivated condition. These, however, are of rare +occurrence, and do not play any important part in the economy of nature. + +In order to attain a better solution of the problem we must take a +broader view of the facts. The wide range of variability of +ever-sporting varieties is due to the presence of two antagonistic +characters which cannot be evolved at the same time and in the same +organ, because they exclude one another. Whenever one is active, the +other must be latent. But latency is not absolute inactivity and may +often only operate to encumber the evolution of the antagonistic +character, and to produce large numbers of lesser grades of its +development. The antagonism however, is not such in the exact meaning of +the word; it is rather a mutual exclusion, because one of the opponents +simply takes the place of the other when absent, or supplements [432] it +to the extent that it may be only imperfectly developed. This completion +ordinarily occurs in all possible degrees and thus causes the wide range +of the variability. Nevertheless it may be wanting, and in the case of +the double stocks only the two extremes are present. + +It is rather difficult to get a clear conception of the substitution, +and it seems necessary to designate the peculiar relationship between +the two characters forming such a pair by a simple name. They might be +termed alternating, if only it were clearly understood that the +alternation may be complete, or incomplete in all degrees. Complete +alternation would result in the extremes, the incomplete condition in +the intermediate states. In some cases as with the stocks, the first +prevails, while in other cases, as with the poppies, the very extremes +are only rarely met with. + +Taking such an alternation as a real character of the ever-sporting +varieties, a wide range of analogous cases is at once revealed among the +normal qualities of wild plants. Alternation is here almost universal. +It is the capacity of young organs to develop in two diverging +directions. The definitive choice must be made in extreme youth, or +often at a relatively late period of development. Once made, this [433] +choice is final, and a further change does not occur in the normal +course of things. + +The most curious and most suggestive instance of such an alternation is +the case of the water-persicaria or _Polygonum amphibium_. It is known +to occur in two forms, one aquatic and the other terrestrial. These are +recorded in systematic works as varieties, and are described under the +names of _P. amphibium_ var. _natans_ Moench, and _P. amphibium_ var. +_terrestre_ Leers or _P. amphibium_ var. _terrestris_ Moench. Such +authorities as Koch in his German flora, and Grenier and Godron in their +French flora agree in the conception of the two forms as varieties. + +Notwithstanding this, the two varieties may often be observed to sport +into one another. They are only branches of the same plant, grown under +different conditions. The aquatic form has floating or submerged stems +with oblong or elliptic leaves, which are glabrous and have long +petioles. The terrestrial plants are erect, nearly simple, more or less +hispid throughout, with lanceolate leaves and short petioles, often +nearly sessile. The aquatic form flowers regularly, producing its +peduncle at right angles from the floating stems, but the terrestrial +specimens are ordinarily seen without flower-spikes, which are but +rarely met with, at least as far as my own experience goes. Intermediate +[434] forms are very rare, perhaps wholly wanting, though in swamps the +terrestrial plants may often vary widely in the direction of the +floating type. + +That both types sport into each other has long been recognized in +field-observations, and has been the ground for the specific name of +_amphibium_, though in this respect herbarium material seems usually to +be scant. The matter has recently been subjected to critical and +experimental studies by the Belgian botanist Massart, who has shown that +by transplanting the forms into the alternate conditions, the change may +always be brought about artificially. If floating plants are established +on the shore they make ascending hairy stems, and if the terrestrial +shoots are submerged, their buds grow into long and slack, aquatic +stems. Even in such experiments, intermediates are rare, both types +agreeing completely with the corresponding models in the wild state. + +Among all the previously described cases of horticultural plants and +monstrosities there is no clearer case of an ever-sporting variety than +this one of the water-persicaria. The var. _terrestris_ sports into the +var. _natans_, and as often as the changing life conditions may require +it. It is-true that ordinary sports occur without our discerning the +cause and without [435] any relation to adaptation. This however is +partly due to our lack of knowledge, and partly to the general rule that +in nature only such sports as are useful are spared by natural +selection, and what is useful we ordinarily term adaptive. + +Another side of the question remains to be considered. The word variety, +as is now becoming generally recognized, has no special meaning +whatever. But here it is assumed in the clearly defined sense of a +systematic variety, which includes all subdivisions of species. Such +subdivisions may be, from a biological point of view, elementary species +and also be eversporting varieties. They may be retrograde varieties, +and the two alternating types may be described as separate varieties. + +It is readily granted that many writers would not willingly accept this +conclusion. But it is simply impossible to avoid it. The two forms of +the water-persicaria must remain varieties, though they are only types +of the different branches of a single plant. + +If not, hundreds and perhaps thousands of analogous cases are at once +exposed to doubt, and the whole conception of systematic varieties would +have to be thrown over. Biologists of course would have no objection to +this, but the student of the flora of any given country [436] or region +requires the systematic subdivisions and should always use his utmost +efforts to keep them as they are. There is no intrinsic difficulty in +the statement that different parts of the same plant should constitute +different varieties. + +In some cases different branches of the same plant have been described +as species. So for instance with the climbing forms of figs. Under the +name of _Ficus repens_ a fine little plant is quite commonly cultivated +as a climber in flower baskets. It is never seen bearing figs. On the +other hand a shrub of our hothouses called _Ficus stipulata_, is +cultivated in pots and makes a small tree which produces quite large, +though non-edible figs. Now these two species are simply branches of the +same plant. If the _repens_ is allowed to climb up high along the walls +of the hothouses, it will at last produce stipulate branches with the +corresponding fruits. _Ficus radicans_ is another climbing form, +corresponding to the shrub _Ficus ulmifolia_ of our glasshouses. And +quite the same thing occurs with ivy, the climbing stems of which never +flower, but always first produce erect and free branches with rhombic +leaves. These branches have often been used as cuttings and yield little +erect and richly flowering shrubs, which are known in [437] horticulture +under the varietal name of _Hedera Helix arborea_. + +Manifestly this classification is as nearly right as that of the two +varieties of the water-persicaria. Going one step further, we meet with +the very interesting case of alpine plants. The vegetation of the higher +regions of mountains is commonly called alpine, and the plants show a +large number of common features, differentiating them from the flora of +lower stations. The mountain plants have small and dense foliage, with +large and brightly-colored flowers. The corresponding forms of the +lowlands have longer and weaker stems, bearing their leaves at greater +distances, the leaves themselves being more numerous. The alpine forms, +if perennial, have thick, strongly developed and densely branched +rootstocks with heavy roots, in which a large amount of food material is +stored up during the short summer, and is available during the long +winter months of the year. + +Some species are peculiar to such high altitudes, while many forms from +the lowlands have no corresponding type on the mountains. But a large +number of species are common to both regions, and here the difference of +course is most striking. _Lotus corniculatus_ and _Calamintha Acinos_, +_Calluna vulgaris_ and _Campanula_ [438] _rotundifolia_ may be quoted as +instances, and every botanist who has visited alpine regions may add +other examples. Even the edelweiss of the Swiss Alps, _Gnaphalium +Leontopodium_, loses its alpine characters, if cultivated in lowland +gardens. Between such lowland and alpine forms intermediates regularly +occur. They may be met with whenever the range of the species extends +from the plains upward to the limit of eternal snow. + +In this case the systematists formerly enumerated the alpine plants as +_forma alpestris_, but whenever the intermediate is lacking the term +_Varietas alpestris_ was often made use of. + +It is simply impossible to decide concerning the real relation between +the alpine and lowland types without experiments. About the middle of +the last century it was quite a common thing to collect plants not only +for herbarium-material, but also for the purpose of planting them in +gardens and thus to observe their behavior under new conditions. This +was done with the acknowledged purpose of investigating the systematic +significance of observed divergencies. Whenever these held good in the +garden they were considered to be reliable, but if they disappeared they +were regarded as the results of climatic conditions, or of the influence +of soil or nourishment. Between [439] these two alternatives, many +writers have tried to decide, by transplanting their specimens after +some time in the garden, into arid or sandy soil, in order to see +whether they would resume their alpine character. + +Among the systematists who tested plants in this way, Nageli especially, +directed his attention to the hawkweeds or _Hieracium_. On the Swiss +Alps they are very small and exhibit all the characters of the pure +alpine type. Thousands of single plants were cultivated by him in the +botanical garden of Munich, partly from seed and partly from introduced +rootstocks. Here they at once assumed the tall stature of lowland forms. +The identical individual, which formerly bore small rosettes of basal +leaves, with short and unbranched flower-stalks, became richly leaved +and often produced quite a profusion of flower-heads on branched stems. +If then they were transplanted to arid sand, though remaining in the +same garden and also under the same climatic conditions they resumed +their alpine characters. This proved nutrition to be the cause of the +change and not the climate. + +The latest and most exact researches on this subject are due to Bonnier, +who has gone into all the details of the morphologic as well as of the +physiologic side of the problem. [440] His purpose was the study of +partial variability under the influence of climate and soil. In every +experiment he started from a single individual, divided it into two +parts and planted one half on a mountain and the other half on the +plain. The garden cultures were made chiefly at Paris and Fontainebleau, +the alpine cultures partly in the Alps, partly in the Pyrenees. From +time to time the halved plants were compared with each other, and the +cultures lasted, as a rule, during the lifetime of the individual, often +covering many years. + +The common European frostweed or _Helianthemum vulgare_ will serve to +illustrate his results. A large plant growing in the Pyrenees in an +altitude of 2,400 meters was divided. One half was replanted on the same +spot, and the other near Cadeac, at the base of the mountain range (740 +M.). In order to exclude the effect of a change of soil, a quantity of +the earth from the original locality was brought into the garden and the +plant put therein. Further control experiments were made at Paris. As +soon as the two halved individuals commenced to grow and produced new +shoots, the influence of the different climates made itself felt. On the +mountain, the underground portions remained strong and dense, the leaves +and internodes small and hairy, the flowering stems nearly [441] +procumbent, the flowers being large and of a deep yellow. At Cadeac and +at Paris the whole plant changed at once, the shoots becoming elongated +and loose, with broad and flattened, rather smooth leaves and numerous +pale-hued flowers. The anatomical structure exhibited corresponding +differences, the intercellular spaces being small in the alpine plant +and large in the one grown in the lowlands, the wood-tissues strong in +the first and weak in the second case. + +The milfoil (_Achillea Millefolium_) served as a second example, and the +experiments were carried on in the same localities. The long and thick +rootstocks of the alpine plant bearing short stems only with a few dense +corymbs contrasted markedly with the slender stems, loose foliage and +rich groups of flowerheads of the lowland plant. The same differences, +in inner and outer structures were observed in numerous instances, +showing that the alpine type in these cases is dependent on the climate, +and that the capacity for assuming the antagonistic characters is +present in every individual of the species. The external conditions +decide which of them becomes active and which remains inactive, and the +case seems to be exactly parallel to that of the water-persicaria. + +In the experiments of Bonnier the influence of the soil was, as a rule, +excluded by transplanting [442] part of the original earth with the +transplanted half of the plant. From this he concluded that the observed +changes were due to the inequality of the climate. This involved three +main factors, light, moisture and temperature. On the mountains the +light is more intense, the air drier and cooler. Control-experiments +were made on the mountains, depriving the plants of part of the light. +In various ways they were more or less shaded, and as a rule responded +to this treatment in the same way as to transplantation to the plain +below. Bonnier concluded that, though more than one factor takes part in +inciting the morphologic changes, light is to be considered as the chief +agency. The response is to be considered as a useful one, as the whole +structure of the alpine varieties is fitted to produce a large amount of +organic material in a short time, which enables the plants to thrive +during the short summers and long winters of their elevated stations. + +In connection with these studies on the influences of alpine climates, +Bonnier has investigated the internal structure of arctic plants, and +made a series of experiments on growth in continuous electric light. The +arctic climate is cold, but wet, and the structure of the leaves is +correspondingly loose, though the plants become [443] as small as on the +Alps. Continuous electric light had very curious effects; the plants +became etiolated, as if growing in darkness, with the exception that +they assumed a deep green tinge. They showed more analogy with the +arctic than with the alpine type. + +The influence of the soil often produces changes similar to that of +climate. This was shown by the above cited experiments of Nageli with +the hawkweeds, and may easily be controlled in other cases. The +ground-honeysuckle or _Lotus corniculatus_ grows in Holland partly on +the dry and sandy soil of the dunes, and occasionally in meadows. It is +small and dense in the first case, with orange and often very darkly +colored petals, while it is loose and green in the meadows, with +yellower flowers. Numerous analogous cases might be given. On mountain +slopes in South Africa, and especially in Natal, a species of composite +is found, which has been introduced into culture and is used as a +hanging plant. It is called _Othonna crassifolia_ and has fleshy, nearly +cylindrical leaves, and exactly mimics some of the crassulaceous +species. On dry soil the leaves become shorter and thicker and assume a +reddish tinge, the stems remain short and woody and bear their leaves in +dense rosettes. On moist and rich garden-soil this aspect becomes [444] +changed at once, the stems grow longer and of a deeper green. +Intermediates occur, but notwithstanding this the two extremes +constitute clearly antagonistic types. + +The flora of the deserts is known to exhibit a similar divergent type. +Or rather two types, one adapted to paucity of water, and the other to a +storage of fluid at one season in order to make use of it at other +times, as is the case with the cactuses. Limiting ourselves to the +alternate group, we observe a rich and dense branching, small and +compact leaves and extraordinarily long roots. Here the analogy with the +alpine varieties is manifest, and the dryness of the soil evidently +affects the plants in a similar way, as do the conditions of life in +alpine regions. The question at once comes up as to whether here too we +have only instances of partial variability, and whether many of the +typical desert-species would lose their peculiar character by +cultivation under ordinary conditions. The varieties of _Monardella +macrantha_, described by Hall, from the San Jacinto Mountain, Cal., are +suggestive of such an intimate analogy with the cases studied by +Bonnier, that it seems probable that they might yield similar results, +if tested by the same method. + +Leaving now the description of these special [445] cases, we may resume +our theoretical discussion of the subject, and try to get a clearer +insight into the analogy of ever-sporting varieties and the wild species +quoted. All of them may be characterized by the general term of +dimorphism. Two types are always present, though not in the same +individual or in the same organ. They exclude one another, and during +their juvenile stage a decision is taken in one direction or in the +other. Now, according to the theory of natural selection, wild species +can only retain useful or at least innocuous qualities, since all +mutations in a wrong direction must perish sooner or later. Cultivated +species on the other hand are known to be largely endowed with +qualities, which would be detrimental in the wild condition. +Monstrosities are equally injurious and could not hold their own if left +to themselves. + +These same principles may be applied to ever-sporting or antagonistic +pairs of characters. According to the theory of mutations such pairs may +be either useful or useless. But only the useful will stand further +test, and if they find suitable conditions will become specific or +varietal characters. On this conclusion it becomes at once clear, why +natural dimorphism is, as a rule, a very useful quality, while the +cultivated dimorphous varieties [446] strike us as something unnatural. +The relation between cause and effect, is in truth other than it might +seem to be at first view, but nevertheless it exists, and is of the +highest importance. + +From this same conclusion we may further deduce some explanation of the +hereditary races characterized by monstrosities. It is quite evident +that the twisted teasels are inadequate for the struggle with their tall +congeners, or with the surrounding plants. Hence the conclusion that a +pure and exclusively twisted race would soon die out. The fact that such +races are not in existence finds its explanation in this circumstance, +and therefore it does not prove the impossibility or even the +improbability that some time a pure twisted race might arise. If chance +should put such an accidental race in the hands of an experimenter, it +could be protected and preserved, and having no straight atavistic +branches, but being twisted in all its organs, might yield the most +curious conceivable monstrosity, surpassing even the celebrated dwarf +twisted shrubs of Japanese horticulturists. + +Such varieties however, do not exist at present. The ordinary twisted +races on the other hand, are found in the wild state and have only to be +isolated and cultivated to yield large numbers [447] of twisted +individuals. In nature they are able to maintain themselves during long +centuries, quite as well as normal species and varieties. But they owe +this quality entirely to their dimorphous character. A twisted race of +teasels might consist of successive generations of tall atavistic +individuals, and produce yearly some twisted specimens, which might be +destroyed every time before ripening their seeds. Reasoning from the +evidence available, and from analogous cases, the variety would, even +under such extreme circumstances, be able to last as long as any other +good variety or elementary species. And it seems to me that this +explanation makes clear how it is possible that varieties, which are +potentially rich in their peculiar monstrosity, are discovered from time +to time among plants when tested by experimental methods. + +Granting these conclusions, monstrosities on the one side, and +dimorphous wild species on the other, constitute the most striking +examples of the inheritance of latent characters. + +The bearing of the phenomena of dimorphism upon the principles of +evolution formulated by Lamarck, and modified by his followers to +constitute Neo-Lamarckianism, remains to be considered. Lamarck assumed +that the external conditions directly affected the organisms in [448] +such a way as to make them better adapted to life, under prevailing +circumstances. Nageli gave to this conception the name "Theory of direct +causation" (Theorie der directen Bewirkung), and it has received the +approval of Von Wettstein, Strasburger and other German investigators. +According to this conception a plant, when migrating from lowlands into +the mountains would slowly be changed and gradually assume alpine +habits. Once acquired this habit would become fixed and attain the rank +of specific characters. In testing this theory by field-observations and +culture-experiments, the defenders of the Nagelian principle could +easily produce evidence upon the first point. The change of +lowland-plants into alpine varieties can be brought about in numerous +cases, and corresponding changes under the influence of soil, or +climate, or life-conditions are on record for the most various +characters and qualities. + +The second point, however, is as difficult to prove as the first is of +easy treatment. If after hundreds and thousands of years of exposure to +alpine or other extreme conditions a fixed change is proved to have +taken place, the question remains unanswered, whether the change has +been a gradual or a sudden one. Darwin pointed out that long periods of +life afford a [449] chance for a sudden change in the desired direction, +as well as for the slow accumulation of slight deviations. Any mutations +in a wrong direction would at once be destroyed, but an accidental +change in a useful way would be preserved, and multiply itself. If in +the course of centuries this occurred, they would be nearly sure to +become established, however rare at the outset. Hence the positive +assertion is scarcely capable of direct proof. + +On the other hand the negative assertion must be granted full +significance. If the alpine climate has done no more than produce a +transitory change, it is clear that thousands of years do not, +necessarily, cause constant and specific alterations. This requirement +is one of the indispensable supports of the Lamarckian theory. The +matter is capable of disproof however, and such disproof seems to be +afforded by the direct evidence of the present condition of the alpine +varieties at large, and by many other similar cases. + +Among these the observations of Holtermann on some desert-plants of +Ceylon are of the highest value. Moreover they touch questions which are +of wide importance for the study of the biology of American deserts. For +this reason I may be allowed to introduce them here at some length. + +[450] The desert of Kaits, in Northern Ceylon, nourishes on its dry and +torrid sands some species, represented by a large number of individuals, +together with some rarer plants. The commonest forms are _Erigeron +Asteroides_, _Vernonia cinerea_, _Laurea pinnatifida_, _Vicoa +auriculata_, _Heylandia latebrosa_ and _Chrysopogon montanus_. In direct +contrast with the ordinary desert-types they have a thin epidermis, with +exposed stomata, features that ordinarily were characteristic of species +of moister regions. They are annuals, growing rapidly, blooming and +ripening their seeds before the height of the dry season. Evidently they +are to be considered as the remainder of the flora of a previous period, +when the soil had not yet become arid. They might be called relics. Of +course they are small and dwarf-like, when compared with allied forms. + +These curious little desert-plants disprove the Nagelian views in two +important points. First, they show that extreme conditions do not +necessarily change the organisms subjected to them, in a desirable +direction. During the many centuries that these plants must have existed +in the desert in annual generations, no single feature in the anatomical +structure has become changed. Hence the conclusion that small leaves, +abundant rootstocks and short [451] stems, a dense foliage, a strongly +cuticularized epidermis, few and narrow air-cavities in the tissues and +all the long range of characteristics of typical desert-plants are not a +simple result of the influence of climate and soil. There is no direct +influence in this sense. + +The second point, in which Nageli's idea is broken down by Holtermann's +observations, results from the behavior of the plants of the Kaits +desert when grown or sown on garden soil. When treated in this way they +at once lose the only peculiarity which might be considered as a +consequence of the desert-life of their ancestors, their dwarf stature. +They behave exactly like the alpine plants in Bonnier's experiments, and +with even more striking differences. In the desert they attain a height +of a few centimeters, but in the garden they attain half a meter and +more in height. Nothing in the way of stability has resulted from the +action of the dry soil, not even in such a minor point as the height of +the stems. + +From the facts and discussions we may conclude that double adaptation is +not induced by external influences, at least not in any way in which it +might be of use to the plant. It may arise by some unknown cause, or may +not be incited at all. In the first case the plant becomes capable of +living under the alternating [452] circumstances, and if growing near +the limits of such regions it will overlap and get into the new area. +All other species, which did not acquire the double habit, are of course +excluded, with such curious exceptions as those of Kaits. The typical +vegetation under such extreme conditions however, finds explanation +quite as well by the one as by the other view. + +Leaving these obvious cases of double adaptation, there still remains +one point to be considered. It is the dwarf stature of so many desert +and alpine plants. Are these dwarfs only the extremes of the normal +fluctuating variability, or is their stature to be regarded as the +expression of some peculiar adaptive but latent quality? It is as yet +difficult to decide this question, because statistical studies of this +form of variability are still wanting. The capacity of ripening the seed +on individuals of dwarf stature however, is not at all a universal +accompaniment of a variable height. Hence it cannot be considered as a +necessary consequence of it. On the other hand the dwarf varieties of +numerous garden-plants, as for instance: of larkspurs, snapdragon, +opium-poppies and others are quite stable and thence are obviously due +to peculiar characteristics. Such characteristics, if combined with tall +stature into a pair of antagonists, would yield a double [453] +adaptation, and on such a base a hypothetical explanation could no doubt +be rested. Instead of discussing this problem from the theoretical side, +I prefer to compare those species which are capable of assuming a dwarf +stature under less uncommon conditions than those of alpine and +desert-plants. Many weeds of our gardens and many wild species have this +capacity. They become very tall, with large leaves, richly branched +stems and numerous flowers in moist and rich soil. On bad soil, or if +germinating too late, when the season is drier, they remain very small, +producing only a few leaves and often limiting themselves to one +flower-head. This is often seen with thorn-apples and amaranths, and +even with oats and rye, and is notoriously the case with buckwheat. +Gauchery has observed that the extremes differ often as much from one +another as 1:10. In the case of the Canadian horseweed or _Erigeron +canadensis_, which is widely naturalized in Europe, the tallest +specimens are often twenty-five times as tall as the smallest, the +difference increasing to greater extremes, if besides the main stem, the +length of the numerous branches of the tall plants are taken into +consideration. Other instances studied by the French investigator are +_Erythraea pulchella_ and _Calamintha Acinos_. + +[454] Dimorphism is of universal occurrence in the whole vegetable +kingdom. In some cases it is typical, and may easily be discerned from +extreme fluctuating variability. In others the contrast is not at all +obvious, and a closer investigation is needed to decide between the two +possibilities. Sometimes the adaptive quality is evident, in other cases +it is not. A large number of plants bear two kinds of leaves linked with +one another by intermediate forms. Often the first leaves of a shoot, or +those of accidentally strong shoots, exhibit deviating shapes, and the +usefulness of such occurrences seems to be quite doubtful. The +elongation of stems and linear leaves, and the reduction of lateral +organs in darkness, is manifestly an adaptation. Many plants have +stolons with double adaptations which enable them to retain their +character of underground stems with bracts or to exchange it for the +characteristics of erect stems with green leaves according to the outer +circumstances. In some shrubs and trees the capacity of a number of buds +to produce either flowers or shoots with leaves seems to be in the same +condition. The capacity of producing spines is also a double adaptation, +active on dry and arid soil and latent in a moist climate or under +cultivation, as with the wild and cultivated apple, and in the +experiments of Lothelier [455] with _Berberis_, _Lycium_ and other +species, which lose their spines in damp air. + +In some conifers the evolution of horizontal branches may be modified by +simply turning the buds upside down. Or the lateral branches can be +induced to become erect stems by cutting off the normal summit of a +tree. Numerous organs and functions lie dormant until aroused by +external agencies, and many other cases could be cited, showing the wide +occurrence of double adaptation. + +There are, however, two points, which should not be passed over without +some mention. One of them is the influence of sun and shade on leaves, +and the other the atavistic forms, often exhibited during the juvenile +period. + +The leaves of many plants, and especially those of some shrubs and +trees, have the capacity of adapting themselves either to intense or to +diffuse light. On the circumference of the crown of a tree the light is +stronger and the leaves a small and thick, with a dense tissue. In the +inner parts of the crown the light is weak and the leaves are broader in +order to get as much of it as possible. They become larger but thinner, +consisting often of a small number of cell layers. The definitive +formation is made in extreme youth, often even during the previous +summer, at the time of the [456] very first evolution of the young +organs within the buds. _Iris_, and _Lactuca Scariola_ or the prickly +lettuce, and many other plants afford similar instances. As the +definitive decision must be made in these cases long before the direct +influence of the conditions which would make the change useful is felt, +it is hardly conceivable how they could be ascribed to this cause. + +It is universally known that many plants show deviating features when +very young, and that these often remind us of the characters of their +probable ancestors. Many plants that must have been derived from their +nearest systematic relatives, chiefly by reductions, are constantly +betraying this relation by a repetition of the ancestral marks during +their youth. + +There can be hardly a doubt that the general law of natural selection +prevails in such cases as it does in others. Or stated otherwise, it is +very probable, that in most cases the atavistic characters have been +retained during youth because of their temporary usefulness. +Unfortunately, our knowledge of utility of qualities is as yet, very +incomplete. Here we must assume that what is ordinarily spared by +natural selection is to be considered as useful, [457] until direct +experimental investigations have been made. + +So it is for instance with the submerged leaves of water-plants. As a +rule they are linear, or if compound, are reduced to densely branching +filiform threads. Hence we may conclude that this structure is of some +use to them. Now two European and some corresponding American species of +water-parsnip, the _Sium latifolium_ and _Berula angustifolia_ with +their allies, are umbellifers, which bear pinnate instead of bi- or +tri-pinnate leaves. But the young plants and even the young shoots when +developing from the rootstocks under water comply with the above rule, +producing very compound, finely and pectinately dissected leaves. From a +systematic point of view these leaves indicate the origin of the +water-parsnips from ordinary umbellifers, which generally have bi- and +tripinnate leaves. + +Similar cases of double adaptation, dependent on external conditions at +different periods of the evolution of the plant are very numerous. They +are most marked among leguminous plants, as shown by the trifoliolate +leaves of the thorn-broom and allies, which in the adult state have +green twigs destitute of leaves. + +As an additional instance of dimorphism and probable double adaptation +to unrecognized external [458] conditions I might point to the genus +_Acacia_. As we have seen in a previous lecture some of the numerous +species of this genus bear bi-pinnate leaves, while others have only +flattened leaf-stalks. According to the prevailing systematic +conceptions, the last must have been derived from the first by the loss +of the blades and the corresponding increase of size and superficial +extension of the stalk. In proof of this view they exhibit, as we have +described, the ancestral characters in the young plantlets, and this +production of bi-pinnate leaves has probably been retained at the period +of the corresponding negative mutations, because of some distinct, +though still unknown use. + +Summarizing the results of this discussion, we may state that useful +dimorphism, or double adaptation, is a substitution of characters quite +analogous to the useless dimorphism of cultivated ever-sporting +varieties and the stray occurrence of hereditary monstrosities. The same +laws and conditions prevail in both cases. + + +[459] + +E. MUTATIONS + +LECTURE XVI + +THE ORIGIN OF THE PELORIC TOAD-FLAX + +I have tried to show previously that species, in the ordinary sense of +the word, consist of distinct groups of units. In systematic works these +groups are all designated by the name of varieties, but it is usually +granted that the units of the system are not always of the same value. +Hence we have distinguished between elementary species and varieties +proper. The first are combined into species whose common original type +is now lost or unknown, and from their characters is derived an +hypothetical image of what the common ancestor is supposed to have been. +The varieties proper are derived in most cases from still existing +types, and therefore are subjoined to them. A closer investigation has +shown that this derivation is ordinarily produced by the loss of some +definite attribute, or by the re-acquisition of an apparently [460] lost +character. The elementary species, on the other hand, must have arisen +by the production of new qualities, each new acquisition constituting +the origin of a new elementary form. + +Moreover we have seen, that such improvements and such losses constitute +sharp limits between the single unit-forms. Every type, of course, +varies around an average, and the extremes of one form may sometimes +reach or even overlap those of the nearest allies, but the offspring of +the extremes always return to the type. The transgression is only +temporary and a real transition of one form to another does not come +within ordinary features of fluctuating variability. Even in the cases +of eversporting varieties, where two opposite types are united within +one race, and where the succeeding individuals are continually swinging +from one extreme to the other, passing through a wide range of +intermediate steps, the limits of the variety are as sharply defined and +as free from real transgression as in any other form. + +In a complete systematic enumeration of the real units of nature, the +elementary species and varieties are thus observed to be discontinuous +and separated by definite gaps. Every unit may have its youth, may lead +a long life in the adult state and may finally die. But through [461] +the whole period of its existence it remains the same, at the end as +sharply defined from its nearest allies as in the beginning. Should some +of the units die out, the gaps between the neighboring ones will become +wider, as must often have been the case. Such segregations, however +important and useful for systematic distinctions, are evidently only of +secondary value, when considering the real nature of the units +themselves. + +We may now take up the other side of the problem. The question arises as +to how species and varieties have originated. According to the Darwinian +theory they have been produced from one another, the more highly +differentiated ones from the simpler, in a graduated series from the +most simple forms to the most complicated and most highly organized +existing types. This evolution of course must have been regular and +continuous, diverging from time to time into new directions, and linking +all organisms together into one common pedigree. All lacunae in our +present system are explained by Darwin as due to the extinction of the +forms, which previously filled them. + +Since Lamarck first propounded the conception of a common origin for all +living beings, much has been done to clear up our ideas as to the real +nature of this process. The broader [462] aspect of the subject, +including the general pedigree of the animal and vegetable kingdom, may +be said to have been outlined by Darwin and his followers, but this +phase of the subject lies beyond the limits of our present discussion. + +The other phase of the problem is concerned with the manner in which the +single elementary species and varieties have sprung from one another. +There is no reason to suppose that the world is reaching the end of its +development, and so we are to infer that the production of new species +and varieties is still going on. In reality, new forms are observed to +originate from time to time, both wild and in cultivation, and such +facts do not leave any doubt as to their origin from other allied types, +and according to natural and general laws. + +In the wild state however, and even with cultivated plants of the field +and garden, the conditions, though allowing of the immediate observation +of the origination of new forms, are by no means favorable for a closer +inquiry into the real nature of the process. Therefore I shall postpone +the discussion of the facts till another lecture, as their bearing will +be more easily understood after having dealt with more complete cases. + +These can only be obtained by direct experimentation. Comparative +studies, of course, [463] are valuable for the elucidation of general +problems and broad features of the whole pedigree, but the narrower and +more practical question as to the genetic relation of the single forms +to one another must be studied in another way, by direct experiment. The +exact methods of the laboratory must be used, and in this case the +garden is the laboratory. The cultures must be guarded with the +strictest care and every precaution taken to exclude opportunities for +error. The parents and grandparents and their offspring must be kept +pure and under control, and all facts bearing upon the birth or origin +of the new types should be carefully recorded. + +Two great difficulties have of late stood in the way of such +experimental investigation. One of them is of a theoretical, the, other +of a practical nature. One is the general belief in the supposed +slowness of the process, the other is the choice of adequate material +for experimental purposes. Darwin's hypothesis of natural selection as +the means by which new types arise, is now being generally interpreted +as stating the slow transformation of ordinary fluctuating divergencies +from the average type into specific differences. But in doing so it is +overlooked that Quetelet's law of fluctuating variability was not yet +discovered at the time, when Darwin propounded his theory. So there +[464] is no real and intimate connection between these two great +conceptions. Darwin frequently pointed out that a long period of time +might be needed for slow improvements, and was also a condition for the +occurrence of rare sports. In any case those writers have been in error, +according to my opinion, who have refrained from experimental work on +the origin of species, on account of this narrow interpretation of +Darwin's views. The choice of the material is quite another question, +and obviously all depends upon this choice. Promising instances must be +sought for, but as a rule the best way is to test as many plants as +possible. Many of them may show nothing of interest, but some might lead +to the desired end. + +For to-day's lecture I have chosen an instance, in which the grounds +upon which the choice was based are very evident. It is the origin of +the peloric toad-flax (_Linaria vulgaris peloria_). + +The ground for this choice lies simply in the fact that the peloric +toad-flax is known to have originated from the ordinary type at +different times and in different countries, under more or less divergent +conditions. It had arisen from time to time, and hence I presumed that +there was a chance to see it arise again. If this should happen under +experimental circumstances [465] the desired evidence might easily be +gathered. Or, to put it in other words, we must try to arrange things so +as to be present at the time when nature produces another of these rare +changes. + +There was still another reason for choosing this plant for observational +work. The step from the ordinary toad-flax to the peloric form is short, +and it appears as if it might be produced by slow conversion. The +ordinary species produces from time to time stray peloric flowers. These +occur at the base of the raceme, or rarely in the midst of it. In other +species they are often seen at the summit. Terminal pelories are usually +regular, having five equal spurs. Lateral pelories are generally of +zygomorphic structure, though of course in a less degree than the normal +bilabiate flowers, but they have unequal spurs, the middle one being of +the ordinary length, the two neighboring being shorter, and those +standing next to the opposite side of the flower being the shortest of +all. This curious remainder of the original, symmetrical structure of +the flower seems to have been overlooked hitherto by the investigators +of peloric toad-flaxes. + +The peloric variety of this plant is characterized by its producing only +peloric flowers. No single bilabiate or one-spurred flower remains. + +[466] I once had a lot of nearly a hundred specimens of this fine +variety, and it was a most curious and beautiful sight to observe the +many thousands of nearly regular flowers blooming at the same time. Some +degree of variability was of course present, even in a large measure. +The number of the spurs varied between four and six, transgressing these +limits in some instances, but never so far as to produce really +one-spurred flowers. Comparing this variety with the ordinary type, two +ways of passing over from the one to the other might be imagined. One +would entail a slow increase of the number of the peloric flowers on +each plant, combined with a decrease of the number of the normal ones, +the other a sudden leap from one extreme to the other without any +intermediate steps. The latter might easily be overlooked in field +observations and their failure may not have the value of direct proof. +They could never be overlooked, on the other hand, in experimental +culture. + +The first record of the peloric toad-flax is that of Zioberg, a student +of Linnaeus, who found it in the neighborhood of Upsala. This curious +discovery was described by Rudberg in his dissertation in the year 1744. +Soon afterwards other localities were discovered by Link near Gottingen +in Germany about 1791 and afterwards [467] in the vicinity of Berlin, as +stated by Ratzeburg, 1825. Many other localities have since been +indicated for it in Europe, and in my own country some have been noted +of late, as for instance near Zandvoort in 1874 and near Oldenzaal in +1896. In both these last named cases the peloric form arose +spontaneously in places which had often been visited by botanists before +the recorded appearance, and therefore, without any doubt, they must +have been produced directly and independently by the ordinary species +which grows in the locality. The same holds good for other occurrences +of it. In many instances the variety has been recorded to disappear +after a certain lapse of time, the original specimens dying out and no +new ones being produced. _Linaria_ is a perennial herb, multiplying +itself easily by buds growing on the roots, but even with this means of +propagation its duration seems to have definite limits. + +There is one other important point arguing strongly for the independent +appearance of the peloric form in its several localities. It is the +difficulty of fertilization and the high degree of sterility, even if +artificially pollinated. Bees and bumble-bees are unable to crawl into +the narrow tubular flowers, and to bring the fertilizing pollen to the +stigma. Ripe capsules with seeds [468] have never been seen in the wild +state. The only writer who succeeded in sowing seeds of the peloric +variety was Wildenow and he got only very few seedlings. But even in +artificial pollination the result is the same, the anthers seeming to be +seriously affected by the change. I tried both self-fertilization and +cross-pollination, and only with utmost care did I succeed in saving +barely a hundred seeds. In order to obtain them I was compelled to +operate on more than a thousand flowers on about a dozen peloric plants. + +The variety being wholly barren in nature, the assumption that the +plants in the different recorded localities might have a common origin +is at once excluded. There must have been at least nearly as many +mutations as localities. This strengthens the hope of seeing such a +mutation happen in one's own garden. It should also be remembered that +peloric flowers are known to have originated in quite a number of +different species of _Linaria_, and also with many of the allied species +within the range of the Labiatiflorae. + +I will now give the description of my own experiment. Of course this did +not give the expected result in the first year. On the contrary, it was +only after eight years' work that I had the good fortune of observing +the mutation. [469] But as the whole life-history of the preceding +generations had been carefully observed and recorded, the exact +interpretation of the fact was readily made. + +My culture commenced in the year 1886. I chose some plants of the normal +type with one or two peloric flowers besides the bilabiate majority +which I found on a locality in the neighborhood of Hilversum in Holland. +I planted the roots in my garden and from them had the first flowering +generation in the following summer. From their seeds I grew the second +generation in three following years. They flowered profusely and +produced in 1889 only one, and in 1890 only two peloric structures. I +saved the seeds in 1889 and had in 1890-1891 the third generation. These +plants likewise flowered only in the second year, and gave among some +thousands of symmetrical blossoms, only one five-spurred flower. I +pollinated this flower myself, and it produced abundant fruit with +enough seeds for the entire culture in 1892, and they only were sown. + +Until this year my generations required two years each, owing to the +perennial habit of the plants. In this way the prospects of the culture +began to decrease, and I proposed to try to heighten my chances by +having a new generation yearly. With this intention I sowed the [470] +selected seeds in a pan in the glasshouse of my laboratory and planted +them out as soon as the young stems had reached a length of some few +centimeters. Each seedling was put in a separate pot, in heavily manured +soil. The pots were kept under glass until the beginning of June, and +the young plants produced during this period a number of secondary stems +from the curious hypocotylous buds which are so characteristic of the +species. These stems grew rapidly and as soon as they were strong +enough, the plants were put into the beds. They all, at least nearly +all, some twenty specimens, flowered in the following month. + +I observed only one peloric flower among the large number present. I +took the plant bearing this flower and one more for the culture of the +following year, and destroyed all others. These two plants grew on the +same spot, and were allowed to fertilize each other by the agency of the +bees, but were kept isolated from any other congener. They flowered +abundantly, but produced only one-spurred bilabiate flowers during the +whole summer. They matured more than 10 cu. cm. of seeds. + +It is from this pair of plants that my peloric race has sprung. And as +they are the ancestors of the first closely observed case of peloric +mutation, [471] it seems worth while to give some details regarding +their fertilization. + +Isolated plants of _Linaria vulgaris_ do not produce seed, even if +freely pollinated by bees. Pollen from other plants is required. This +requirement is not at all restricted to the genus _Linaria_, as many +instances are known to occur in different families. It is generally +assumed that the pollen of any other individual of the same species is +capable of producing fertilization, although it is to be said that a +critical examination has been made in but few instances. + +This, however, is not the case, at least not in the present instance. I +have pollinated a number of plants, grown from seed of the same strain +and combined them in pairs, and excluded the visits of insects, and +pollen other than that of the plant itself and that of the specimen with +which it was paired. The result was that some pairs were fertile and +others barren. Counting these two groups of pairs, I found them nearly +equal in number, indicating thereby that for any given individual the +pollen of half of the others is potent, but that of the other half +impotent. From these facts we may conclude the presence of a curious +case of dimorphy, analogous to that proposed for the primroses, but +without visible differentiating marks in the flowers. At least such +opposite characters [472] have as yet not been ascertained in the case +of our toad-flax. + +In order to save seed from isolated plants it is necessary, for this +reason, to have at least two individuals, and these must belong to the +two physiologically different types. Now in the year 1892, as in other +years, my plants, though separated at the outset by distances of about +20 cm. from each other, threw out roots of far greater length, growing +in such a way as to abolish the strict isolation of the individuals. Any +plot may produce several stems from such roots, and it is manifestly +impossible to decide whether they all belong to one original plant or to +the mixed roots of several individuals. No other strains were grown on +the same bed with my plants however, and so I considered all the stems +of the little group as belonging to one plant. But their perfect +fertility showed, according to the experience described, that there must +have been at least two specimens mingled together. + +Returning now to the seeds of this pair of plants, I had, of course, not +the least occasion to ascribe to it any higher value than the harvest of +former years. The consequence was that I had no reason to make large +sowings, and grew only enough young plants to have about 50 in bloom in +the summer of 1894. Among [473] these, stray peloric flowers were +observed in somewhat larger number than in the previous generations, 11 +plants bearing one or two, or even three such abnormalities. This +however, could not be considered as a real advance, since such plants +may occur in varying, though ordinarily small numbers in every +generation. + +Besides them a single plant was seen to bear only peloric flowers; it +produced racemes on several stems and their branches. All were peloric +without exception. I kept it through the winter, taking care to preserve +a complete isolation of its roots. The other plants were wholly +destroyed. Such annihilation must include both the stems and roots and +the latter of course requires considerable labor. The following year, +however, gave proof of the success of the operation, since my plant +bloomed luxuriously for the second time and remained true to the type of +the first year, producing peloric flowers exclusively. + +Here we have the first experimental mutation of a normal into a peloric +race. Two facts were clear and simple. The ancestry was known for over a +period of four generations, living under the ordinary care and +conditions of an experimental garden, isolated from other toad-flaxes, +but freely fertilized by bees or at times by myself. This ancestry was +quite constant as to [474] the peloric peculiarity, remaining true to +the wild type as it occurs everywhere in my country, and showing in no +respect any tendency to the production of a new variety. + +The mutation took place at once. It was a sudden leap from the normal +plants with very rare peloric flowers to a type exclusively peloric. No +intermediate steps were observed. The parents themselves had borne +thousands of flowers during two summers, and these were inspected nearly +every day, in the hope of finding some pelories and of saving their seed +separately. Only one such flower was seen. If there had been more, say a +few in every hundred flowers, it might be allowable to consider them as +previous stages, showing a preparation of the impending change. But +nothing of this kind was observed. There was simply no visible +preparation for the sudden leap. + +This leap, on the other hand, was full and complete. No reminiscence of +the former condition remained. Not a single flower on the mutated plant +reverted to the previous type. All were thoroughly affected by the new +attribute, and showed the abnormally augmented number of spurs, the +tubular structure of the corolla and the round and narrow entrance of +its throat. The whole plant departed absolutely from the old type of its +progenitors. + +[475] Three ways were open to continue my experiment. The first was +indicated by the abundant harvest from the parent-plants of the +mutation. It seemed possible to compare the numerical proportion of the +mutated seeds with those of normal plants. In order to ascertain this +proportion I sowed the greatest part of my 10 cu. cm. of seed and +planted some 2,000 young plants in little pots with well-manured soil. I +got some 1,750 flowering plants and observed among them 16 wholly +peloric individuals. The numerical proportion of the mutation was +therefore in this instance to be calculated equal to about 1% of the +whole crop. + +This figure is of some importance. For it shows that the chance of +finding mutations requires the cultivation of large groups of +individuals. One plant in each hundred may mutate, and cultures of less +than a hundred specimens must therefore be entirely dependent on chance +for the appearance of new forms, even if such should accidentally have +been produced and lay dormant in the seed. In other cases mutations may +be more numerous, or on the contrary, more rare. But the chance of +mutative changes in larger numbers is manifestly much reduced by this +experiment, and they may be expected to form a very small proportion of +the culture. + +[476] The second question which arose from the above result was this. +Could the mutation be repeated? Was it to be ascribed to some latent +cause which might be operative more than once? Was there some hidden +tendency to mutation, which, ordinarily weak, was strengthened in my +cultures by some unknown influence? Was the observed mutation to be +explained by a common cause with the other cases recorded by +field-observations? To answer this question I had only to continue my +experiment, excluding the mutated individuals from any intercrossing +with their brethren. To this end I saved the seeds from duly isolated +groups in different years and sowed them at different times. For various +causes I was not prepared to have large cultures from these seeds, but +notwithstanding this, the mutation repeated itself. In one instance I +obtained two, in another, one peloric plant with exclusively +many-spurred flowers. As is easily understood, these were related as +"nieces" to the first observed mutants. They originated in quite the +same way, by a sudden leap, without any preparation and without any +intermediate steps. + +Mutation is proved by this experience to be of an iterative nature. It +is the expression of some concealed condition, or as it is generally +[477] called, of some hidden tendency. The real nature of this state of +the hereditary qualities is as yet wholly unknown. It would not be safe +to formulate further conclusions before the evidence offered by the +evening-primroses is considered. + +Thirdly, the question arises, whether the mutation is complete, not only +as to the morphologic character, but also as to the hereditary +constitution of the mutated individuals. But here unfortunately the high +degree of sterility of the peloric plants, as previously noted, makes +the experimental evidence a thing of great difficulty. During the course +of several years I isolated and planted together the peloric individuals +already mentioned, all in all some twenty plants. Each individual was +nearly absolutely sterile when treated with its own pollen, and the aid +of insects was of no avail. I intercrossed my plants artificially, and +pollinated more than a thousand flowers. Not a single one gave a normal +fruit, but some small and nearly rudimentary capsules were produced, +bearing a few seeds. From these I had 119 flowering plants, out of which +106 were peloric and 13 one-spurred. The great majority, some 90%, were +thus shown to be true to their new type. Whether the 10% reverting ones +were truly atavists, or whether they were [478] only vicinists, caused +by stray pollen grains from another culture, cannot of course be decided +with sufficient certitude. + +Here I might refer to the observations concerning the invisible +dimorphous state of the flowers of the normal toad-flax. Individuals of +the same type, when fertilized with each other, are nearly, but not +absolutely, sterile. The yield of seeds of my peloric plants agrees +fairly well with the harvest which I have obtained from some of the +nearly sterile pairs of individuals in my former trial. Hence the +suggestion is forced upon us that perhaps, owing to some unknown cause, +all the peloric individuals of my experiment belonged to one and the +same type, and were sterile for this reason only. If this is true, then +it is to be presumed that all previous investigators have met the same +condition, each having at hand only one of the two required types. And +this discussion has the further advantage of showing the way, in which +perhaps a full and constant race of peloric toad-flaxes may be obtained. +Two individuals of different type are required to start from. They seem +as yet never to have arisen from one group of mutations. But if it were +possible to combine the products of two mutations obtained in different +countries and under different conditions, there would be a chance [479] +that they might belong to the supposed opposite types, and thus be +fertile with one another. My peloric plants are still available, and the +occurrence of this form elsewhere would give material for a successful +experiment. The probability thereof is enhanced by the experience that +my peloric plants bear large capsules and a rich harvest of seeds when +fertilized from plants of the normal one-spurred race, while they remain +nearly wholly barren by artificial fertilization with others. I suppose +that they are infertile with the normal toad-flaxes of their own sexual +disposition, but fertile with those of the opposite constitution. At all +events the fact that they may bear abundant seed when properly +pollinated is an indication of successful experiments on the possibility +of gaining a hereditary race with exclusively peloric flowers. And such +a race would be a distinct gain for sundry physiologic inquiries, and +perhaps not wholly destitute of value from an horticultural point of +view. + +Returning now to the often recorded occurrence of peloric toad-flaxes in +the wild state and recalling our discussion about the improbability of a +dispersion from one locality to another by seed, and the probability of +independent origin for most of these cases, we are confronted with the +conception that a latent [480] tendency to mutation must be universally +present in the whole species. Another observation, although it is of a +negative character, gains in importance from this point of view. I refer +to the total lack of intermediate steps between normal and peloric +individuals. If such links had ordinarily been produced previous to the +purely peloric state they would no doubt have been observed from time to +time. This is so much the more probable as _Linaria_ is a perennial +herb, and the ancestors of a mutation might still be in a flowering +condition together with their divergent offspring. But no such +intermediates are on record. The peloric toad-flaxes are, as a rule, +found surrounded by the normal type, but without intergrading forms. +This discontinuity has already been insisted upon by Hofmeister and +others, even at the time when the theory of descent was most under +discussion, and any link would surely have been produced as a proof of a +slow and continuous change. But no such proof has been found, and the +conclusion seems admissible that the mutation of toad-flaxes ordinarily, +if not universally, takes place by a sudden step. Our experiment may +simply be considered as a thoroughly controlled instance of an often +recurring phenomenon. It teaches us how, in the [481] main, the peloric +mutations must be assumed to proceed. + +This conception may still be broadened. We may include in it all similar +occurrences, in allied and other species. There is hardly a limit to the +possibilities which are opened up by this experience. But it will be +well to refrain from hazardous theorizing, and consider only those cases +which may be regarded as exact repetitions of the same phenomenon and of +which our culture is one of the most recent instances on record. We will +limit ourselves to the probable origin of peloric variations at large, +of which little is known, but some evidence may be derived from the +recorded facts. Only one case can be said to be directly analogous to +our observations. + +This refers to the peloric race of the common snapdragon, or +_Antirrhinum majus_ of our gardens. It is known to produce peloric races +from time to time in the same way as does the toadflax. But the +snapdragon is self-fertile and so is its peloric variety. Some cases are +relatively old, and some of them have been recorded and in part observed +by Darwin. Whence they have sprung and in what manner they were +produced, seems never to have been noted. Others are of later origin, +and among these one or two varieties have been accidentally produced +[482] in the nursery of Mr. Chr. Lorenz in Erfurt, and are now for sale, +the seeds being guaranteed to yield a large proportion of peloric +individuals. The peloric form in this case appeared at once, but was not +isolated, and was left free to visiting insects, which of course crossed +it with the surrounding varieties. Without doubt the existence of two +color-varieties of the peloric type, one of a very dark red, indicating +the "Black prince" variety as the pollen-parent, and the other with a +white tube of the corolla, recalling the form known as "Delila," is due +to these crossings. I had last year (1903) a large lot of plants, partly +normal and partly peloric, but evidently of hybrid origin, from seeds +from this nursery, showing moreover all intermediate steps between +nearly wholly peloric individuals and apparently normal ones. I have +saved the seeds of the isolated types and before seeing the flowers of +their offspring, nothing can be said about the purity and constancy of +the type, when freed from hybrid admixtures. The peloric snapdragon has +five small unequal spurs at the base of its long tube, and in this +respect agrees with the peloric toad-flax. + +Other pelories are terminal and quite regular, and occur in some species +of _Linaria_, where I observed them in _Linaria dalmatica_. The [483] +terminal flowers of many branches were large and beautifully peloric, +bearing five long and equal spurs. About their origin and inheritance +nothing is known. + +A most curious terminal pelory is that of the common foxglove or +_Digitalis purpurea_. As we have seen in a previous lecture, it is an +old variety. It was described and figured for the first time by Vrolik +of Amsterdam, and the original specimens of his plates are still to be +seen in the collections of the botanic garden of that university. Since +his time it has been propagated by seed as a commercial variety, and may +be easily obtained. The terminal flower of the central stem and those of +the branches only are affected, all other flowers being wholly normal. +Almost always it is accompanied by other deviations, among which a +marked increase of the number of the parts of the corolla and other +whorls is the most striking. Likewise supernumerary petals on the outer +side of the corolla, and a production of a bud in the center of the +capsule may be often met with. This bud as a rule grows out after the +fading away of the flower, bursting through the green carpels of the +unripe fruit, and producing ordinarily a secondary raceme of flowers. +This raceme is a weak but exact repetition of the first, bearing +symmetrical foxgloves all [484] along and terminating in a peloric +structure. On the branches these anomalies are more or less reduced, +according to the strength of the branch, and conforming to the rule of +periodicity, given in our lecture on the "five-leaved" clover. Through +all this diminution the peloric type remains unchanged and therefore +becomes so much the purer, the weaker the branches on which it stands. + +I am not sure whether such peloric flowers have ever been purely +pollinated and their seed saved separately, but I have often observed +that the race comes pure from the seed of the zygomorphic flowers. It is +as yet doubtful whether it is a half race or a double race, and whether +it might be purified and strengthened by artificial selection. Perhaps +the determination of the hereditary percentage described when dealing +with the tricotyls might give the clue to the acquisition of a higher +specialized race. The variety is old and widely disseminated, but must +be subjected to quite a number of additional experiments before it can +be said to be sufficiently understood. + +The most widespread peloric variety is that of _Gloxinia_. It has erect +instead of drooping flowers; and with the changed position the structure +is also changed. Like other pelories it has five equal stamens instead +of four unequal [485] ones, and a corolla with five equal segments +instead of an upper and a lower lip. It shows the peloric condition in +all of its flowers and is often combined with a small increase of the +number of the parts of the whorls. It is for sale under the name of +_erecta_, and may be had in a wide range of color-types. It seems to be +quite constant from seed. + +Many other instances of peloric flowers are on record. Indian cress or +_Tropaeolum majus_ loses the spur in some double varieties and with it +most of its symmetrical structure; it seems to be considered justly as a +peloric malformation. Other species produce such anomalies only from +time to time and nothing is known about their hereditary tendency. One +of the most curious instances is the terminal flower of the raceme of +the common laburnum, which loses its whole papilionaceous character and +becomes as regularly quinate as a common buttercup. + +Some families are more liable to pelorism than others. Obviously all the +groups, the flowers of which are not symmetrical, are to be excluded. +But then we find that labiates and their allies among the dicotyledonous +plants, and orchids among the monocotyledonous ones are especially +subjected to this alteration. In both groups many genera and a long list +of species [486] could be quoted as proof. The family of the labiates +seems to be essentially rich in terminal pelories, as for instance in +the wild sage or _Salvia_ and the dead-nettle or _Lamium_. Here the +pelories have long and straight corolla-tubes, which are terminated by a +whorl of four or five segments. Such forms often occur in the wild state +and seem to have a geographic distribution as narrowly circumscribed as +in the case of many small species. Those of the labiates chiefly belong +to southern Europe and are unknown at least in some parts of the other +countries. On the contrary terminal pelories of _Scrophularia nodosa_ +are met with from time to time in Holland. Such facts clearly point to a +common origin, and as only the terminal flowers are affected by the +malformation, the fertility of the whole plant is evidently not +seriously infringed upon. + +Before leaving the labiates, we may cite a curious instance of pelorism +in the toad-flax, which is quite different from the ordinary peloric +variety. This latter may be considered from a morphologic standpoint to +be owing to a five-fold repetition of the middle part of the underlip. +This conception would at once explain the occurrence of five spurs and +of the orange border all around the corolla-tube. We might readily +imagine that any other of the five [487] parts of the corolla could be +repeated five-fold, in which case there would be no spur, and no orange +hue on the upper corolla-ring. Such forms really occur, though they seem +to be more rare than the five-spurred pelories. Very little is known +about their frequency and hereditary qualities. + +Orchids include a large number of peloric monstrosities and moreover a +wild pelory which is systematically described not only as a separate +species but even as a new genus. It bears the name of _Uropedium +lindenii_, and is so closely related to _Cypripedium caudatum_ that many +authors take it for the peloric variety of this plant. It occurs in the +wild state in some parts of Mexico, where the _Cypripedium_ also grows. +Its claims to be a separate genus are lessened by the somewhat monstrous +condition of the sexual organs, which are described as quite abnormal. +But here also, intermediates are lacking, and this fact points to a +sudden origin. + +Many cases of pelorism afford promising material for further studies of +experimental mutations. The peloric toad-flax is only the prototype of +what may be expected in other cases. No opportunity should be lost to +increase the as yet too scanty, evidence on this point. + + +[488] + +LECTURE XVII + +THE PRODUCTION OF DOUBLE FLOWERS + +Mutations occur as often among cultivated plants as among those in the +wild state. Garden flowers are known to vary markedly. Much of their +variability, however, is due to hybridism, and the combination of +characters previously separate has a value for the breeder nearly equal +the production of really new qualities. Nevertheless there is no doubt +that some new characters appear from time to time. + +In a previous lecture we have seen that varietal characters have many +features in common. One of them is their frequent recurrence both in the +same and in other, often very distantly related, species. This +recurrence is an important factor in the choice of the material for an +experimental investigation of the nature of mutations. + +Some varieties are reputed to occur more often and more readily than +others. White-colored varieties, though so very common, seem for the +most part to be of ancient date, but only few [489] have a known origin, +however. Without any doubt many of them have been found in a wild state +and were introduced into culture. On the other hand double flowers are +exceedingly rare in the wild state, and even a slight indication of a +tendency towards doubling, the stray petaloid stamens, are only rarely +observed growing wild. In cultivation, however, double flowers are of +frequent occurrence; hence the conclusion that they have been produced +in gardens and nurseries more frequently than perhaps any other type of +variety. + +In the beginning of my experimental work I cherished the hope of being +able to produce a white variety. My experiments, however, have not been +successful, and so I have given them up temporarily. Much better chances +for a new double variety seemed to exist, and my endeavors in this +direction have finally been crowned with success. + +For this reason I propose to deal now with the production of double +flowers, to inquire what is on record about them in horticultural +literature, and to give a full description of the origin thereof in an +instance which it was my good fortune to observe in my garden. + +Of course the historical part is only a hasty survey of the question and +will only give such evidence as may enable us to get an idea of the +[490] chances of success for the experimental worker. In the second half +of the seventeenth century (1671), my countryman, Abraham Munting, +published a large book on garden plants with many beautiful figures. It +is called "Waare Oeffeninge der Planters," or "True Exercises With +Plants." The descriptions pertain to ordinary typical species in greater +part, but garden varieties receive special attention. Among these a long +list of double flowers are to be seen. Double varieties of poppies, +liverleaf (Hepatica), wallflowers (_Cheiranthus_), violets, _Caltha_, +_Althaea_, _Colchicum_, and periwinkles (_Vinca_), and a great many +other common flowers were already in cultivation at that time. + +Other double forms have been since added. Many have been introduced from +Japan, especially the Japanese marigold, _Chrysanthemum indicum_. Others +have been derived from Mexico, as for instance the double zinnias. The +single dahlias only seem to have been originally known to the +inhabitants of Mexico. They were introduced into Spain at about 1789, +and the first double ones were produced in Louvain, Belgium, in 1814. +The method of their origin has not been described, and probably escaped +the originators themselves. But in historical records we find the +curious statement that it took place after three years' work. This +indicates [491] a distinct plan, and the possibility of carrying it to a +practical conclusion within a few years' time. + +Something more is known about other cases. Garden anemones, _Anemone +coronaria_, are said to have become double in the first half of the last +century in an English nursery. The owner, Williamson, observing in his +beds a flower with a single broadened stamen, saved its seeds +separately, and in the next generations procured beautifully filled +flowers. These he afterwards had crossed by bees with a number of +colored varieties, and in this way succeeded in producing many new +double types of anemone. + +The first double petunia is known to have suddenly and accidentally +arisen from ordinary seed in a private garden at Lyons about 1855. From +this one plant all double races and-varieties have been derived by +natural and partly by artificial crosses. Carriere, who reported this +fact, added that likewise other species were known at that time to +produce new double varieties rapidly. The double fuchsias originated +about the same time (1854) and ten years later the range of double +varieties of this plant had become so large that Carriere found it +impossible to enumerate all of them. + +Double carnations seem to be relatively old, double corn-flowers and +double blue-bells being [492] of a later period. A long list could +easily be made, to show that during the whole history of horticulture +double varieties have arisen from time to time. As far as we can judge, +such appearances have been isolated and sudden. Sometimes they sprang +into existence in the full display of their beauty, but most commonly +they showed themselves for the first time, exhibiting only spare +supernumerary petals. Whenever such sports were worked up, a few years +sufficed to reach the entire development of the new varietal attribute. + +From this superficial survey of historical facts, the inference is +forced upon us that the chance of producing a new double variety is good +enough to justify the attempt. It has frequently succeeded for practical +purposes, why should it not succeed as well for purely scientific +investigation? At all events the type recommends itself to the student +of nature, both on account of its frequency, and of the apparent +insignificance of the first step, combined with the possibility of +rapidly working up from this small beginning of one superfluous petal +towards the highest degree of duplication. + +Compared with the tedious experimental production of the peloric +toad-flax, the attempt to produce a double flower has a distinct +attraction. The peloric toad-flax is nothing new; the [493] experiment +was only a repetition of what presumably takes place often within the +same species. To attempt to produce a double variety we may choose any +species, and of course should select one which as yet has not been known +to produce double flowers. By doing so we will, if we succeed, produce +something new. Of course, it does not matter whether the new variety has +an horticultural interest or not, and it seems preferable to choose a +wild or little cultivated species, to be quite sure that the variety in +question is not already in existence. Finally the prospect of success +seems to be enhanced if a species is chosen, the nearest allies of which +are known to have produced double flowers. + +For these reasons and others I chose for my experiment the +corn-marigold, or _Chrysanthemum segetum_. It is also called the golden +cornflower. In the wheat and rye fields of central Europe it associates +with the blue-bottle or blue corn-flower. It is sometimes cultivated and +the seeds are offered for sale by many nurserymen. It has a cultivated +variety, called _grandiflorum_, which is esteemed for its brilliancy and +long succession of golden bloom. This variety has larger flower-heads, +surrounded with a fuller border of ray-florets. The species belongs to a +genus many species of which have produced [494] double varieties. One of +them is the Japanese marigold, others are the _carinatum_ and the +_imbricatum_ species. Nearly allied are quite a number of garden-plants +with double flower-heads, among which are the double camomiles. + +My attention was first drawn to the structure of the heads and +especially to the number of the ray-florets of the corn-marigold. The +species appertains to that group of composites which have a head of +small tubular florets surrounded by a broad border of rays. These rays, +when counted, are observed to occur in definite numbers, which are +connected with each other by a formula, known as "the series" of Braun +and Schimper. In this formula, which commences with 1 and 2, each number +is equal to the sum of the two foregoing figures. Thus 5, 8 and 13 are +very frequent occurrences, and the following number, 21, is a most +general one for apparently full rays, such as in daisies, camomiles, +_Arnica_ and many other wild and cultivated species. + +These numbers are not at all constant. They are only the averages, +around which the real numbers fluctuate. There may even be an +overlapping of the extremes, since the fluctuation around 13 may even go +beyond 8 and 21, and so on. But such extremes are only found in stray +flowers, occurring on the same [495] individuals with the lesser degrees +of deviation. + +Now the marigold averages 13, and the _grandiflorum_ 21 rays. The wild +species is pure in this respect, but the garden-variety is not. The +seeds which are offered for sale usually contain a mixture of both forms +and their hybrids. So I had to isolate the pure types from this mixture +and to ascertain their constancy and mutual independency. To this end I +isolated from the mixture first the 13-rayed, and afterwards the +21-rayed types. As the marigolds are not sufficiently self-fertile, and +are not easily pollinated artificially, it seemed impossible to carry on +these two experiments at the same time and in the same garden. I devoted +the first three years to the lower form, isolated some individuals with +12-13 rays out of the mixture of 1892 and counted the ray-florets on the +terminal head of every plant of the ensuing generation next year. I +cultivated and counted in this way above 150 individuals and found an +average of exactly 13 with comparatively few individuals displaying 14 +or only 12 rays, and with the remainder of the plants grouped +symmetrically around this average. I continued the experiment for still +another year and found the same group of figures. I was then satisfied +as to the purity of the isolated strain. Next year I sowed a new mixture +in [496] order to isolate the reputed pure _grandiflorum_ type. During +the beginning of the flowering period I ruthlessly threw away all plants +displaying less than 21 rays in the first or terminal head. But this +selection was not to be considered as complete, because the 13-rayed +race may eventually transgress its boundary and come over to the 21 and +more. This made a second selection necessary. On the selected plants all +the secondary heads were inspected and their ray-florets counted. Some +individuals showed an average of about 13 and were destroyed. Others +gave doubtful figures and were likewise eliminated, and only 6 out of a +lot of nearly 300 flowering plants reached an average of 21 for all of +the flowers. + +Our summer is a short one, compared with the long and beautiful summer +of California, and it was too late to cut off the faded and the open +flowers, and await new ones, which might be purely fertilized after the +destruction of all minor plants. So I had to gather the seed from +flowers, which might have been partially fertilized by the wrong pollen. +This however, is not so great a drawback in selection experiments as +might be supposed at first sight. The selection of the following year is +sure to eliminate the offspring of such impure parentage. + +[497] A far more important principle is that of the hereditary +percentage, already discussed in our lecture on the selection of +monstrosities. In our present case it had to be applied only to the six +selected plants of 1895. To this end the seeds of each of them were sown +separately, the ray-florets of the terminal heads of each of the new +generation were counted, and curves and averages were made up for the +six groups. Five of them gave proof of still being mixtures and were +wholly rejected. The children of the sixth parent, however, formed a +group of uniform constitution, all fluctuating around the desired +average of 21. All in all the terminal heads of over 1,500 plants have +been subjected to the somewhat tedious work of counting their +ray-florets. And this not in the laboratory, but in the garden, without +cutting them off. Otherwise it would obviously have been impossible to +recognize the best plants for preservation. I chose only two plants +which in addition recommended themselves by the average number of rays +on their secondary heads, sowed their seeds next year separately and +compared the numerical constitution of their offspring. Both groups +averaged 21 and were distributed very symmetrically around this mean. +This result [498] showed that no further selection could be of any +avail, and that I had succeeded in purifying the 21-rayed _grandiflorum_ +variety. + +It is from this _grandiflorum_ that I have finally produced my double +variety. In the year 1896 I selected from among the above quoted 1,500 +plants, 500 with terminal heads bearing 21 or more rays. On these I +counted the rays of all the secondary heads about the middle of August +(1896) and found that they had, as a rule, retrograded to lower figures. +On many thousands of heads only two were found having 22 rays. All +others had the average number of 21 or even less. I isolated the +individual which bore these two heads, allowed them to be fertilized by +insects with the pollen of some of the best plants of the same group, +but destroyed the remainder. + +This single exceptional plant has been the starting point of my double +variety. It was not remarkable for its terminal head, which exhibited +the average number of rays of the 21-rayed race. Nor was it +distinguished by the average figure for all its heads. It was only +selected because it was the one plant which had some secondary heads +with one ray more than all the others. This indication was very slight, +and could not have been detected save by the counting of the rays of +thousands of heads. + +[499] But the rarity of the anomaly was exactly the indication wanted, +and the same deviation would have had no signification whatever, had it +occurred in a group fluctuating symmetrically around the average figure. +On the other hand, the observed anomaly was only an indication, and no +guarantee of future developments. + +Here it should be remarked that the indication alluded to was not the +appearance of the expected character of doubling in ever so slight a +measure. It was only a guide to be followed in further work. The real +character of double flower-heads among composites lies in the production +of rays on the disk. No increase of the number of the outer rays can +have the same significance. A hasty inspection of double flower heads +may convey the idea that all rays are arranged around a little central +cluster of disk-florets, the remainder of the original disk-florets but +a closer investigation will always reveal the fallacy of this +conclusion. Hidden between the inner rays, and covered by them, lie the +little tubular and fertile florets everywhere on the disk. They may not +be easily seen, but if the supernumerary rays are pulled out, the disk +may be seen to bear numerous small florets at intervals. But these +intervals are not at all numerous, showing thereby that only a +relatively small number of tubes has been [500] converted into rays. +This conversion is obviously the true mark of the doubling, and before +traces of it are found, no assertion whatever can be given as to the +issue of the pedigree experiment. + +Three more years were required before this first, but decisive trace was +discovered. During these years I subjected my strain to the same sharp +selection as has already been described. The chosen ancestor of the race +had flowered in 1896, and the next year I sowed its seeds only. From +this generation I chose the one plant with the largest number of rays in +its terminal head, and repeated this in the following year. + +The consequence was that the average number of rays increased rapidly, +and with it the absolute maximum of the whole strain. The average came +up from 21 to 34. Brighter and brighter crowns of the yellow rays +improved my race, until it became difficult and very time consuming to +count all the large rays of the borders. The largest numbers determined +in the succeeding generations increased by leaps from 21 to 34 in the +first year, and thence to 48 and 66 in the two succeeding summers. Every +year I was able to save enough seed from the very best plant and to use +it only for the continuance of the race. Before the selected plants were +allowed to open the flowers from which the seed [501] was to be +gathered, nearly the whole remaining culture was exterminated, excepting +only some of the best examples, in order to have the required material +for cross-pollination by insects. Each new generation was thereby as +sharply selected as possible with regard to both parents. + +All flower-heads were of course closely inspected. Not the slightest +indication of real doubling was discovered, even in the summer of 1899 +in the fourth generation of my selected race. But among the best the new +character suddenly made its appearance. It was at the commencement of +September (1899), too late to admit of the seeds ripening before winter. +An inspection of the younger heads was made, which revealed three heads +with some few rays in the midst of the disk on one plant, the result of +the efforts of four years. Had the germ of the mutation lain hidden +through all this time? Had it been present, though dormant in the +original sample of seed? Or had an entirely new creation taken place +during my continuous endeavors? Perhaps as their more or less immediate +result? It is obviously impossible to answer these questions, before +further and similar experiments shall have been performed, bringing to +light other details that will enable us to reach a more definite +conclusion. + +[502] The fact that the origination of such forms is accessible to +direct investigation is proven quite independently of all further +considerations. The new variety came into existence at once. The leap +may have been made by the ancestor of the year 1895, or by the plant of +1899, which showed the first central rays, or the sport may have been +gradually built up during those four years. In each case there was a +leap, contrasting with the view which claims a very long succession of +years for the development of every new character. + +Having discovered this first trace of doubling, it was to be expected +that the new variety would be at once as pure and as rich as other +double composites usually are. Some effect of the crossing with the +other seed-bearing individuals might still disturb this uniformity in +the following year, but another year's work would eliminate even this +source of impurity. + +These two years have given the expected result. The average number of +the rays, which had already arisen from 13 to 34 now at once came up to +47 and 55, the last figure being the sum of 21 and 34 and therefore the +probable uttermost limit to be reached before absolute doubling. The +maximum numbers came as high as 100 in 1900, and reached even 200 in +1901. Such heads are as completely double as are the [503] brightest +heads of the most beautiful double commercial varieties of composites. +Even the best white camomiles (_Chrysanthemum inodorum_) and the +gold-flowers or garden-marigolds (_Calendula officinalis_) do not come +nearer to purity since they always have scores of little tubular florets +between the rays on their disks. + +Real atavists or real reversionists were seen no more after the first +purification of the race. I have continued my culture and secured last +summer (1903) as many and as completely doubled heads as previously. The +race has at once become permanent and constant. It has of course a wide +range of fluctuating variability, but the lower limit has been worked up +to about 34 rays, a figure never reached by the _grandiflorum_ parent, +from which my new variety is thus sharply separated. + +Unfortunately the best flowers and even the best individuals of my race +are wholly barren. Selection has reached its practical limit. Seeds must +be saved from less dense heads, and no way has been found of avoiding +it. The ray-florets are sterile, even in the wild species, and when +growing in somewhat large numbers on the disk, they conceal the fertile +flowers from the visiting insects, and cause them also to be sterile. +The same is the case with the best cultivated forms. Their showiest +individuals are [504] barren, and incapable of the reproduction of the +race. + +This last is therefore, of necessity, always continued by means of +individuals whose deviation from the mean average is the least. But in +many cases the varieties are so highly differentiated that selection has +become quite superfluous for practical purposes. I have already +discussed the question as to the actual moment, in which the change of +the _grandiflorum_ variety into the new _plenum_ form must be assumed to +have taken place. In this respect some stress is to be laid on the fact +that the improvement through selection has been gradual and continuous, +though very rapid from the first moment. But with the appearance of the +first stray rays within the disk, this continuity suddenly changed. All +the children of this original mutated plant showed the new character, +the rays within the disk, without exception. Not on all the heads, nor +even on the majority of the heads on some individuals, but on some heads +all gave clear proof of the possession of the new attribute. This was +present in all the representatives of the new race, and had never been +seen in any of their parents and grandparents. Here there was evidently +a sudden leap, at least in the external form of the plants. And it seems +to me to be the most simple conception, [505] that this visible leap +directly corresponded to that inner change, which brought about the +complete inheritability of the new peculiarity. It is very interesting +to observe how completely my experience agrees with the results of the +observations of breeders at large. No doubt a comparison is difficult, +and the circumstances are not adequate to a close study. + +Isolation and selection have been applied commonly only so far as was +consistent with the requirements of practical horticulture, and of +course a determination of the hereditary percentage was never made. The +disregard of this feature made necessary a greater length of time and a +larger number of generations to bring about the desired changes. +Notwithstanding this, however, it has been seen that double varieties +are produced suddenly. This may have occurred unexpectedly or after a +few years' effort toward the end desired. Whether this sudden appearance +is the consequence of a single internal differentiating step, or of the +rapid succession of lesser changes, cannot yet be made out. The extreme +variability of double flowers and the chance of their appearance with +only slight indications of the previous petaloid alterations of a few +stamens may often result in their origin being overlooked, while +subsequent generations may come in for full notice. [506] In the greater +number of cases recorded it remains doubtful whether the work said to be +done to obtain a new double variety was done before the appearance of +these preliminary indications or afterward. + +In the first case, it would correspond with our selection of large +numbers of florets in the outer rays, in the second however, with the +ordinary purification of new races from hybrid mixtures. + +In scientific selection-experiments such crosses are of course avoided, +and the process of purification is unnecessary, even as in the +_Chrysanthemum_ culture. The first generation succeeding the original +plant with disk-rays was in this respect wholly uniform and true to the +new type. + +In practice the work does not start from such slight indications, and is +done with no other purpose in view than to produce double flowers in +species in which they did not already exist. Therefore it is of the +highest importance to know the methods used and the chances of success. +Unfortunately the evidence is very scanty on both points. + +Lindley and other writers, on horticultural theory and practice assert +that a large amount of nourishment tends to produce double flowers, +while a culture under normal conditions, [507] even if the plants are +very strong and healthy, has no such effect. But even here it remains +doubtful whether it applies to the period before or after the internal +mutation. On the other hand success is not at all to be relied upon, nor +is the work to be regarded as easy. The instances of double flowers said +to be obtainable at will, are too rare in comparison with the number of +cases, where the first indication of them was found accidentally. + +Leaving all these doubtful points, which will have to be cleared up by +further scientific investigation, the high degree of variability +requires further discussion. It may be considered from three different +points of view according to the limit of the deviation from the average, +to the dependency on external conditions and to periodicity. It seems +best to take up the last two points first. + +On a visit to a nursery at Erfurt I once inspected an experiment with a +new double variety of the common blue-bottle or blue corn-flower. The +plants were dependent on the weather to a high degree. Bad weather +increased the number of poorly filled flower-heads, while warm and sunny +days were productive of beautiful double flowers. The heads that are +borne by strong branches have a greater tendency to become double than +those of the weaker ones, [508] and towards the autumn, when all those +of the first group are faded away, and only a weak though large section +of the heads is still flowering, the whole aspect of the variety +gradually retrogrades. The same law of dependency and periodicity is +prevalent everywhere. In my own cultures of the improved field-marigold +I have observed it frequently. The number of the ray-florets may be +considered as a direct response to nourishment, both when this is +determined by external circumstances, and when it depends on the +particular strength of the branch, which bears the head in question. It +is a case exactly similar to that of the supernumerary carpels of the +pistilloid poppy, and the deductions arrived at with that variety may be +applied directly to double flowers. + +This dependency upon nourishment is of high practical importance in +combination with the usual effect of the doubling which makes the +flowers sterile. It is a general rule that the most perfect flowers do +not produce seed. At the height of the flowering period the external +circumstances are the most favorable, and the flowering branches still +constitute the stronger axes of the plants. Hence we may infer that +sterility will prevail precisely in this period. Many varieties are +known to yield only seeds from the very last flowers, as for instance +some [509] double begonias. Others bear only seed on their weaker +lateral branches, as the double camomile, or become fertile only towards +the fall, as is often the case with the above quoted Erfurt variety of +the blue-bottle. As far as I have been able to ascertain, such seeds are +quite adequate for the reproduction and perpetuation of the double +varieties, but the question whether there are differences between the +seeds of the more or less double flowers of the same plants still +remains open. It is very probable, from a theoretical point of view, +that such differences exist, but perhaps they are so slight, as to have +practically no bearing on the question. + +On the ground of their wide range of variability, the double varieties +must be regarded as pertaining to the group of ever-sporting forms. On +one side they fluctuate in the direction towards such petalomanous +flowers as are borne by the stocks and others, which we have previously +discussed. Here no trace of the fertile organs is left. But this extreme +is never reached by petaloid double flowers. A gap remains which, often +overlooked, always exists, and which sharply separates the two types. On +the other hand the alteration of the stamens gradually relapses to +perfectly single flowers. Here the analogy with the pistillody of the +poppies and with the "five-leaved" clover is obvious. + +[510] This conception of the inner nature of double flowers explains the +fact that the varietal mark is seldom seen to be complete throughout +larger groups of individuals, providing these have not been already +selected by this character. _Tagetes africana_ is liable to produce some +poorly filled specimens, and some double varieties of carnations are +offered for sale with the note that the seed yields only 80% of doubles. +With _Chrysanthemum coronarium_ and blue-bottles this figure is often +announced to be only about 50%. No doubt it is partly due to impurities, +caused by vicinism, but it is obviously improbable that the effect of +these impurities should be so large. + +Some cases of partial reversion may be interpreted in the same way. +Among the garden anemones, _Anemone coronaria_, there is a variety +called the "Bride," on account of its pure white dowers. It is for sale +with single and with double flowers, and these two forms are known to +sport into one another, although they are multiplied in the vegetative +way. Such cases are known to be of quite ordinary occurrence. Of course +such sports must be considered as partial, and the same stem may bear +both types of flowers. It even happens that some particular flower is +partly double and partly single. Mr. Krelage, of Haarlem, had the +kindness to [511] send me such a curious flower. One half of it was +completely double, while the other half was entirely single, bearing +normal and fertile stamens in the ordinary number. + +The same halfway doubling is recorded to occur among composites +sometimes, and from the same source I possess in my collection a head of +_Pyrethrum roseum_, bearing on half of its disk elongated corolla tubes, +and on the other half the small disk-florets of the typical species. + +It is a current belief, that varieties are improved by continued +culture. I have never been able to ascertain the grounds on which this +conviction rests. It may be referred either to the purity of the race or +to the complete development of the varietal character. In the first case +it is a question of hybrid mixtures from which many young varieties must +be freed before being placed on the market. But as we have already seen +in a former lecture, this requires only three or four years, and +afterwards the degree of purity is kept up to the point which proves to +be the most suitable for practical purposes. The complete development of +the varietal character is a question restricted to ever-sporting +varieties, since in white flowers and other constant varieties this +degree is variable in a very small and unimportant measure. [512] Hence +the double flowers seem to afford a very good example for this +discussion. + +It can be decided by two facts. First by a consideration of the oldest +double varieties, and secondly by that of the very youngest. Are the +older ones now in a better condition than at the outset? Have they +really been gradually improved during the centuries of their existence? +Obviously this can only be answered by a comparison of the figures given +by older writers, with the varieties as they are now in culture. +Munting's drawings and descriptions are now nearly two centuries and a +half old, but I do not find any real difference between his double +varieties and their present representatives. So it is in other cases in +which improvements by crossing or the introduction of new forms does not +vitiate the evidence. Double varieties, as a rule, are exactly the same +now, as they were at the time of their first introduction. + +If this were otherwise one would expect that young double varieties +should in the main display only slight grades of the anomaly, and that +they would require centuries to reach their full development. Nothing of +the kind is on record. On the contrary the newest double sorts are said +to be not only equal to their predecessors, but to excel them. As a rule +such claims may be exaggerated, but not to any great extent. [513] This +is proven in the simplest way by the result of our own experiment. + +In the double field-marigold we have the very first generation of a +variety of pure and not hybrid origin. It shows the new attribute in its +full development. It has flower-heads nearly as completely filled as the +best double varieties of allied cultivated composites. In the second +generation it reached heads with 200 rays each, and much larger numbers +will seldom be seen in older species on heads of equal size. I have +compared my novelty with the choicest double camomiles and others, but +failed to discover any real difference. Improvement of the variety +developed in the experiments carried on by myself seems to be excluded +by the fact that it comes into conflict with the same difficulty that +confronts the older cultivated species, viz.: the increasing sterility +of the race. + +It is perfectly evident that this double marigold is now quite constant. +Continuously varying about a fixed average it may live through +centuries, but the mean and the limits will always remain the same, as +in the case of the ever-sporting varieties. + +Throughout this lecture I have spoken of double flowers and double +flower-heads of composites as of one single group. They are as nearly +related from the hereditary point of [514] view, as they are divergent +in other respects. It would be superfluous to dwell any longer upon the +difference between heads and flowers. But it is as well to point out, +that the term double flowers indicates a motley assemblage of different +phenomena. The hen-and-chicken daisy, and the corresponding variety of +the garden cineraria (_Cineraria cruenta_), are extremes on one side. +The hen-and-chicken type occurs even in other families and is known to +produce most curious anomalies, as with _Scabiosa_, the supernumerary +heads of which may be produced on long stalks and become branched +themselves in the same manner. + +Petalody of the stamens is well known to be the ordinary type of +doubling. But it is often accompanied by a multiplication of the organs, +both of the altered stamens and of the petals themselves. This +proliferation may consist in median or in lateral cleavages, and in both +cases the process may be repeated one or more times. It would be quite +superfluous to give more details, which may be gathered from any +morphologic treatise on double flowers. But from the physiologic point +of view all these cases are to be considered as one large group, +complying with previously given definitions of the ever-sporting +varieties. They are very variable and wholly permanent. Obviously this +[515] permanency agrees perfectly with the conception of their sudden +origin. + + +[516] + +LECTURE XVIII + +NEW SPECIES OF _OENOTHERA_ + +In our experiments on the origin of peloric varieties and double flowers +we were guided in the choice of our material by a survey of the evidence +already at hand. We chose the types known to be most commonly produced +anew, either in the wild state or under the conditions of cultivation. +In both instances our novelty was a variety in the ordinary sense of the +word. Our pedigree-culture was mainly an experimental demonstration of +the validity of conclusions, which had previously been deduced from such +observations as can be made after the accidental birth of new forms. + +From these facts, and even from these pedigree-experiments, it is +scarcely allowable to draw conclusions as to the origin of real species. +If we want to know how species originate, it is obviously necessary to +have recourse to direct observation. The question is of the highest +importance, both for the theory of descent, and for our conception of +the real nature of [517] systematic affinities at large. Many authors +have tried to solve it on the ground of comparative studies and of +speculations upon the biologic relations of plants and animals. But in +vain. Contradiction and doubt still reign supreme. All our hopes now +rest on the result of experiments. + +Unfortunately such experiments seemed simply impossible a few years ago. +What is to guide us in the choice of the material? The answer may only +be expected from a consideration of elementary species. For it is +obvious that they only can be observed to originate, and that the +systematic species, because they are only artificial groups of lower +unities, can never become the subject of successful experimental +inquiry. + +In previous lectures we tried to clear up the differences existing +between nearly related elementary species. We have seen that they affect +all of the attributes of the plants, each of them changing in some +measure all of the organs. Nevertheless they were due to distinct +unities and of the lowest possible degree. Such unit-steps may therefore +be expected to become visible some time or other by artificial means. On +the other hand, mutations as a rule make their appearance in groups, and +there are many systematic species which on close inspection [518] have +been shown to be in reality composite assemblages. Roses and brambles, +hawkweeds and willows are the best known examples. Violets and _Draba +verna_, dandelions and helianthemums and many other instances were dealt +with in previous lectures. Even wheat and barley and corn afford +instances of large groups of elementary species. Formerly mixed in the +fields, they became separated during the last century, and now +constitute constant races, which, for brevity's sake, are dealt with +under the name of varieties. + +In such groups of nearly allied forms the single members must evidently +be of common origin. It is not necessary for them to have originated all +in the same place or at the same time. In some cases, as with _Draba +verna_, the present geographic distribution points to a common +birthplace, from whence the various forms may about the same period have +radiated in all directions. The violets on the other hand seem to +include widely diffused original forms, from which branches have started +at different times and in different localities. + +The origin of such groups of allied forms must therefore be the object +of our research. Perhaps we might find a whole group, perhaps only part +of it. In my opinion we have the right to assume that if _Draba_ and +violets and [519] others have formerly mutated in this way, other +species must at present be in the same changeable condition. And if +mutations in groups, or such periodic mutations should be the rule, it +is to be premised that these periods recur from time to time, and that +many species must even now be in mutating condition, while others are +not. + +It is readily granted that the constant condition of species is the +normal one, and that mutating periods must be the exception. This fact +does not tend to increase our prospect of discovering a species in a +state of mutability. Many species will have to be tested before finding +an instance. On the other hand, a direct trial seems to be the only way +to reach the goal. No such special guides as those that led us to the +choice of pelories and double flowers are available. The only indication +of value is the presumption that a condition of mutability might be +combined with a general state of variability at large, and that groups +of plants of very uniform features might be supposed to be constant in +this respect too. On the contrary, anomalies and deviations if existent +in the members of one strain, or found together in one native locality +of a species, might be considered as an indication in the desired +direction. + +Few plants vary in the wild state in such a [520] measure as to give +distinct indications. All have to be given a trial in the garden under +conditions as similar as possible to their natural environments. +Cultivated plants are of course to be excluded. Practically they have +already undergone the experience in question and can not be expected to +change their habits soon enough. Moreover they are often of hybrid +origin. The best way is to experiment with the native plants of one's +own country. + +I have made such experiments with some hundred species that grow wild in +Holland. Some were very variable, as for instance, the jointed charlock +(_Raphanus Raphanistrum_) and the narrow-leaved plantain (_Plantago +lanceolata_). Others seemed more uniform, but many species, collected +without showing any malformation, subsequently produced them in my +garden, either on the introduced plants themselves or among their +offspring. From this initial material I have procured a long series of +hereditary races, each with some peculiar anomaly for its special +character. But this result was only a secondary gain, a meager +consolation for the negative fact that no real mutability could be +discovered. + +My plants were mostly annuals or biennials, or such perennials as under +adequate treatment might produce flowers and seeds during their [521] +first summer. It would be of no special use to enumerate them. The +negative result does not apply to the species as such, but only to the +individual strain, which I collected and cultivated. Many species, which +are quite constant with us, may be expected to be mutable in other parts +of their range. + +Only one of all my tests met my expectations. This species proved to be +in a state of mutation, producing new elementary forms continually, and +it soon became the chief member of my experimental garden. It was one of +the evening primroses. + +Several evening-primroses have at different times been introduced into +European gardens from America. From thence they have spread into the +vicinity, becoming common and exhibiting the behavior of indigenous +types. _Oenothera biennis_ was introduced about 1614 from Virginia, or +nearly three centuries ago. _O. muricata_, with small corollas and +narrow leaves, was introduced in the year 1789 by John Hunneman, and _O. +suaveolens_, or sweet-scented primrose, a form very similar to the +_biennis_, about the same time, in 1778, by John Fothergill. This form +is met with in different parts of France, while the _biennis_ and +_muricata_ are very common in the sandy regions of Holland, where I have +observed them for [522] more than 40 years. They are very constant and +have proven so in my experiments. Besides these three species, the +large-flowered evening-primrose, or _Oenothera lamarckiana_, is found in +some localities in Holland and elsewhere. We know little concerning its +origin. It is supposed to have come from America in the same way as its +congeners, but as yet I have not been able to ascertain on what grounds +this supposition rests. As far as I know, it has not been seen growing +wild in this country, though it may have been overlooked. The fact that +the species of this group are subject to many systematic controversies +and are combined by different writers into systematic species in +different ways, being often considered as varieties of one or two types, +easily accounts for it having been overlooked. However, it would be of +great interest to ascertain whether _O. lamarckiana_ yet grows in +America, and whether it is in the same state of mutability here as it is +in Holland. + +The large-flowered evening-primrose was also cultivated about the +beginning of the last century in the gardens of the Museum d'Histoire +Naturelle, at Paris, where it was noticed by Lamarck, who at once +distinguished it as an undescribed species. He wrote a complete +description [523] of it and his type specimens are still preserved in +the herbarium of the Museum, where I have compared them with the plants +of my own culture. Shortly afterwards it was renamed by Seringe, in +honor of its eminent discoverer, whose name it now bears. So Lamarck +unconsciously discovered and described himself the plant, which after a +century, was to become the means of an empirical demonstration of his +far-reaching views on the common origin of all living beings. + +_Oenothera lamarckiana_ is considered in Europe as a garden-plant, much +prized for parks and ornamental planting. It is cultivated by +seed-merchants and offered for sale. It has escaped from gardens, and +having abundant means for rapid multiplication, has become wild in many +places. As far as I know its known localities are small, and it is to be +presumed that in each of them the plant has escaped separately from +culture. It was in this state that I first met with this beautiful +species. + +Lamarck's evening-primrose is a stately plant, with a stout stem, +attaining often a height of 1.6 meters and more. When not crowded the +main stem is surrounded by a large circle of smaller branches, growing +upwards from its base so as often to form a dense bush. These branches +in their turn have numerous lateral [524] branches. Most of them are +crowned with flowers in summer, which regularly succeed each other, +leaving behind them long spikes of young fruits. The flowers are large +and of a bright yellow color, attracting immediate attention, even from +a distance. They open towards evening, as the name indicates, and are +pollinated by humble-bees and moths. On bright days their duration is +confined to one evening, but during cloudy weather they may still be +found open on the following morning. Contrary to their congeners they +are dependent on visiting insects for pollination. _O. biennis_ and _O. +muricata_ have their stigmas in immediate contact with the anthers +within the flower-buds, and as the anthers open in the morning preceding +the evening of the display of the petals, fecundation is usually +accomplished before the insects are let in. But in _O. lamarckiana_ no +such self-fertilization takes place. The stigmas are above the anthers +in the bud, and as the style increases in length at the time of the +opening of the corolla, they are elevated above the anthers and do not +receive the pollen. Ordinarily the flowers remained sterile if not +visited by insects or pollinated by myself, although rare instances of +self-fertilization were seen. + +In falling off, the flowers leave behind them a stout ovary with four +cells and a large number [525] of young seeds. The capsule when ripe, +opens at its summit with four valves, and contains often from two to +three hundred seeds. A hundred capsules on the main stem is an average +estimate, and the lateral branches may ripen even still more fruits, by +which a very rapid dissemination is ensured. + +This striking species was found in a locality near Hilvers, in the +vicinity of Amsterdam, where it grew in some thousands of individuals. +Ordinarily biennial, it produces rosettes in the first, and stems in the +second year. Both the stems and the rosettes were at once seen to be +highly variable, and soon distinct varieties could be distinguished +among them. + +The first discovery of this locality was made in 1886. Afterwards I +visited it many times, often weekly or even daily during the first few +years, and always at least once a year up to the present time. This +stately plant showed the long-sought peculiarity of producing a number +of new species every year. Some of them were observed directly on the +field, either as stems or as rosettes. The latter could be transplanted +into my garden for further observation, and the stems yielded seeds to +be sown under like control. Others were too weak to live a sufficiently +long time in the field. They were discovered by sowing seed from +indifferent plants [526] of the wild locality in the garden. A third and +last method of getting still more new species from the original strain, +was the repetition of the sowing process, by saving and sowing the seed +which ripened on the introduced plants. These various methods have led +to the discovery of over a dozen new types, never previously observed or +described. + +Leaving the physiologic side of the relations of these new forms for the +next lecture, it would be profitable to give a short description of the +several novelties. To this end they may be combined under five different +heads, according to their systematic value. The first head includes +those which are evidently to be considered as varieties, in the narrower +sense of the word, as previously given. The second and third heads +indicate the real progressive elementary species, first those which are +as strong as the parent-species, and secondly a group of weaker types, +apparently not destined to be successful. Under the fourth head I shall +include some inconstant forms, and under the last head those that are +organically incomplete. + +Of varieties with a negative attribute, or real retrograde varieties, I +have found three, all of them in a flowering condition in the field. I +have given them the names of _laevifolia_, _brevistylis_ and _nanella_. + +[527] The _laevifolia_, or smooth-leaved variety, was one of the very +first deviating types found in the original field. This was in the +summer of 1887, seventeen years ago. It formed a little group of plants +growing at some distance from the main body, in the same field. I found +some rosettes and some flowering stems and sowed some seed in the fall. +The variety has been quite constant in the field, neither increasing in +number of individual plants nor changing its place, though now closely +surrounded by other _Lamarckiana_s. In my garden it has proved to be +constant from seed, never reverting to the original _lamarckiana_, +provided intercrossing was excluded. + +It is chiefly distinguished from Lamarck's evening-primrose by its +smooth leaves, as the name indicates. The leaves of the original form +show numerous sinuosities in their blades, not at the edge, but anywhere +between the veins. The blade shows numbers of convexities on either +surface, the whole surface being undulated in this manner; it lacks also +the brightness of the ordinary evening-primrose or _Oenothera biennis_. + +These undulations are lacking or at least very rare on the leaves of the +new _laevifolia_. Ordinarily they are wholly wanting, but at times +single leaves with slight manifestations of this [528] character may +make their appearance. They warn us that the capacity for such +sinuosities is not wholly lost, but only lies dormant in the new +variety. It is reduced to a latent state, exactly as are the apparently +lost characters of so many ordinary horticultural varieties. + +Lacking the undulations, the _laevifolia_ leaves are smooth and bright. +They are a little narrower and more slender than those of the +_lamarckiana_. The convexities and concavities of leaves are said to be +useful in dry seasons, but during wet summers, such as those of the last +few years, they must be considered as very harmful, as they retain some +of the water which falls on the plants, prolonging the action of the +water on the leaves. This is considered by some writers to be of some +utility after slight showers, but was observed to be a source of +weakness during wet weather in my garden, preventing the leaves from +drying. Whether the _laevifolia_ would do better under such +circumstances, remains to be tested. + +The flowers of the _laevifolia_ are also in a slight degree different +from those of _lamarckiana_. The yellow color is paler and the petals +are smoother. Later, in the fall, on the weaker side branches these +differences increase. The _laevifolia_ petals become smaller and are +often not emarginated at the apex, becoming ovate [529] instead of +obcordate. This shape is often the most easily recognized and most +striking mark of the variety. In respect to the reproductive organs, the +fertility and abundance of good seed, the _laevifolia_ is by no means +inferior or superior to the original species. + +_O. brevistylis_, or the short-styled evening primrose, is the most +curious of all my new forms. It has very short styles, which bring the +stigmas only up to the throat of the calyx tube, instead of upwards of +the anthers. The stigmas themselves are of a different shape, more +flattened and not cylindrical. The pollen falls from the anthers +abundantly on them, and germinates in the ordinary manner. + +The ovary which in _lamarckiana_ and in all other new forms is wholly +underneath the calyx-tube, is here only partially so. This tube is +inserted at some distance under its summit. The insertion divides the +ovary into two parts: an upper and a lower one. The upper part is much +reduced in breadth and somewhat attenuated, simulating a prolongation of +the base of the style. The lower part is also reduced, but in another +manner. At the time of flowering it is like the ovary of _lamarckiana_, +neither smaller nor larger. But it is reached by only a very few +pollen-tubes, and is therefore always incompletely fertilized. It does +[530] not fall off after the fading away of the flower, as unfertilized +ovaries usually do; neither does it grow out, nor assume the upright +position of normal capsules. It is checked in its development, and at +the time of ripening it is nearly of the same length as in the +beginning. Many of them contain no good seeds at all; from others I have +succeeded in saving only a hundred seeds from thousands of capsules. + +These seeds, if purely pollinated, and with the exclusion of the visits +of insects, reproduce the variety, entirely and without any reversion to +the _lamarckiana_ type. + +Correlated with the detailed structures is the form of the flower-buds. +They lack the high stigma placed above the anthers, which in the +_lamarckiana_, by the vigorous growth of the style, extends the calyx +and renders the flower bud thinner and more slender. Those of the +_brevistylis_ are therefore broader and more swollen. It is quite easy +to distinguish the individuals by this striking character alone, +although it differs from the parent in other particulars. + +The leaves of the _O. brevistylis_ are more rounded at the tip, but the +difference is only pronounced at times, slightly in the adult rosettes, +but more clearly on the growing summits of the stems and branches. By +this character, the plants [531] may be discerned among the others, some +weeks before the flowers begin to show themselves. But the character by +which the plants may be most easily recognized from a distance in the +field is the failure of the fruits. They were found there nearly every +year in varying, but always small numbers. + +Leaving the short-styled primrose, we come now to the last of our group +of retrograde varieties. This is the _O. nanella_, or the dwarf, and is +a most attractive little plant. It is very short of stature, reaching +often a height of only 20-30 cm., or less than one-fourth of that of the +parent. It commences flowering at a height of 10-15 cm., while the +parent-form often measures nearly a meter at this stage of its +development. Being so very dwarfed the large flowers are all the more +striking. They are hardly inferior to those of the _lamarckiana_, and +agree with them in structure. When they fade away the spike is rapidly +lengthened, and often becomes much longer than the lower or vegetative +part of the stem. + +The dwarfs are one of the most common mutations in my garden, and were +observed in the native locality and also grown from seeds saved there. +Once produced they are absolutely constant. I have tried many thousands +of seeds from various dwarf mutants, and never observed [532] any trace +of reversion to the _lamarckiana_ type. I have also cultivated them in +successive generations with the same result. In a former lecture we have +seen that contrary to the general run of horticultural belief, varieties +are as constant as the best species, if kept free from hybrid +admixtures. This is a general rule, and the exceptions, or cases of +atavism are extremely rare. In this respect it is of great interest to +observe that this constancy is not an acquired quality, but is to be +considered as innate, because it is already fully developed at the very +moment when the original mutation takes place. + +From its first leaves to the rosette period, and through this to the +lengthening of the stem, the dwarfs are easily distinguished from any +other of their congeners. The most remarkable feature is the shape of +the leaves. They are broader and shorter, and especially at the base +they are broadened in such a way as to become apparently sessile. The +stalk is very brittle, and any rough treatment may cause the leaves to +break off. The young seedlings are recognizable by the shape of the +first two or three leaves, and when more of them are produced, the +rosettes become dense and strikingly different from others. Later leaves +are more nearly like the parent-type, but the petioles remain short. The +bases of the blades are frequently [533] almost cordate, the laminae +themselves varying from oblong-ovate to ovate in outline. The stems are +often quite unbranched, or branched only at the base of the spike. +Strong secondary stems are a striking attribute of the _lamarckiana_ +parent, but they are lacking, or almost so in the dwarfs. The stem is +straight and short, and this, combined with the large crown of bright +flowers, makes the dwarfs eminently suitable for bed or border plants. +Unfortunately they are very sensitive, especially to wet weather. + +_Oenothera gigas_ and _O. rubrinervis_, or the giant, and the red-veined +evening-primroses, are the names given to two robust and stout species, +which seem to be equal in vigor to the parent-plant, while diverging +from it in striking characters. Both are true elementary species, +differentiated from _lamarckiana_ in nearly all their organs and +qualities, but not showing any preponderating character of a retrograde +nature. Their differences may be compared with those of the elementary +species of other genera, as for instance, of _Draba_, or of violets, as +will be seen by their description. + +The giant evening-primrose, though not taller in stature than _O. +lamarckiana_, deserves its name because it is so much stouter in all +respects. [534] The stems are robust, often with twice the diameter of +_lamarckiana_ throughout. The internodes are shorter, and the leaves +more numerous, covering the stems with a denser foliage. This shortness +of the internodes extends itself to the spike, and for this reason the +flowers and fruits grow closer together than on the parent-plant. Hence +the crown of bright flowers, opening each evening, is more dense and +more strikingly brilliant, so much the more so as the individual flowers +are markedly larger than those of the parents. In connection with these +characters, the flower-buds are seen to be much stouter than those of +_lamarckiana_. The fruits attain only half the normal size, but are +broader and contain fewer, but larger seeds. + +The _rubrinervis_ is in many respects a counterpart to the _gigasv, but +its stature is more slender. The spikes and flowers are those of the +_lamarckianav, but the bracts are narrower. Red veins and red streaks on +the fruits afford a striking differentiating mark, though they are not +absolutely lacking in the parent-species. A red hue may be seen on the +calyx, and even the yellow color of the petals is somewhat deepened in +the same way. Young plants are often marked by the pale red tinge of the +mid-veins, but in adult rosettes, or from lack of sunshine, this hue is +often very faint. + +[535] The leaves are narrow, and a curious feature of this species is +the great brittleness of the leaves and stems, especially in annual +individuals, especially in those that make their stem and flowers in the +first year. High turgidity and weak development of the mechanical and +supporting tissues are the anatomical cause of this deficiency, the +bast-fibers showing thinner walls than those of the parent-type under +the microscope. Young stems of _rubrinervis_ may be broken off by a +sharp stroke, and show a smooth rupture across all the tissues, while +those of _lamarckiana_ are very tough and strong. + +Both the giant and the red-veined species are easily recognized in the +rosette-stage. Even the very young seedlings of the latter are clearly +differentiated from the _lamarckiana_, but often a dozen leaves are +required, before the difference may be seen. Under such circumstances +the young plants must reach an age of about two months before it is +possible to discern their characters, or at least before these +characters have become reliable enough to enable us to judge of each +individual without doubt. But the divergencies rapidly become greater. +The leaves of _O. gigas_ are broader, of a deeper green, the blade more +sharply set off against the stalk, all the rosettes [536] becoming stout +and crowded with leaves. Those of _O. rubrinervis_ on the contrary are +thin, of a paler green and with a silvery white surface; the blades are +elliptic, often being only 2 cm. or less in width. They are acute at the +apex and gradually narrowed into the petiole. + +It is quite evident that such pale narrow leaves must produce smaller +quantities of organic food than the darker green and broad organs of the +_gigas_. Perhaps this fact is accountable partly, at least, for the more +robust growth of the giant in the second year. Perhaps also some +relation exists between this difference in chemical activity and the +tendency to become annual or biennial. The _gigas_, as a rule, produces +far more, and the _rubrinervis_ far less biennial plants than the +_lamarckiana_. Annual culture for the one is as unreliable as biennial +culture for the other. _Rubrinervis_ may be annual in apparently all +specimens, in sunny seasons, but _gigas_ will ordinarily remain in the +state of rosettes during the entire first summer. It would be very +interesting to obtain a fuller insight into the relation of the length +of life to other qualities, but as yet the facts can only be detailed as +they stand. + +Both of these stout species have been found [537] quite constant from +the very first moment of their appearance. I have cultivated them from +seed in large numbers, and they have never reverted to the +_lamarckiana_. From this they have inherited the mutability or the +capacity of producing at their turn new mutants. But they seem to have +done so incompletely, changing in the direction of more absolute +constancy. This was especially observed in the case of _rubrinervis_, +which is not of such rare occurrence as _O. gigas_, and which it has +been possible to study in large numbers of individuals. So for instance, +the "red-veins" have never produced any dwarfs, notwithstanding they are +produced very often by the parent-type. And in crossing experiments also +the red-veins gave proof of the absence of a mutative capacity for their +production. + +Leaving the robust novelties, we may now take up a couple of forms, +which are equally constants and differentiated from the parent species +in exactly the same manner, though by other characters, but which are so +obviously weak as to have no manifest chance of self maintenance in the +wild state. These are the whitish and the oblong-leaved +evening-primroses or the _Oenothera albida_ and _oblonga_. + +_Oenothera albida_ is a very weak species, with whitish, narrow leaves, +which are evidently incapable [538] of producing sufficient quantities +of organic food. The young seedling-plants are soon seen to lag behind, +and if no care is taken of them they are overgrown by their neighbors. +It is necessary to take them out, to transplant them into pots with +richly manured soil, and to give them all the care that should be given +to weak and sickly plants. If this is done fully grown rosettes may be +produced, which are strong enough to keep through the winter. In this +case the individual leaves become stronger and broader, with oblong +blades and long stalks, but retain their characteristic whitish color. + +In the second year the stems become relatively stout. Not that they +become equal to those of _lamarckiana_, but they become taller than +might have been expected from the weakness of the plants in the previous +stages. The flowers and racemes are nearly as large as those of the +parent-form, the fruits only a little thinner and containing a smaller +quantity of seed. From these seeds I have grown a second and a third +generation, and observed that the plants remain true to their type. + +_O. oblonga_ may be grown either as an annual, or as a biennial. In the +first case it is very slender and weak, bearing only small fruits and +few seeds. In the alternative case however, it [539] becomes densely +branched, bearing flowers on quite a number of racemes and yielding a +full harvest of seeds. But it always remains a small plant, reaching +about half the height of that of _lamarckiana_. + +When very young it has broader leaves, but in the adult rosettes the +leaves become very narrow, but fleshy and of a bright green color. They +are so crowded as to leave no space between them unoccupied. The +flowering spikes of the second year bear long leaf-like bracts under the +first few flowers, but those arising later are much shorter. Numerous +little capsules cover the axis of the spike after the fading away of the +petals, constituting a very striking differentiating mark. This species +also was found to be quite constant, if grown from pure seed. + +We have now given the descriptions of seven new forms, which diverge in +different ways from the parent-type. All were absolutely constant from +seed. Hundreds or thousands of seedlings may have arisen, but they +always come true and never revert to the original _O. lamarckiana_ type. +From this they have inherited the condition of mutability, either +completely or partly, and according to this they may be able to produce +new forms themselves. But this occurs only rarely, and combinations of +more than one [540] type in one single plant seem to be limited to the +admixture of the dwarf stature with the characters of the other new +species. + +These seven novelties do not comprise the whole range of the new +productions of my _O. lamarckiana_. But they are the most interesting +ones. Others, as the _O. semilata_ and the _O. leptocarpa_ are quite as +constant and quite as distinct, but have no special claims for a closer +description. Others again were sterile, or too weak to reach the adult +stage and to yield seeds, and no reliable description or appreciation +can be given on the ground of the appearance of a single individual. + +Contrasted with these groups of constant forms are three inconstant +types which we now take up. They belong to two different groups, +according to the cause of their inconstancy. In one species which I call +_O. lata_, the question of stability or instability must remain wholly +unsolved, as only pistillate flowers are produced, and no seed can be +fertilized save by the use of the pollen of another form, and therefore +by hybridization. The other head comprises two fertile forms, _O. +scintillans_ and _O. elliptica_, which may easily be fertilized with +their own pollen, but which gave a progeny only partly similar to the +parents. + +The _Oenothera lata_ is a very distinct form [541] which was found more +than once in the field, and recently (1902) in a luxuriant flowering +specimen. It has likewise been raised from seeds collected in different +years at the original station. It is also wholly pistillate. Apparently +the anthers are robust, but they are dry, wrinkled and nearly devoid of +contents. The inner wall of cells around the groups of pollen grow out +instead of being resorbed, partly filling the cavity which is left free +by the miscarriage of the pollen-grains. This miscarriage does not +affect all the grains in the same degree, and under the microscope a few +of them with an apparently normal structure may be seen. But the +contents are not normally developed, and I have tried in vain to obtain +fertilization with a large number of flowers. Only by +cross-fertilization does _O. lata_ produce seeds, and then as freely as +the other species when self-fertilized. Of course its chance of ever +founding a wild type is precluded by this defect. + +_O. lata_ is a low plant, with a limp stem, bent tips and branches, all +very brittle, but with dense foliage and luxuriant growth. It has bright +yellow flowers and thick flower-buds. But for an unknown reason the +petals are apt to unfold only partially and to remain wrinkled +throughout the flowering time. The stigmas are slightly divergent from +the normal type, [542] also being partly united with one another, and +laterally with the summit of the style, but without detriment to their +function. + +Young seedlings of _lata_ may be recognized by the very first leaves. +They have a nearly orbicular shape and are very sharply set off against +their stalk. The surface is very uneven, with convexities and +concavities on both sides. This difference is lessened in the later +leaves, but remains visible throughout the whole life of the plant, even +during the flowering season. Broad, sinuate leaves with rounded tips are +a sure mark of _O. lata_. On the summits of the stems and branches they +are crowded so as to form rosettes. + +Concerning inheritance of these characteristics nothing can be directly +asserted because of the lack of pollen. The new type can only be +perpetuated by crosses, either with the parent form or some other +mutant. I have fertilized it, as a rule, with _lamarckiana_ pollen, but +have often also used that from _nanella_ and others. In doing so, the +_lata_ repeats its character in part of its offspring. This part seems +to be independent of the nature of the pollen used, but is very variable +according to external circumstances. On the average one-fourth of the +offspring become _lata_, the others assuming the type of the +pollen-parent, if this was a _lamarckiana_ or [543] partly this type and +partly that of any other of the new species derived from _lamarckiana_, +that might have been used as the pollen-parent. This average seems to be +a general rule, recurring in all experiments, and remaining unchanged +through a long series of successive generations. The fluctuations around +this mean go up to nearly 50% and down nearly to 1%, but, as in other +cases, such extreme deviations from the average are met with only +exceptionally. + +The second category includes the inconstant but perfectly fertile +species. I have already given the names of the only two forms, which +deserve to be mentioned here. + +One of them is called _scintillans_ or the shiny evening-primrose, +because its leaves are of a deep green color with smooth surfaces, +glistening in the sunshine. On the young rosettes these leaves are +somewhat broader, and afterwards somewhat narrower than those of _O. +lamarckiana_ at the corresponding ages. The plants themselves always +remain small, never reaching the stature of the ancestral type. They are +likewise much less branched. They can easily be cultivated in annual +generations, but then do not become as fully developed and as fertile, +as when flowering in the second year. The flowers have the same +structure as those of the _lamarckiana_, but are of a smaller size. + +[544] Fertilizing the flowers artificially with their own pollen, +excluding the visiting insects by means of paper bags, and saving and +sowing the seed of each individual separately, furnishes all the +requisites for the estimation of the degree of stability of this +species. In the first few weeks the seed-pans do not show any +unequality, and often the young plants must be replanted at wider +intervals, before anything can be made out with certainty. But as soon +as the rosettes begin to fill it becomes manifest that some of them are +more backward than others in size. Soon the smaller ones show their +deeper green and broader leaves, and thereby display the attributes of +the _scintillans_. The other grow faster and stronger and exhibit all +the characteristics of ordinary _lamarckiana_s. + +The numerical proportion of these two groups has been found different on +different occasions. Some plants give about one-third _scintillans_ and +two-thirds _lamarckiana_, while the progeny of individuals of another +strain show exactly the reverse proportion. + +Two points deserve to be noticed. First the progeny of the _scintillans_ +appears to be mutable in a large degree, exceeding even the +_lamarckiana_. The same forms that are produced most often by the +parent-family are also most ordinarily [545] met with among the +offspring of the shiny evening-primrose. They are _oblonga_, _lata_ and +_nanella_. _Oblonga_ was observed at times to constitute as much as 1% +or more of the sowings of _scintillans_, while _lata_ and _nanella_ were +commonly seen only in a few scattering individuals, although seldom +lacking in experiments of a sufficient size. + +Secondly the instability seems to be a constant quality, although the +words themselves are at first sight, contradictory. I mean to convey the +conception that the degree of instability remains unchanged during +successive generations. This is a very curious fact, and strongly +reminds us of the hereditary conditions of striped-flower varieties. +But, on the contrary, the atavists, which are here the individuals with +the stature and the characteristics of the _lamarckiana_, have become +_lamarckiana_s in their hereditary qualities, too. If their seed is +saved and sown, their progeny does not contain any _scintillans_, or at +least no more than might arise by ordinary mutations. + +One other inconstant new species is to be noted, but as it was very rare +both in the field and in my cultures, and as it was difficult of +cultivation, little can as yet be said about it. It is the _Oenothera +elliptica_, with narrow elliptical leaves and also with elliptical +petals. It repeats [546] its type only in a very small proportion of its +seed. + +All in all we thus have a group of a dozen new types, springing from an +original form in one restricted locality, and seen to grow there, or +arising in the garden from seeds collected from the original locality. +Without any doubt the germs of the new types are fully developed within +the seed, ready to be evolved at the time of germination. More favorable +conditions in the field would no doubt allow all of the described new +species to unfold their attributes there, and to come into competition +with each other and with the common parents. But obviously this is only +of secondary importance, and has no influence on the fact that a number +of new types, analogous to the older swarms of _Draba_, _Viola_ and of +many other polymorphous species, have been seen to arise directly in the +wild state. + + +[547] + +LECTURE XIX + +EXPERIMENTAL PEDIGREE-CULTURES + +The observation of the production of mutants in the field at Hilversum, +and the subsequent cultivation of the new types in the garden at +Amsterdam, gives ample proof of the mutability of plants. Furthermore it +furnishes an analogy with the hypothetical origin of the swarms of +species of _Draba_ and _Viola_. Last but not least important it affords +material for a complete systematic and morphologic study of the newly +arisen group of forms. + +The physiologic laws, however, which govern this process are only very +imperfectly revealed by such a study. The instances are too few. +Moreover the seeds from which the mutants spring, escape observation. It +is simply impossible to tell from which individual plants they have been +derived. The laevifolia and the brevistylis have been found almost every +year, the first always recurring on the same spot, the second on various +parts of the original field. It is therefore allowable to assume a +common [548] origin for all the observed individuals of either strain. +But whether, besides this, similar strains are produced anew by the old +_lamarckiana_ group, it is impossible to decide on the sole ground of +these field-observations. + +The same holds good with the other novelties. Even if one of them should +germinate repeatedly, without ever opening its flowers, the possibility +could not be excluded that the seeds might have come originally from the +same capsule but lain dormant in the earth during periods of unequal +length. + +Other objections might be cited that can only be met by direct and fully +controlled experiments. Next to the native locality comes the +experimental garden. Here the rule prevails that every plant must be +fertilized with pollen of its own, or with pollen of other individuals +of known and recorded origin. The visits of insects must be guarded +against, and no seeds should be saved from flowers which have been +allowed to open without this precaution. Then the seeds of each +individual must be saved and sown separately, so as to admit of an +appreciation, and if necessary, a numerical determination of the nature +of its progeny. And last but not least the experiments should be +conducted in a similar manner during a series of successive years. + +[549] I have made four such experiments, each comprising the handling of +many thousands of individual plants, and lasting through five to nine +generations. At the beginning the plants were biennial, as in the native +locality, but later I learned to cultivate them in annual generations. +They have been started from different plants and seeds, introduced from +the original field into my garden at Amsterdam. + +It seems sufficient to describe here one of these pedigree-cultures, as +the results of all four were similar. In the fall of 1886 I took nine +large rosettes from the field, planted them together on an isolated spot +in the garden, and harvested their seeds the next year. These nine +original plants are therefore to be considered as constituting the first +generation of my race. The second generation was sown in 1888 and +flowered in 1889. It at once yielded the expected result. 15,000 +seedlings were tested and examined, and among them 10 showed diverging +characters. They were properly protected, and proved to belong to two +new types. 5 of them were _lata_ and 5 _nanella_. They flowered next +year and displayed all the characters as described in our preceding +lecture. Intermediates between them and the general type were not found, +and no indication of their appearance was noted in their parents. [550] +They came into existence at once, fully equipped, without preparation or +intermediate steps. No series of generations, no selection, no struggle +for existence was needed. It was a sudden leap into another type, a +sport in the best acceptation of the word. It fulfilled my hopes, and at +once gave proof of the possibility of the direct observation of the +origin of species, and of the experimental control thereof. + +The third generation was in the main a repetition of the second. I tried +some 10,000 seedlings and found three _lata_ and three _nanella_, or +nearly the same proportion as in the first instance. But besides these a +_rubrinervis_ made its appearance and flowered the following year. This +fact at once revealed the possibility that the instability of +_lamarckiana_ might not be restricted to the three new types now under +observation. Hence the question arose how it would be possible to obtain +other types or to find them if they were present. It was necessary to +have better methods of cultivation and examination of the young plants. +Accordingly I devoted the three succeeding years to working on this +problem. + +I found that it was not at all necessary to sow any larger quantities of +seed, but that the young plants must have room enough to develop into +full and free rosettes. Moreover I observed [551] that the attributes of +_lata_ and _nanella_, which I now studied in the offspring of my first +mutants, were clearly discernible in extreme youth, while those of +_rubrinervis_ remained concealed some weeks longer. Hence I concluded +that the young plants should be examined from time to time until they +proved clearly to be only normal _lamarckiana_. Individuals exhibiting +any deviation from the type, or even giving only a slight indication of +it, were forthwith taken out of the beds and planted separately, under +circumstances as favorable as possible. They were established in pots +with well-manured soil and kept under glass, but fully exposed to +sunshine. As a rule they grew very fast, and could be planted out early +in June. Some of them, of course, proved to have been erroneously taken +for mutants, but many exhibited new characters. + +All in all I had 334 young plants which did not agree with the parental +type. As I examined some 14,000 seedlings altogether, the result was +estimated at about 2.5%. This proportion is much larger than in the +yields of the two first generations and illustrates the value of +improved methods. No doubt many good mutations had been overlooked in +the earlier observations. + +As was to be expected, _lata_ and _nanella_ [552] were repeated in this +third generation (1895). I was sure to get nearly all of them, without +any important exceptions, as I now knew how to detect them at almost any +age. In fact, I found many of them; as many as 60 _nanella_ and 73 +_lata_, or nearly 5% of each. _Rubrinervis_ also recurred, and was seen +in 8 specimens. It was much more rare than the two first-named types. + +But the most curious fact in that year was the appearance of _oblonga_. +No doubt I had often seen it in former years, but had not attached any +value to the very slight differences from the type, as they then seemed +to me. I knew now that any divergence was to be esteemed as important, +and should be isolated for further observation. This showed that among +the selected specimens not less than 176, or more than 1% belonged to +the _oblonga_ type. This type was at that time quite new to me, and it +had to be kept through the winter, to obtain stems and flowers. It +proved to be as uniform as its three predecessors, and especially as +sharply contrasted with _lamarckiana_. The opportunity for the discovery +of any intermediates was as favorable as could be, because the +distinguishing marks were hardly beyond doubt at the time of the +selection and removal of the young plants. But no connecting links were +found. + +[553] The same holds good for _albida_, which appeared in 15 specimens, +or in 0.1%, of the whole culture. By careful cultivation these plants +proved not to be sickly, but to belong to a new, though weak type. It +was evident that I had already seen them in former years, but having +failed to recognize them had allowed them to be destroyed at an early +age, not knowing how to protect them against adverse circumstances. Even +this time I did not succeed in getting them strong enough to keep +through the winter. + +Besides these, two new types were observed, completing the range of all +that have since been recorded to regularly occur in this family. They +were _scintillans_ and _gigas_. The first was obtained in the way just +described. The other hardly escaped being destroyed, not having showed +itself early enough, and being left in the bed after the end of the +selection. But as it was necessary to keep some rosettes through the +winter in order to have biennial flowering plants to furnish seeds, I +selected in August about 30 of the most vigorous plants, planted them on +another bed and gave them sufficient room for their stems and branches +in the following summer. Most of them sent up robust shoots, but no +difference was noted till the first flowers opened. One plant had a much +larger crown of bright blossoms than any of the others. [554] As soon as +these flowers faded away, and the young fruits grew out, it became clear +that a new type was showing itself. On that indication I removed all the +already fertilized flowers and young fruits, and protected the buds from +the visits of insects. Thus the isolated flowers were fertilized with +their own pollen only, and I could rely upon the purity of the seed +saved. This lot of seeds was sown in the spring of 1897 and yielded a +uniform crop of nearly 300 young _gigas_ plants. + +Having found how much depends upon the treatment, I could gradually +decrease the size of my cultures. Evidently the chance of discovering +new types would be lessened thereby, but the question as to the repeated +production of the same new forms could more easily and more clearly be +answered in this way. In the following year (1896) I sowed half as many +seeds as formerly, and the result proved quite the same. With the +exception of _gigas_ all the described forms sprang anew from the purely +fertilized ancestry of normal _lamarckiana_s. It was now the fifth +generation of my pedigree, and thus I was absolutely sure that the +descendants of the mutants of this year had been pure and without +deviation for at least four successive generations. + +Owing partly to improved methods of selection, [555] partly no doubt to +chance, even more mutants were found this year than in the former. Out +of some 8,000 seedlings I counted 377 deviating ones, or nearly 5%, +which is a high proportion. Most of them were _oblonga_ and _lata_, the +same types that had constituted the majority in the former year. + +_Albida_, _nanella_ and _rubrinervis_ appeared in large numbers, and +even _scintillans_, of which I had but a single plant in the previous +generation, was repeated sixfold. + +New forms did not arise, and the capacity of my strain seemed exhausted. +This conclusion was strengthened by the results of the next three +generations, which were made on a much smaller scale and yielded the +same, or at least the mutants most commonly seen in previous years. + +Instead of giving the figures for these last two years separately, I +will now summarize my whole experiment in the form of a pedigree. In +this the normal _lamarckiana_ was the main line, and seeds were only +sown from plants after sufficient isolation either of the plants +themselves, or in the latter years by means of paper bags enclosing the +inflorescences. I have given the number of seedlings of _lamarckiana_ +which were examined each year in the table below. Of course by far the +largest number of them were [556] thrown away as soon as they showed +their differentiating characters in order to make room for the remaining +ones. At last only a few plants were left to blossom in order to +perpetuate the race. I have indicated for each generation the number of +mutants of each of the observed forms, placing them in vertical columns +underneath their respective heads. The three first generations were +biennial, but the five last annual. + + + PEDIGREE OF A MUTATING FAMILY + OF _OENOTHERA LAMARCKIANA_ IN THE + EXPERIMENTAL GARDEN AT AMSTERDAM + + Gener: O.gig. albida obl. rubrin. Lam. nanella lata. scint. + VIII. 5 1 0 1700 21 1 + VII. 9 0 3000 11 + VI. 11 29 3 1800 9 5 1 + V. 25 135 20 8000 49 142 6 + IV. 1 15 176 8 14000 60 73 1 + III. 1 10000 3 3 + II. 15000 5 5 + I. 9 + +It is most striking that the various mutations of the evening-primrose +display a great degree of regularity. There is no chaos of forms, no +indefinite varying in all degrees and in all directions. Quite on the +contrary, it is at once evident that very simple rules govern the whole +phenomenon. + +I shall now attempt to deduce these laws from [557] my experiment. +Obviously they apply not only to our evening-primroses, but may be +expected to be of general validity. This is at once manifest, if we +compare the group of new mutants with the swarms of elementary forms +which compose some of the youngest systematic species, and which, as we +have seen before, are to be considered as the results of previous +mutations. The difference lies in the fact that the evening-primroses +have been seen to spring from their ancestors and that the _drabas_ have +not. Hence the conclusion that in comparing the two we must leave out +the pedigree of the evening-primroses and consider only the group of +forms as they finally show themselves. If in doing so we find sufficient +similarity, we are justified in the conclusion that the _drabas_ and +others have probably originated in the same way as the +evening-primroses. Minor points of course will differ, but the main +lines cannot have complied with wholly different laws. All so-called +swarms of elementary species obviously pertain to a single type, and +this type includes our evening-primroses as the only controlled case. + +Formulating the laws of mutability for the evening-primroses we +therefore assume that they hold good for numerous other corresponding +cases. + + +[558] I. The first law is, that new elementary species appear suddenly, +without intermediate steps. + +This is a striking point, and the one that is in the most immediate +contradiction to current scientific belief. The ordinary conception +assumes very slow changes, in fact so slow that centuries are supposed +to be required to make the differences appreciable. If this were true, +all chance of ever seeing a new species arise would be hopelessly small. +Fortunately the evening-primroses exhibit contrary tendencies. One of +the great points of pedigree-culture is the fact that the ancestors of +every mutant have been controlled and recorded. Those of the last year +have seven generations of known _lamarckiana_ parents preceding them. If +there had been any visible preparation towards the coming mutation, it +could not have escaped observation. Moreover, if visible preparation +were the rule, it could hardly go on at the same time and in the same +individuals in five or six diverging directions, producing from one +parent, _gigas_ and _nanella_, _lata_ and _rubrinervis_, _oblonga_ and +_albida_ and even _scintillans_. + +On the other hand the mutants, that constitute the first representatives +of their race, exhibit all the attributes of the new type in full +display at once. No series of generations, no selection, [559] no +struggle for existence are needed to reach this end. In previous +lectures I have mentioned that I have saved the seeds of the mutants +whenever possible, and have always obtained repetitions of the prototype +only. Reversions are as absolutely lacking as is also a further +development of the new type. Even in the case of the inconstant forms, +where part of the progeny yearly return to the stature of _lamarckiana_, +intermediates are not found. So it is also with _lata_, which is +pistillate and can only be propagated by cross-fertilization. But though +the current belief would expect intermediates at least in this case, +they do not occur. I made a pedigree-culture of lata during eight +successive generations, pollinating them in different ways, and always +obtained cultures which were partly constituted of _lata_ and partly of +_lamarckiana_ specimens. But the _lata_s remained _lata_ in all the +various and most noticeable characters, never showing any tendency to +gradually revert into the original form. + +Intermediate forms, if not occurring in the direct line from one species +to another, might be expected to appear perhaps on lateral branches. In +this case the mutants of one type, appearing in the same year, would not +be a pure type, but would exhibit different degrees of deviation from +the parent. The best would then have to [560] be chosen in order to get +the new type in its pure condition. Nothing of the kind, however, was +observed. All the _oblonga_-mutants were pure _oblongas_. The pedigree +shows hundreds of them in the succeeding years, but no difference was +seen and no material for selection was afforded. All were as nearly +equal as the individuals of old elementary species. + + +II. New forms spring laterally from the main stem. + +The current conception concerning the origin of species assumes that +species are slowly converted into others. The conversion is assumed to +affect all the individuals in the same direction and in the same degree. +The whole group changes its character, acquiring new attributes. By +inter-crossing they maintain a common line of progress, one individual +never being able to proceed much ahead of the others. + +The birth of the new species necessarily seemed to involve the death of +the old one. This last conclusion, however, is hard to understand. It +may be justifiable to assume that all the individuals of one locality +are ordinarily intercrossed, and are moreover subjected to the same +external conditions. They might be supposed to vary in the same +direction if these conditions were changed slowly. But this could of +course have no possible influence on the plants of the [561] same +species growing in distant localities, and it would be improbable they +should be affected in the same way. Hence we should conclude that when a +species is converted into a new type in one locality this is only to be +considered as one of numerous possible ones, and its alteration would +not in the least change the aspect of the remainder of the species. + +But even with this restriction the general belief is not supported by +the evidence of the evening-primroses. There is neither a slow nor a +sudden change of all the individuals. On the contrary, the vast majority +remain unchanged; thousands are seen exactly repeating the original +prototype yearly, both in the native field and in my garden. There is no +danger that _lamarckiana_ might die out from the act of mutating, nor +that the mutating strain itself would be exposed to ultimate destruction +from this cause. + +In older swarms, such as _Draba_ or _Helianthemum_, no such center, +around which the various forms are grouped, is known. Are we to conclude +therefore that the main strain has died out? Or is it perhaps concealed +among the throng, being distinguished by no peculiar character? If our +_gigas_ and _rubrinervis_ were growing in equal numbers with the +_lamarckiana_ in the native field, would it be possible to decide [562] +which of them was the progenitor of the others? Of course this could be +done by long and tedious crossing experiments, showing atavism in the +progeny, and thereby indicating the common ancestor. But even this +capacity seems to be doubtful and connected only with the state of +mutability and to be lost afterwards. Therefore if this period of +mutation were ended, probably there would be no way to decide concerning +the mutual relationship of the single species. + +Hence the lack of a recognizable main stem in swarms of elementary +species makes it impossible to answer the question concerning their +common origin. + +Another phase of the opposition between the prevailing view and my own +results seems far more important. According to the current belief the +conversion of a group of plants growing in any locality and flowering +simultaneously would be restricted to one type. In my own experiments +several new species arose from the parental form at once, giving a wide +range of new forms at the same time and under the same conditions. + + +III. New elementary species attain their full constancy at once. + +Constancy is not the result of selection or of improvement. It is a +quality of its own. It can neither be constrained by selection if it is +absent [563] from the beginning, nor does it need any natural or +artificial aid if it is present. Most of my new species have proved +constant from the first. Whenever possible, the original mutants have +been isolated during the flowering period and artificially +self-fertilized. Such plants have always given a uniform progeny, all +children exhibiting the type of the parent. No atavism was observed and +therefore no selection was needed or even practicable. + +Briefly considering the different forms, we may state that the full +experimental proof has been given for the origin of _gigas_ and +_rubrinervis_, for _albida_ and _oblonga_, and even for _nanella_, which +is to be considered as of a varietal nature; with _lata_ the decisive +experiment is excluded by its unisexuality. _laevifolia_ and +_brevistylis_ were found originally in the field, and never appeared in +my cultures. No observations were made as to their origin, and seeds +have only been sown from later generations. But these have yielded +uniform crops, thereby showing that there is no ground for the +assumption that these two older varieties might behave otherwise than +the more recent derivatives. + +_Scintillans_ and _elliptica_ constitute exceptions to the rule given. +They repeat their character, from pure seed, only in part of the +offspring. I have tried to deliver the _scintillans_ from this [564] +incompleteness of heredity, but in vain. The succeeding generations, if +produced from true representatives of the new type, and with pure +fertilization, have repeated the splitting in the same numerical +proportions. The instability seems to be here as permanent a quality as +the stability in other instances. Even here no selection has been +adequate to change the original form. + + +IV. Some of the new strains are evidently elementary species, while +others are to be considered as retrograde varieties. + +It is often difficult to decide whether a given form belongs to one or +another of these two groups. I have tried to show that the best and +strictest conception of varieties limits them to those forms that have +probably originated by retrograde or degressive steps. Elementary +species are assumed to have been produced in a progressive way, adding +one new element to the store. Varieties differ from their species +clearly in one point, and this is either a distinct loss, or the +assumption of a character, which may be met with in other species and +genera. _laevifolia_ is distinguished by the loss of the crinkling of +the leaves, _brevistylis_ by the partial loss of the epigynous qualities +of the flowers, and _nanella_ is a dwarf. These three new forms are +therefore [565] considered to constitute only retrograde steps, and no +advance. This conclusion has been fully justified by some crossing +experiments with _brevistylis_, which wholly complies with Mendel's law, +and in one instance with _nanella_, which behaves in the same manner, if +crossed with _rubrinervis_. + +On the other hand, _gigas_ and _rubrinervis_, _oblonga_ and _albida_ +obviously bear the characters of progressive elementary species. They +are not differentiated from _lamarckiana_ by one or two main features. +They diverge from it in nearly all organs, and in all in a definite +though small degree. They may be recognized as soon as they have +developed their first leaves and remain discernible throughout life. +Their characters refer chiefly to the foliage, but no less to the +stature, and even the seeds have peculiarities. There can be little +doubt but that all the attributes of every new species are derived from +one principal change. But why this should affect the foliage in one +manner, the flowers in another and the fruits in a third direction, +remains obscure. To gain ever so little an insight into the nature of +these changes, we may best compare the differences of our +evening-primroses with those between the two hundred elementary species +of _Draba_ and other similar instances. In doing so we find the same +main [566] feature, the minute differences in nearly all points. + + +V. The same new species are produced in a large number of individuals. + +This is a very curious fact. It embraces two minor points, viz: the +multitude of similar mutants in the same year, and the repetition +thereof in succeeding generations. Obviously there must be some common +cause. This cause must be assumed to lie dormant in the _Lamarckiana_s +of my strain, and probably in all of them, as no single parent-plant +proved ever to be wholly destitute of mutability. Furthermore the +different causes for the sundry mutations must lie latent together in +the same parent-plant. They obey the same general laws, become active +under similar conditions, some of them being more easily awakened than +others. The germs of the _oblonga_, _lata_ and _nanella_ are especially +irritable, and are ready to spring into activity at the least summons, +while those of _gigas_, _rubrinervis_ and _scintillans_ are far more +difficult to arouse. + +These germs must be assumed to lie dormant during many successive +generations. This is especially evident in the case of _lata_ and +_nanella__, which appeared in the first year of the pedigree culture and +which since have been repeated yearly, and have been seen to arise by +mutation [567] also during the last season (1903). Only _gigas_ appeared +but once, but then there is every reason to assume that in larger +sowings or by a prolongation of the experiments it might have made a +second appearance. + +Is the number of such germs to be supposed to be limited or unlimited? +My experiment has produced about a dozen new forms. Without doubt I +could easily have succeeded in getting more, if I had had any definite +reason to search for them. But such figures are far from favoring the +assumption of indefinite mutability. The group of possible new forms is +no doubt sharply circumscribed. Partly so by the morphologic +peculiarities of _lamarckiana_, which seem to exclude red flowers, +composite leaves, etc. No doubt there are more direct reasons for these +limits, some changes having taken place initially and others later, +while the present mutations are only repetitions of previous ones, and +do not contribute new lines of development to those already existing. +This leads us to the supposition of some common original cause, which +produced a number of changes, but which itself is no longer at work, but +has left the affected qualities, and only these, in the state of +mutability. + +In nature, repeated mutations must be of far greater significance than +isolated ones. How [568] great is the chance for a single individual to +be destroyed in the struggle for life? Hundreds of thousands of seeds +are produced by _lamarckiana_ annually in the field, and only some slow +increase of the number of specimens can be observed. Many seeds do not +find the proper circumstances for germination, or the young seedlings +are destroyed by lack of water, of air, or of space. Thousands of them +are so crowded when becoming rosettes that only a few succeed in +producing stems. Any weakness would have destroyed them. As a matter of +fact they are much oftener produced in the seed than seen in the field +with the usual unfavorable conditions; the careful sowing of collected +seeds has given proof of this fact many times. + +The experimental proof of this frequency in the origin of new types, +seems to overcome many difficulties offered by the current theories on +the probable origin of species at large. + + +VI. The relation between mutability and fluctuating variability has +always been one of the chief difficulties of the followers of Darwin. +The majority assumed that species arise by the slow accumulation of +slight fluctuating deviations, and the mutations were only to be +considered as extreme fluctuations, obtained, in the main, by a +continuous selection of small differences in a constant direction. + +[569] My cultures show that quite the opposite is to be regarded as +fact. All organs and all qualities of _lamarckiana_ fluctuate and vary +in a more or less evident manner, and those which I had the opportunity +of examining more closely were found to comply with the general laws of +fluctuation. But such oscillating changes have nothing in common with +the mutations. Their essential character is the heaping up of slight +deviations around a mean, and the occurrence of continuous lines of +increasing deviations, linking the extremes with this group. Nothing of +the kind is observed in the case of mutations. There is no mean for them +to be grouped around and the extreme only is to be seen, and it is +wholly unconnected with the original type. It might be supposed that on +closer inspection each mutation might be brought into connection with +some feature of the fluctuating variability. But this is not the case. +The dwarfs are not at all the extreme variants of structure, as the +fluctuation of the height of the _lamarckiana_ never decreases or even +approaches that of the dwarfs. There is always a gap. The smallest +specimens of the tall type are commonly the weakest, according to the +general rule of the relationship between nourishment and variation, but +the tallest dwarfs are of course the most robust specimens of their +group. [570] Fluctuating variability, as a rule, is subject to +reversion. The seeds of the extremes do not produce an offspring which +fluctuates around their parents as a center, but around some point on +the line which combines their attributes with the corresponding +characteristic of their ancestors, as Vilmorin has put it. No reversion +accompanies mutation, and this fact is perhaps the completest contrast +in which these two great types of variability are opposed to each other. + +The offspring of my mutants are, of course, subject to the general laws +of fluctuating variability. They vary, however, around their own mean, +and this mean is simply the type of the new elementary species. + + +VII. The mutations take place in nearly all directions. + +Many authors assume that the origin of species is directed by unknown +causes. These causes are assumed to work in each single case for the +improvement of the animals and plants, changing them in a manner +corresponding in a useful way to the changes that take place in their +environment. It is not easy to imagine the nature of these influences +nor how they would bring about the desired effect. + +This difficulty was strongly felt by Darwin, and one of the chief +purposes of his selection theory may be said to have been the attempt +[571] to surmount it. Darwin tried to replace the unknown cause by +natural agencies, which lie under our immediate observation. On this +point Darwin was superior to his predecessors, and it is chiefly due to +the clear conception of this point that his theory has gained its +deserved general acceptance. According to Darwin, changes occur in all +directions, quite independently of the prevailing circumstances. Some +may be favorable, others detrimental, many of them without significance, +neither useful nor injurious. Some of them will sooner or later be +destroyed, while others will survive, but which of them will survive, is +obviously dependent upon whether their particular changes agree with the +existing environic conditions or not. This is what Darwin has called the +struggle for life. It is a large sieve, and it only acts as such. Some +fall through and are annihilated, others remain above and are selected, +as the phrase goes. Many are selected, but more are destroyed; daily +observation does not leave any doubt upon this point. + +How the differences originate is quite another question. It has nothing +to do with the theory of natural selection nor with the struggle for +life. These have an active part only in the accumulation of useful +qualities, and only in so [572] far as they protect the bearers of such +characters against being crowded out by their more poorly constituted +competitors. + +However, the differentiating characteristics of elementary species are +only very small. How widely distant they are from the beautiful +adaptative organizations of orchids, of insectivorous plants and of so +many others! Here the difference lies in the accumulation of numerous +elementary characters, which all contribute to the same end. Chance must +have produced them, and this would seem absolutely improbable, even +impossible, were it not for Darwin's ingenious theory. Chance there is, +but no more than anywhere else. It is not by mere chance that the +variations move in the required direction. They do go, according to +Darwin's view, in all directions, or at least in many. If these include +the useful ones, and if this is repeated a number of times, cumulation +is possible; if not, there is simply no progression, and the type +remains stable through the ages. Natural selection is continually acting +as a sieve, throwing out the useless changes and retaining the real +improvements. Hence the accumulation in apparently predisposed +directions, and hence the increasing adaptations to the more specialized +conditions of life. It must be obvious to any one who can free himself +from the current ideas, [573] that this theory of natural selection +leaves the question as to how the changes themselves are brought about, +quite undecided. There are two possibilities, and both have been +propounded by Darwin. One is the accumulation of the slight deviations +of fluctuating variability, the other consists of successive sports or +leaps taking place in the same direction. + +In further lectures a critical comparison of the two views will be +given. Today I have only to show that the mutations of the +evening-primroses, though sudden, comply with the demands made by Darwin +as to the form of variability which is to be accepted as the cause of +evolution and as the origin of species. + +Some of my new types are stouter and others weaker than their parents, +as shown by _gigas_ and _albida_. Some have broader leaves and some +narrower, _lata_ and _oblonga_. Some have larger flowers (_gigas_) or +deeper yellow ones (_rubrinervis_), or smaller blossoms (_scintillans_), +or of a paler hue (_albida_). In some the capsules are longer +(_rubrinervis_), or thicker (_gigas_), or more rounded (_lata_), or +small (_oblonga_), and nearly destitute of seeds (_brevistylis_). The +unevenness of the surface of the leaves may increase as in _lata_, or +decrease as in _laevifolia_. The tendency to become annual prevails in +_rubrinervis_, but _gigas_ tends to become [574] biennial. Some are rich +in pollen, while _scintillans_ is poor. Some have large seeds, others +small. _Lata_ has become pistillate, while _brevistylis_ has nearly lost +the faculty to produce seeds. Some undescribed forms were quite sterile, +and some I observed which produced no flowers at all. From this +statement it may be seen that nearly all qualities vary in opposite +directions and that our group of mutants affords wide material for the +sifting process of natural selection. On the original field the +_laevifolia_ and _brevistylis_ have held their own during sixteen years +and probably more, without, however, being able to increase their +numbers to any noticeable extent. Others perish as soon as they make +their appearance, or a few individuals are allowed to bloom, but +probably leave no progeny. + +But perhaps the circumstances may change, or the whole strain may be +dispersed and spread to new localities with different conditions. Some +of the latter might be found to be favorable to the robust _gigas_, or +to _rubrinervis_, which requires a drier air, with rainfall in the +springtime and sunshine during the summer. It would be worth while to +see whether the climate of California, where neither _O. lamarckiana_ +nor _O. biennis_ are found wild, would not exactly [575] suit the +requirements of the new species _rubrinervis_ and _gigas_. + +NOTE. _Oenothera_s are native to America and all of the species growing +in Europe have escaped from gardens directly, or may have arisen by +mutation, or by hybridization of introduced species. A fixed hybrid +between _O. cruciata_ and _O. biennis_ constituting a species has been +in cultivation for many years. The form known as _O. biennis_ in Europe, +and used by de Vries in all of the experiments described in these +lectures, has not yet been found growing wild in America and is not +identical with the species bearing that name among American botanists. +Concerning this matter Professor de Vries writes under date of Sept. 12, +1905: "The '_biennis_' which I collected in America has proved to be a +motley collection of forms, which at that time I had no means of +distinguishing. No one of them, so far as they are now growing in my +garden is identical with our _biennis_ of the sand dunes." The same +appears to be the case with _O. muricata_. Plants from the Northeastern +American seaboard, identifiable with the species do not entirely agree +with those raised from seed received from Holland. + +_O. lamarckiana_ has not been found growing wild in America in recent +years although the evidence at hand seems to favor the conclusion that +it was seen and collected in the southern states in the last century. +(See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the +_Oenotheras_. Publication 24. Carnegie Institution. Washington, D.C., +1905.) EDITOR. + + + +[576] +LECTURE XX + +THE ORIGIN OF WILD SPECIES AND VARIETIES + +New species and varieties occur from time to time in the wild state. +Setting aside all theoretical conceptions as to the common origin of +species at large, the undoubted fact remains that new forms are +sometimes met with. In the case of the peloric toad-flax the mutations +are so numerous that they seem to be quite regular. The production of +new species of evening-primroses was observed on the field and +afterwards duplicated in the garden. There is no reason to think that +these cases are isolated instances. Quite on the contrary they seem to +be the prototypes of repeated occurrences in nature. + +If this conception is granted, the question at once arises, how are we +to deal with analogous cases, when fortune offers them, and what can we +expect to learn from them? + +A critical study of the existing evidence seems to be of great +importance in order to ascertain the best way of dealing with new facts, +and of estimating the value of the factors concerned. [577] It is +manifest that we must be very careful and conservative in dealing with +new facts that are brought to our attention, and every effort should be +made to bring additional evidence to light. Many vegetable anomalies are +so rare that they are met with only by the purest chance, and are then +believed to be wholly new. When a white variety of some common plant is +met with for the first time we generally assume that it originated on +that very spot and only a short time previously. The discovery of a +second locality for the same variety at once raises the question as to a +common origin in the two instances. Could not the plants of the second +locality have arisen from seeds transported from the first? + +White varieties of many species of blue-bells and gentians are found not +rarely, white-flowering plants of heather, both of _Erica Tetralix_ and +_Calluna vulgaris_ occur on European heaths; white flowers of _Brunella +vulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen in +many localities in the habitats of the colored species. Pelories of +labiates seem to occur often in Austria, but are rare in Holland; white +bilberries (_Vaccinium Myrtillus_) have many known localities throughout +Europe, and nearly all the berry-bearing species in the large heath +family are recorded as having white varieties. + +[578] Are we to assume a single origin for all the representatives of +such a variety, as we have done customarily for all the representatives +of a wild species? Or can the same mutation have been repeated at +different times and in distant localities? If a distinct mutation from a +given species is once possible, why should it not occur twice or thrice? + +A variety which seems to be new to us may only appear so, because the +spot where it grows had hitherto escaped observation. _Lychnis preslii_ +is a smooth variety of _Lychnis diurna_ and was observed for the first +time in the year 1842 by Sekera. It grew abundantly in a grove near +Munchengratz in southern Hungary. It was accompanied by the ordinary +hairy type of the species. Since then it has been observed to be quite +constant in the same locality, and some specimens have been collected +for me there lately by Dr. Nemec, of Prague. No other native localities +of this variety have been discovered, and there can be no doubt that it +must have arisen from the ordinary campion near the spot where it still +grows. But this change may have taken place some years before the first +discovery, or perhaps one or more centuries ago. This could only be +known if it could be proved that the locality had been satisfactorily +investigated previously, and that the variety had not [579] been met +with. Even in this case only something would be discovered about the +time of the change, but nothing about its real nature. + +So it is in many cases. If a variety is observed in a number of +specimens at the time of its first discovery, and at a locality not +studied previously, it takes the aspect of an old form of limited +distribution, and little can be learned as to the circumstances under +which it arose. If on the contrary it occurs in very small numbers or +perhaps even in a single individual, and if the spot where it is found +is located so that it could hardly have escaped previous observation, +then the presumption of a recent origin seems justified. + +What has to be ascertained on such occasions to give them scientific +value? Three points strike me as being of the highest importance. First, +the constancy of the new type; secondly, the occurrence or lack of +intermediates, and last, but not least, the direct observation of a +repeated production. + +The first two points are easily ascertained. Whether the new type is +linked with its more common supposed ancestor by intermediate steps is a +query which at once strikes the botanist. It is usually recorded in such +cases, and we may state at once that the general result is, that such +intermediates do not occur. This is [580] of the highest importance and +admits of only two explanations. One is that intermediates may be +assumed to have preceded the existent developed form, and to have died +out afterwards. But why should they have done so, especially in cases of +recent changes? On the other hand the intermediates may be lacking +because they have never existed, the change having taken place by a +sudden leap, such as the mutations described in our former lectures. It +is manifest that the assumption of hypothetical intermediates could only +gain some probability if they had been found in some instance. Since +they do not occur, the hypothesis seems wholly unsupported. + +The second point is the constancy of the new type. Seeds should be saved +and sown. If the plant fertilizes itself without the aid of insects, as +do some evening-primroses, the seed saved from the native locality may +prove wholly pure, and if it does give rise to a uniform progeny the +constancy of the race may be assumed to be proved, provided that +repeated trials do not bring to light any exceptions. If the offspring +shows more than one type, cross-fertilization is always to be looked to +as the most probable cause, and should be excluded, in order to sow pure +seeds. Garden-experiments of this kind, and repeated trials, should +always be combined [581] with the discovery of a presumed mutation. In +many instances the authors have realized the importance of this point, +and new types have been found constant from the very beginning. Many +cases are known which show no reversions and even no partial reversions. +This fact throws a distinct light on our first point, as it makes the +hypothesis of a slow and gradual development still more improbable. + +My third point is of quite another nature and has not as yet been dealt +with. But as it appeals to me as the very soul of the problem, it seems +necessary to describe it in some detail. It does not refer to the new +type itself, nor to any of its morphologic or hereditary attributes, but +directly concerns the presumed ancestors themselves. + +The peloric toad-flax in my experiment was seen to arise thrice from the +same strain. Three different individuals of my original race showed a +tendency to produce peloric mutations, and they did so in a number of +their seeds, exactly as the mutations of the evening-primroses were +repeated nearly every year. Hence the inference, that whenever we find a +novelty which is really of very recent date, the parent-strain which has +produced it might still be in existence on the same spot. In the case of +shrubs or perennials the very parents might yet be found. [582] But it +seems probable, and is especially proved in the case of the +evening-primroses, that all or the majority of the representatives of +the whole strain have the same tendency to mutate. If this were a +general rule, it would suffice to take some pure seeds from specimens of +the presumed parents and to sow and multiply the individuals to such an +extent that the mutation might have a chance to be repeated. + +Unfortunately, this has not as yet been done, but in my opinion it +should be the first effort of any one who has the good luck to discover +a new wild mutation. Specimens of the parents should be transplanted +into a garden and fertilized under isolated conditions. Seeds saved from +the wild plant would have little worth, as they might have been partly +fertilized by the new type itself. + +After this somewhat lengthy discussion of the value of observations +surrounding the discovery of new wild mutations, we now come to the +description of some of the more interesting cases. As a first example, I +will take the globular fruited shepherd's purse, described by Solms +Laubach as _Capsella heegeri_. Professor Heeger discovered one plant +with deviating fruits, in a group of common shepherd's purses in the +market-place near Landau in Germany, in the fall of 1897. They were +nearly spherical, [583] instead of flat and purse-shaped. Their valves +were thick and fleshy, while those of the ordinary form are +membranaceous and dry. The capsules hardly opened and therefore differed +in this point from the shepherd's purse, which readily loosens both its +valves as soon as it is ripe. + +Only one plant was observed; whence it came could not be determined, nor +whether it had arisen from the neighboring stock of C_apsella_ or not. +The discoverer took some seed to his garden and sent some to the +botanical garden at Strassburg, of which Solms-Laubach is the director. +The majority of the seeds of course were sowed naturally on the original +spot. The following year some of the seeds germinated and repeated the +novelty. The leaves, stems and flowers were those of the common +shepherd's purse, but no decision could be reached concerning the type +of this generation before the first flowers had faded and the rounded +capsules had developed. Then it was seen that the _heegeri_ came true +from seed. It did so both in the gardens and on the market-place, where +it was observed to have multiplied and spread in some small measure. The +same was noted the following year, but then the place was covered with +gravel and all the plants destroyed. It is not recorded to have been +seen wild since. + +[584] Intermediate forms have not been met with. Some slight reversions +may occur in the autumn on the smallest and weakest lateral branches. +Such reversions, however, seem to be very rare, as I have tried in vain +to produce them on large and richly branched individuals, by applying +all possible inducements in the form of manure and of cutting, to +stimulate the production of successive generations of weaker side +branches. + +This constancy was proved by the experiments of Solms-Laubach, which I +have repeated in my own garden during several years with seed received +from him. No atavists or deviating specimens have been found among many +hundreds of flowering plants. + +It is important to note that within the family of the crucifers the form +of the capsule and the attributes of the valves and seeds are usually +considered to furnish the characteristics of genera, and this point has +been elucidated at some length by Solms-Laubach. There is, however, no +sufficient reason to construe a new genus on the ground of Heeger's +globular fruited shepherd's purse; but as a true elementary species, and +even as a good systematic species it has proved itself, and as such it +is described by Solms-Laubach, who named it in honor of its discoverer. + +Exactly analogous discoveries have been [586] instead of displaying a +bright yellow cup. _O. cruciata_ grows in the Adirondack Mountains, in +the states of New York and Vermont, and seems to be abundant there. It +has been introduced into botanical gardens and yielded a number of +hybrids, especially with _O. biennis and _O. lamarckiana_, and the +narrow petals of the parent-species may be met with in combination with +the stature and vegetative characteristics of these last named species. +_O. cruciata_ has a purple foliage, while _biennis_ and _lamarckiana_ +are green, and many of the hybrids may instantly be recognized by their +purple color. + +The curious attribute of the petals is not to be considered simply as a +reduction in size. On anatomical inquiry it has been found that these +narrow petals bear some characteristics which, on the normal plants, are +limited to the calyx. Stomata and hairs, and the whole structure of the +surface and inner tissues on some parts of these petals are exactly +similar to those of the calyx, while on others they have retained the +characteristics of petals. Sometimes there may even be seen by the naked +eye green longitudinal stripes of calyx-like structure alternating with +bright yellow petaloid parts. For these reasons the cruciata character +may be considered as a case of sepalody of the petals, or of the petals +being partly converted into sepals. + +[587] It is worth while to note that as a monstrosity this occurrence is +extremely rare throughout the whole vegetable kingdom, and only very few +instances have been recorded. + +Two cases of sudden mutations have come to my knowledge, producing this +same anomaly in allied species. One has been already alluded to; it +pertains to the common evening-primrose or _Oenothera biennis_, and one +is a species belonging to another genus of the same family, the great +hairy willow-herb or _Epilobium _hirsutum_. I propose to designate both +new forms by the varietal name of _cruciata_, or _cruciatum_. + +_Oenothera biennis cruciata_ was found in a native locality of the _O. +biennis itself. It consisted of only one plant, showing in all its +flowers the _cruciata_ marks. In all other respects it resembled wholly +the _biennis_, especially in the pure green color of its foliage, which +at once excluded all suspicion of hybrid origin with the purple _O. +cruciata_. Moreover in our country this last occurs only in the +cultivated state in botanical gardens. + +Intermediates were not seen, and as the plant bore some pods, it was +possible to test its constancy. I raised about 500 plants from its +seeds, out of which more than 100 flowered in the first year. The others +were partly kept through the winter and flowered next year. Seeds saved +in [588] both seasons were sown on a large scale. Both the first and the +succeeding generations of the offspring of the original plant came true +without any exception. Intermediates are often found in hybrid cultures, +and in them the character is a very variable one, but as yet they were +not met with in progeny of this mutant. All these plants were exactly +like _O. biennis_, with the single exception of their petals. + +_Epilobium hirsutum cruciatum_ was discovered by John Rasor near +Woolpit, Bury St. Edmunds, in England. It flowered in one spot, +producing about a dozen stems, among large quantities of the +parent-species, which is very common there, as it is elsewhere in +Europe. This species is a perennial, multiplying itself by underground +runners, and the stems of the new variety were observed to stand so +close to each other that they might be considered as the shoots of one +individual. In this case this specimen might probably be the original +mutant, as the variety had not been seen on that spot in previous years, +even as it has not been found elsewhere in the vicinity. + +Intermediates were not observed, though the difference is a very +striking one. In the cruciate flowers the broad and bright purple petals +seem at first sight to be wholly wanting. They are too weak to expand +and to reflex the calyx [589] as in the normal flowers of the species. +The sepals adhere to one another, and are only opened at their summit by +the protruding pistils. Even the stamens hardly come to light. At the +period of full bloom the flowers convey only the idea of closed buds +crowned by the conspicuous white cross of the stigma. Any intermediate +form would have at once betrayed itself by larger colored petals, coming +out of the calyx-sheath. The cruciate petals are small and linear and +greenish, recalling thereby the color of the sepals. + +Mr. Rasor having sent me some flowers and some ripe capsules of his +novelty, I sowed the latter in my experimental garden, where the plant +flowered in large numbers and with many thousands of flowers both in +1902 and 1903. All of these plants and all of these flowers repeated the +cruciate type exactly, and not the slightest impurity or tendency to +partial reversion has been observed. + +Thus true and constant cruciate varieties have been produced from +accidentally observed initial plants, and because of their very curious +characters they will no doubt be kept in botanical gardens, even if they +should eventually become lost in their native localities. + +At this point I might note another observation made on the wild species +of _Oenothera cruciata_ [590] from the Adirondacks. Through the kindness +of Dr. MacDougal, of the New York Botanical Garden, I received seeds +from Sandy Hill near Lake George. When the plants, grown from these +seeds, flowered, they were not a uniform lot, but exhibited two distinct +types. Some had linear petals and thin flower-buds, and in others the +petals were a little broader and the buds more swollen. The difference +was small, but constant on all the flowers, each single plant clearly +belonging to one or the other of the two types. Probably two elementary +species were intermixed here, but whether one is the systematic type and +the other a mutation, remains to be seen. + +Nor seem these two types to exhaust the range of variability of +_Oenothera cruciata_. Dr. B.L. Robinson of Cambridge, Mass., had the +kindness to send me seeds from another locality in the same region. The +seeds were collected in New Hampshire and in my garden produced a true +and constant _cruciata_, but with quite different secondary characters +from both the aforesaid varieties. The stems and flower-spikes and even +the whole foliage were much more slender, and the calyx-tubes of the +flowers were noticeably more elongated. It seems not improbable that +_Oenothera cruciata_ includes a group of lesser unities, and may prove +to comprise a [591] swarm of elementary species, while the original +strain might even now be still in a condition of mutability. A close +scrutiny in the native region is likely to reveal many unexpected +features. + +A very interesting novelty has already been described in a former +lecture. It is the _Xanthium wootoni_, discovered in the region about +Las Vegas, New Mexico, by T.D.A. Cockerell. It is similar in all +respects to _X. commune_, but the burrs are more slender and the +prickles much less numerous, and mostly stouter at their base. It grows +in the same localities as the _X. commune_, and is not recorded to occur +elsewhere. Whether it is an old variety or a recent mutation it is of +course impossible to decide. In a culture made in my garden from the +seed sent me by Mr. Cockerell, I observed (1903) that both forms had a +subvariety with brownish foliage, and, besides this, one of a pure +green. Possibly this species, too, is still in a mutable condition. + +Perhaps the same may be asserted concerning the beautiful shrub, +_Hibiscus Moscheutos_, observed in quite a number of divergent types by +John W. Harshberger. They grew in a small meadow at Seaside Park, New +Jersey, in a locality which had been undisturbed for years. They +differed from each other in nearly all the [592] organs, in size, in the +diameter of the stems, which were woody in some and more fleshy in +others, in the shape of the foliage and in the flowers. More than twenty +types could be distinguished and seeds were saved from a number of them, +in order to ascertain whether they are constant, or whether perhaps a +main stem in a mutating condition might be found among them. If this +should prove to be the case, the relations between the observed forms +would probably be analogous to those between the _O. lamarckiana_ and +its derivatives. + +Many other varieties have sprung from the type-species under similar +conditions from time to time. A fern-leaved mercury, _Mercurialis annua +laciniata_, was discovered in the year 1719 by Marchant. The type was +quite new at the time and maintained itself during a series of years. +The yellow deadly nightshade or _Atropa Belladonna lutea_ was found +about 1850 in the Black Forest in Germany in a single spot, and has +since been multiplied by seeds. It is now dispersed in botanical +gardens, and seems to be quite constant. A dwarf variety of a bean, +_Phaseolus lunatus_, was observed to spring from the ordinary type by a +sudden leap about 1895 by W.W. Tracy, and many similar cases could be +given. + +The annual habit is not very favorable for [593] the discovery of new +forms in the wild state. New varieties may appear, but may be crowded +out the first year. The chances are much greater with perennials, and +still greater with shrubs or trees. A single aberrant specimen may live +for years and even for centuries, and under such conditions is pretty +sure to be discovered sooner or later. Hence it is no wonder that many +such cases are on record. They have this in common that the original +plant of the variety has been found among a vast majority of +representatives of the corresponding species. Nothing of course is +directly known about its origin. Intermediate links have as a rule been +wanting, and the seeds, which have often been sown, have not yielded +reliable results, as no care was taken to preserve the blossoms from +intercrossing with their parent-forms. + +Stress should be laid upon one feature of these curious occurrences. +Relatively often the same novelty has been found twice or thrice, or +even more frequently, and under conditions which make it very improbable +that any relation between such occurrences might exist. The same +mutation must have taken place more than once from the same main stem. + +The most interesting of these facts are connected with the origin of the +purple beech, which [594] is now so universally cultivated. I take the +following statements from an interesting historical essay of Prof. +Jaggi. He describes three original localities. One is near the Swiss +village, Buch am Irchel, and is located on the Stammberg. During the +17th century five purple beeches are recorded to have grown on this +spot. Four of them have died, but one is still alive. Seedlings have +germinated around this little group, and have been mostly dug up and +transplanted into neighboring gardens. Nothing is known about the real +origin of these plants, but according to an old document, it seems that +about the year 1190 the purple beeches of Buch were already enjoying +some renown, and attracting large numbers of pilgrims, owing to some old +legend. The church of Embrach is said to have been built in connection +with this legend, and was a goal for pilgrimages during many centuries. + +A second native locality of the purple beech is found in a forest near +Sondershausen in Thuringen, Germany, where a fine group of these trees +is to be seen. They were mentioned for the first time in the latter half +of the eighteenth century, but must have been old specimens long before +that time. The third locality seems to be of much later origin. It is a +forest near Roveredo in South Tyrol, where a new [595] university is +being erected. It is only a century ago that the first specimens of the +purple beech were discovered there. + +As it is very improbable that the two last named localities should have +received their purple beeches from the first named forest, it seems +reasonable to assume that the variety must have been produced at least +thrice. + +The purple beech is now exceedingly common in cultivation. But Jaggi +succeeded in showing that all the plants owe their origin to the +original trees mentioned above, and are, including nearly all cultivated +specimens with the sole exception of the vicinity of Buch, probably +derived from the trees in Thuringen. They are easily multiplied by +grafting, and come true from seed, at least often, and in a high +proportion. Whether the original trees would yield a pure progeny if +fertilized by their own pollen has as yet not been tested. The young +seedlings have purple seed-leaves, and may easily be selected by this +character, but they seem to be always subjected in a large measure to +vicinism. + +Many other instances of trees and shrubs, found in accidental specimens +constituting a new variety in the wild state, might be given. The +oak-leaved beech has been found in a forest of Lippe-Detmold in Germany +and near Versailles, [596] whence it was introduced into horticulture by +Carriere. Similarly divided and cleft leaves seem to have occurred more +often in the wild state, and cut-leaved hazels are recorded from Rouen +in France, birches and alders from Sweden and Lapland, where both are +said to have been met with in several forests. The purple barberry was +found about 1830 by Bertin, near Versailles. Weeping varieties of ashes +were found wild in England and in Germany, and broom-like oaks, _Quercus +pedunculata fastigiata_, are recorded from Hessen-Darmstadt, Calabria, +the Pyrenees and other localities. About the real origin of all these +varieties nothing is definitely known. + +The "single-leaved" strawberry is a variety often seen in botanical +gardens, as it is easily propagated by its runners. It was discovered +wild in Lapland at the time of Linnaeus, and appeared afterwards +unexpectedly in a nursery near Versailles. This happened about the year +1760 and Duchesne tested it from seeds and found it constant. This +strain, however, seems to have died out before the end of the 18th +century. In a picture painted by Holbein (1495-1543), strawberry leaves +can be seen agreeing exactly with the monophyllous type. The variety may +thus be assumed to have arisen independently [597] at least thrice, at +different periods and in distant localities. + +From all these statements and a good many others which can be found in +horticultural and botanical literature, it may be inferred that +mutations are not so very rare in nature as is often supposed. Moreover +we may conclude that it is a general rule that they are neither preceded +nor accompanied by intermediate steps, and that they are ordinarily +constant from seed from the first. + +Why then are they not met with more often? In my opinion it is the +struggle for life which is the cause of this apparent rarity; which is +nothing else than the premature death of all the individuals that so +vary from the common type of their species as to be incapable of +development under prevailing circumstances. It is obviously without +consequence whether these deviations are of a fluctuating or of a +mutating nature. Hence we may conclude that useless mutations will soon +die out and will disappear without leaving any progeny. Even if they are +produced again and again by the same strain, but under the same +unfavorable conditions, there will be no appreciable result. + +Thousands of mutations may perhaps take place yearly among the plants of +our immediate vicinity without any chance of being discovered. [598] We +are trained to the appreciation of the differentiating marks of +systematic species. When we have succeeded in discerning these as given +by our local flora lists, we rest content. Meeting them again we are in +the habit of greeting them with their proper names. Such is the +satisfaction ensuing from this knowledge that we do not feel any +inclination for further inquiry. Striking deviations, such as many +varietal characters, may be remarked, but then they are considered as +being of only secondary interest. Our minds are turned from the +delicately shaded features which differentiate elementary species. + +Even in the native field of the evening-primroses, no botanist would +have discovered the rosettes with smaller or paler leaves, constituting +the first signs of the new species. Only by the guidance of a distinct +theoretical idea were they discovered, and having once been pointed out +a closer inspection soon disclosed their number. + +Variability seems to us to be very general, but very limited. The limits +however, are distinctly drawn by the struggle for existence. Of course +the chance for useful mutations is a very small one. We have seen that +the same mutations are as a rule repeated from time to time by the same +species. Now, if a useful mutation, [599] or even a wholly indifferent +one, might easily be produced, it would have been so, long ago, and +would at the present time simply exist as a systematic variety. If +produced anew somewhere the botanist, would take it for the old variety +and would omit to make any inquiry as to its local origin. + +Thousands of seeds with perhaps wide circles of variability are ripened +each year, but only those that belong to the existing old narrow circles +survive. How different would Nature appear to us if she were free to +evolve all her potentialities! + +Darwin himself was struck with this lack of harmony between common +observations and the probable real state of things. He discussed it in +connection with the cranesbill of the Pyrenees (_Geranium pyrenaicum_). +He described how this fine little plant, which has never been +extensively cultivated, had escaped from a garden in Staffordshire and +had succeeded in multiplying itself so as to occupy a large area. + +In doing so it had evidently found place for an uncommonly large number +of plantlets from its seeds and correspondingly it had commenced to vary +in almost all organs and qualities and nearly in all imaginable +directions. It displayed under these exceptional circumstances a +capacity which never had been exceeded and [600] which of course would +have remained concealed if its multiplication had been checked in the +ordinary way. + +Many species have had occasion to invade new regions and cover them with +hundreds of thousands of individuals. First are to be cited those +species which have been introduced from America into Europe since the +time of Columbus, or from Europe into this country. Some of them have +become very common. In my own country the evening-primroses and Canada +fleabane or are examples, and many others could be given. They should be +expected to vary under these circumstances in a larger degree. Have they +done so? Manifestly they have not struck out useful new characters that +would enable their bearers to found new elementary species. At least +none have been observed. But poor types might have been produced, and +periods of mutability might have been gone through similar to that which +is now under observation for Lamarck's evening primrose in Holland. + +From this discussion we may infer that the chances of discovering new +mutating species are great enough to justify the utmost efforts to +secure them. It is only necessary to observe large numbers of plants, +grown under circumstances which allow the best opportunities for [601] +all the seeds. And as nature affords such opportunities only at rare +intervals, we should make use of artificial methods. Large quantities of +seed should be gathered from wild plants and sowed under very favorable +conditions, giving all the nourishment and space required to the young +seedlings. It is recommended that they be sown under glass, either in a +glass-house or protected against cold and rain by glass-frames. The same +lot of seed will be seen to yield twice or thrice as many seedlings if +thus protected, compared with what it would have produced when sown in +the field or in the garden. I have nearly wholly given up sowing seeds +in my garden, as circumstances can be controlled and determined with +greater exactitude when the sowing is done in a glasshouse. + +The best proof perhaps, of the unfavorable influence of external +conditions for slightly deteriorated deviations is afforded by +variegated leaves. Many beautiful varieties are seen in our gardens and +parks, and even corn has a variety with striped leaves. They are easily +reproduced, both by buds and by seeds, and they are the most ordinary of +all varietal deviations. They may be expected to occur wild also. But no +real variegated species, nor even good varieties with this attribute +occurs in nature. [602] On the other hand occasional specimens with a +single variegated leaf, or with some few of them, are actually met with, +and if attention is once drawn to this question, perhaps a dozen or so +instances might be brought together in a summer. But they never seem to +be capable of further evolution, or of reproducing themselves +sufficiently and of repeating their peculiarity in their progeny. They +make their appearance, are seen during a season, and then disappear. +Even this slight incompleteness of some spots on one or two leaves may +be enough to be their doom. + +It is a common belief that new varieties owe their origin to the direct +action of external conditions and moreover it is often assumed that +similar deviations must have similar causes, and that these causes may +act repeatedly in the same species, or in allied, or even systematically +distant genera. No doubt in the end all things must have their causes, +and the same causes will lead under the same circumstances to the same +results. But we are not justified in deducing a direct relation between +the external conditions and the internal changes of plants. These +relations may be of so remote a nature that they cannot as yet be +guessed at. Therefore only direct experience may be our guide. Summing +up the result of our facts and discussions [603] we may state that wild +new elementary species and varieties are recorded to have appeared from +time to time. Invariably this happened by sudden leaps and without +intermediates. The mutants are constant when propagated by seed, and at +once constitute a new race. In rare instances this may be of sufficient +superiority to win a place for itself in nature, but more often it has +qualities which have led to its introduction into gardens as an +ornamental plant or into botanical gardens by reason of the interest +afforded by their novelty, or by their anomaly. + +Many more mutations may be supposed to be taking place all around us, +but artificial sowings on a large scale, combined with a close +examination of the seedlings and a keen appreciation of the slightest +indications of deviation seem required to bring them to light. + + +[604] +LECTURE XXI + +MUTATIONS IN HORTICULTURE + +It is well known that Darwin based his theory of natural selection to a +large extent upon the experience of breeders. Natural and artificial +selection exhibit the same general features, yet it was impossible in +Darwin's time to make a critical and comparative analysis of the two +processes. + +In accordance with our present conception there is selection of species +and selection within the species. The struggle for life determines which +of a group of elementary species shall survive and which shall +disappear. In agricultural practice the corresponding process is usually +designated by the name of variety-testing. Within the species, or within +the variety, the sieve of natural selection is constantly eliminating +poor specimens and preserving those that are best adapted to live under +the given conditions. Some amelioration and some local races are the +result, but this does not appear to be of much importance. On the +contrary, the selection [605] within the race holds a prominent place in +agriculture, where it is known by the imposing term, race-breeding. + +Experience and methods in horticulture differ from those in agriculture +in many points. Garden varieties have been tested and separated for a +long time, but neither vegetables nor flowers are known to exhibit such +motley groups of types as may be seen in large forage crops. + +New varieties which appear from time to time may be ornamental or +otherwise in flowers, and more or less profitable than their parents in +vegetables and fruits. In either case the difference is usually +striking, or if not, its culture would be unprofitable. + +The recognition of useful new varieties being thus made easy, the whole +attention of the breeder is reduced to isolating the seeds of the +mutants that are to be saved and sown separately, and this process must +be repeated during a few years, in order to produce the quantity of seed +that is needed for a profitable introduction of the variety into +commerce. In proportion to the abundance of the harvest of each year +this period is shorter for some and longer for other species. + +Isolation in practice is not so simple nor so easy an affair as it is in +the experimental garden. Hence we have constant and nearly unavoidable +[606] cross-fertilizations with the parent form or with neighboring +varieties, and consequent impurity of the new strain. This impurity we +have called vicinism, and in a previous lecture have shown its effects +upon the horticultural races on one hand, and on the other, on the +scientific value that can be ascribed to the experience of the breeder. +We have established the general rule that stability is seldom met with, +but that the observed instability is always open to the objection of +being the result of vicinism. Often this last agency is its sole cause; +or it may be complicated with other factors without our being able to +discern them. + +Though our assertion that the practice of the horticulturist in +producing new varieties is limited to isolation, whenever chance affords +them, is theoretically valid, it is not always so. We may discern +between the two chief groups of varieties. The retrograde varieties are +constant, the individuals not differing more from one another than those +of any ordinary species. The highly variable varieties play an important +part in horticulture. Double flowers, striped flowers, variegated leaves +and some others yield the most striking instances. Such forms have been +included in previous lectures among the ever-sporting varieties, because +their peculiar characters oscillate between two extremes, viz: [607] the +new one of the variety and the corresponding character of the original +species. + +In such cases isolation is usually accompanied by selection: rarely has +the first of a double, striped or variegated race well filled or richly +striped flowers or highly spotted leaves. Usually minor degrees of the +anomaly are seen first, and the breeder expects the novelty to develop +its features more completely and more beautifully in subsequent +generations. Some varieties need selection only in the beginning, in +others the most perfect specimens must be chosen every year as +seed-bearers. For striped flowers, it has been prescribed by Vilmorin, +that seeds should be taken only from those with the smallest stripes, +because there is always reversion. Mixed seed or seed from medium types +would soon yield plants with too broad stripes, and therefore less +diversified flowers. + +In horticulture, new varieties, both retrograde and ever-sporting, are +known to occur almost yearly. Nevertheless, not every novelty of the +gardener is to be considered as a mutation in the scientific sense of +the word. First of all, the novelties of perennial and woody species are +to be excluded. Any extreme case of fluctuating variability may be +preserved and multiplied in the vegetative way. Such types are +designated [608] in horticulture as varieties, though obviously they are +of quite another nature than the varieties reproduced by seed. Secondly, +a large number, no doubt the greater number of novelties, are of hybrid +origin. Here we may discern two cases. Hybrids may be produced by the +crossing of old types, either of two old cultivated forms or newly +introduced species, or ordinarily between an old and an introduced +variety. Such novelties are excluded from our present discussion. +Secondly, hybrids may be produced between a true, new mutation and some +of the already existing varieties of the same species. Examples of this +obvious and usual practice will be given further on, but it must be +pointed out now that by such crosses a single mutation may produce as +many novelties as there are available varieties of the same species. + +Summarizing these introductory remarks we must lay stress on the fact +that only a small part of the horticultural novelties are real +mutations, although they do occur from time to time. If useful, they are +as a rule isolated and multiplied, and if necessary, improved by +selection. They are in many instances, as constant from seed as the +unavoidable influence of vicinism allows them to be. Exact observations +on the origin, or on the degree of constancy, are usually lacking, [609] +the notes being ordinarily made for commercial purposes, and often only +at the date of introduction into trade, when the preceding stages of the +novelty may have been partly forgotten. + +With this necessary prelude I will now give a condensed survey of the +historical facts relating to the origin of new horticultural varieties. +An ample description has been given recently by Korshinsky, a Russian +writer, who has brought together considerable historical material as +evidence of the sudden appearance of novelties throughout the whole +realm of garden plants. + +The oldest known, and at the same time one of the most accurately +described mutations is the origin of the cut-leaved variety of the +greater celandine or _Chelidonium majus_. This variety has been +described either as such, or as a distinct species, called _Chelidonium +laciniatum_ Miller. + +It is distinguished from the ordinary species, by the leaves being cut +into narrow lobes, with almost linear tips, a character which is, as we +have seen on a previous occasion, repeated in the petals. It is at +present nearly as commonly cultivated in botanical gardens as the _C. +majus_, and has escaped in many localities and is observed to thrive as +readily as the native wild [610] plants. It was not known until a few +years before the close of the 16th century. Its history has been +described by the French botanist, Rose. It was seen for the first time +in the garden of Sprenger, an apothecary of Heidelberg, where the _C. +majus_ had been cultivated for many years. Sprenger discovered it in the +year 1590, and was struck by its peculiar and sharply deviating +characters. He was anxious to know whether it was a new plant and sent +specimens to Clusius and to Plater, the last of whom transmitted them to +Caspar Bauhin. These botanists recognized the type as quite new and +Bauhin described it some years afterwards in his Phytopinax under the +name of _Chelidonium majus foliis quernis_, or oak-leaved celandine. The +new variety soon provoked general interest and was introduced into most +of the botanical gardens of Europe. It was recognized as quite new, and +repeated search has been made for it in a wild state, but in vain. No +other origin has been discovered than that of Sprenger's garden. +Afterwards it became naturalized in England and elsewhere, but there is +not the least doubt as to its derivation in all the observed cases. + +Hence its origin at Heidelberg is to be considered as historically +proven, and it is of course only legitimate to assume that it originated +in [611] the year 1590 from the seeds of the _C. majus_. Nevertheless, +this was not ascertained by Sprenger, and some doubt as to a possible +introduction from elsewhere might arise. If not, then the mutation must +have been sudden, occurring without visible preparation and without the +appearance of intermediates. + +From the very first, the cut-leaved celandine has been constant from +seed. Or at least it has been propagated by seed largely and without +difficulty. Nothing, however, is known about it in the first few years +of its existence. Later careful tests were made by Miller, Rose and +others and later by myself, which have shown its stability to be +absolute and without reversion, and it has probably been so from the +beginning. The fact of its constancy has led to its specific distinction +by Miller, as varieties were in his time universally, and up to the +present time not rarely, though erroneously, believed to be less stable +than true species. + +Before leaving the laciniate celandine it is to be noted that in crosses +with _C. majus_ it follows the law of Mendel, and for this reason should +be considered as a retrograde variety, the more so, as it is also +treated as such from a morphological point of view by Stahl and others. + +We now come to an enumeration of those cases in which the date of the +first appearance [612] of a new horticultural variety has been recorded, +and I must apologize for the necessity of again quoting many variations, +which have previously been dealt with from another point of view. In +such cases I shall limit myself as closely as possible to historical +facts. They have been recorded chiefly by Verlot and Carriere, who wrote +in Paris shortly after the middle of the past century, and afterwards by +Darwin, Korshinsky, and others. It is from their writings and from +horticultural literature at large that the following evidence is brought +together. + +A very well-known instance is that of the dwarf variety of _Tagetes +signata_, which arose in the nursery of Vilmorin in the year 1860. It +was observed for the first time in a single individual among a lot of +the ordinary _Tagetes signata_. It was found impossible to isolate it, +but the seeds were saved separately. The majority of the offspring +returned to the parental type, but two plants were true dwarfs. From +these the requisite degree of purity for commercial purposes was +reached, the vicinists not being more numerous than 10% of the entire +number. The same mutation had been observed a year earlier in the same +nursery in a lot of _Saponaria calabrica_. The seeds of this dwarf +repeated the variety in the next generation, but in the third none were +observed. Then the variety was [613] thought to be lost, and the culture +was given up, as the Mendelian law of the splitting of varietal hybrids +was not known. According to our present knowledge we might expect the +atavistic descendants of the first dwarf to be hybrids, and to be liable +to split in their progeny into one-fourth dwarfs and three-fourths +normal specimens. From this it is obvious that the dwarfs would have +appeared a second time if the strain had been continued by means of the +seeds of the vicinistic progeny. + +In order to avoid a return to this phase of the question, another use of +the vicinists should at once be pointed out. It is the possibility of +increasing the yield of the new variety. If space admits of sowing the +seeds of the vicinists, a quarter of the progeny may be expected to come +true to the new type, and if they were partly pollinated by the dwarfs, +even a larger number would do so. Hence it should be made a rule to sow +these seeds also, at least when those of the true representatives of the +novelty do not give seed enough for a rapid multiplication. + +Other dwarfs are recorded to have sprung from species in the same sudden +and unexpected manner, as for instance _Ageratum coeruleum_ of the same +nursery, further _Clematis Viticella nana_ and _Acer campestre nanum_. +_Prunus Mahaleb nana_ was discovered in 1828 in one [614] specimen near +Orleans by Mme. LeBrun in a large culture of Mahaleb. _Lonicera tatarica +nana_ appeared in 1825 at Fontenay-aux Roses. A tall variety of the +strawberry is called "Giant of Zuidwijk" and originated at Boskoop in +Holland in the nursery of Mr. van de Water, in a lot of seedlings of the +ordinary strawberry. It was very large, but produced few runners, and +was propagated with much difficulty, for after six years only 15 plants +were available. It proved to be a late variety with abundant large +fruit, and was sold at a high price. For a long time it was prominent in +cultures in Holland only. + +Varieties without prickles are known to have originated all of a sudden +in sundry cases. _Gleditschia sinensis_, introduced in 1774 from China, +gave two seedlings without spines in the year 1823, in the nursery of +Caumzet. It is curious in being one of the rare instances where a +simultaneous mutation in two specimens is acknowledged, because as a +rule, such records comply with the prevailing, though inexact, belief +that horticultural mutations always appear in single individuals. + +From Korshinsky's survey of varieties with cut leaves or laciniate forms +the following cases may be quoted. In the year 1830 a nurseryman named +Jacques had sown a large lot of elms, [615] _Ulmus pedunculata_. One of +the seedlings had cut leaves. He multiplied it by grafting and gave it +to the trade under the name of _U. pedunculata urticaefolia_. It has +since been lost. + +Laciniate alders seem to have been produced by mutation at sundry times. +Mirbel says that the _Alnus glutinosa laciniata_ is found wild in +Normandy and in the forests of Montmorency near Paris. A similar variety +has been met with in a nursery near Orleans in the year 1855. In +connection with this discovery some discussion has arisen concerning the +question whether it was probable that the Orleans strain was a new +mutation, or derived in some way from the trees cited by Mirbel. Of +course, as always in such cases, any doubt, once pronounced, affects the +importance of the observation for all time, since it is impossible to +gather sufficient historical evidence to fully decide the point. The +same variety had appeared under similar circumstances in a nursery at +Lyons previously (1812). + +Laciniated maples are said to be of relatively frequent occurrence in +nurseries, among seedlings of the typical species. Loudon says that once +100 laciniated seedlings were seen to originate from seed of some normal +trees. But in this case it is rather probable that the presumed [616] +normal parents were in reality hybrids between the type and the +laciniated form, and simply split according to Mendel's law. This +hypothesis is partly founded on general considerations and partly on +experiments made by myself with the cut-leaved celandine, previously +alluded to, which I crossed with the type. The hybrids repeated the +features of the species and showed no signs of their internal hybrid +constitution. But the following year one-fourth of their progeny +returned to the cut-leaved form. If the same thing has taken place in +the case of Loudon's maples, but without their hybrid origin being +known, the result would have been precisely what he observed. + +_Broussonetia papyriffera dissecta_ originated about 1830 at Lyons, and +a second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts, +birches, beeches and others have mostly been found in the wild state, as +I have already pointed out in a previous lecture. A similar variety of +the elder, _Sambucus nigra laciniata_, and its near ally, _Sambucus +racemosa laciniata_, are often to be seen in our gardens. They have been +on record since 1886 and come true from seed, but their exact origin +seems to have been forgotten. Cut-leaved walnuts have been known since +1812; they come true from seed, but are extremely liable to vicinism, a +nuisance which is [617] ascribed by some authors to the fact that often +on the same tree the male catkins flower and fall off several weeks +before the ripening of the pistils of the other form of flowers. + +Weeping varieties afford similar instances. _Sophora japonica pendula_ +originated about 1850, and _Gleditschia triacanthos pendula_ some time +later in a nursery at Chateau-Thierry (Aisne, France). In the year 1821 +the bird's cherry, or _Prunus Padus_, produced a weeping variety, and in +1847 the same mutation was observed for the allied _Prunus Mahaleb_. +Numerous other instances of the sudden origin of weeping trees, both of +conifers and of others, have been brought together in Korshinsky's +paper. This striking type of variation includes perhaps the best +examples of the whole historical evidence. As a rule they appear in +large sowings, only one, or only a few at a time. Many of them have not +been observed during their youth, but only after having been planted out +in parks and forests, since the weeping characters show only after +several years. + +The monophyllous bastard-acacia originated in the same way. Its +peculiarities will be dealt with on another occasion, but the +circumstances of its birth may as well be given here. In 1855 in the +nursery of Deniau, at Brain-sur-l'Authion (Maine et Loire), it appeared +in a lot of [618] seedlings of the typical species in a single +individual. This was transplanted into the Jardin des Plantes at Paris, +where it flowered and bore seeds in 1865. It must have been partly +pollinated by the surrounding normal representatives of the species, +since the seeds yielded only one-fourth of true offspring. This +proportion, however, has varied in succeeding years. Briot remarks that +the monophyllous bastard acacia is liable to petaloid alterations of its +stamens, which deficiency may encroach upon its fertility and +accordingly upon the purity of its offspring. + +Broom-like varieties often occur among trees, and some are known for +their very striking reversions by buds, as we have seen on a previous +occasion. They are ordinarily called pyramidal or fastigiate forms, and +as far as their history goes, they arise suddenly in large sowings of +the normal species. The fastigiate birch was produced in this way by +Baumann, the _Abies concolor fastigiata_ by Thibault and Keteleer at +Paris, the pyramidal cedar by Paillat, the analogous form of +_Wellingtonia_ by Otin. Other instances could easily be added, though of +course some of the most highly prized broom-like trees are so old that +nothing is known about their origin. This, for instance, is the case +with the pyramidal yew-tree, _Taxus baccata fastigiata_. [619] Others +have been found wild, as already mentioned in a former lecture. + +An analogous case is afforded by the purpleleaved plums, of which the +most known form is Prunus Pissardi. It is said to be a purple variety of +_Prunus cerasifera_, and was introduced at the close of the seventies +from Persia, where it is said to have been found in Tabris. A similar +variety arose independently and unexpectedly in the nursery of Spath, +near Berlin, about 1880, but it seems to differ in some minor points +from the Persian prototype. + +A white variety of _Cyclamen vernum_ made its appearance in the year +1836 in Holland. A single individual was observed for the first time +among a large lot of seedlings, in a nursery near Haarlem. It yielded a +satisfactory amount of seed, and the progeny was true to the new type. +Such plants propagate slowly, and it was only twenty-seven years later +(1863) that the bulbs were offered for sale by the Haarlem firm of +Krelage & Son. The price of each bulb was $5.00 in that year, but soon +afterwards was reduced to $1.00 each, which was about thrice the +ordinary price of the red variety. + +The firm of Messrs. Krelage & Son has brought into commerce a wide range +of new bulb-varieties, all due to occasional mutations, some by seed and +others by buds, or to the accidental [620] transference of new qualities +into the already existing varieties by cross-pollination through the +agency of insects. Instead of giving long lists of these novelties, I +may cite the black tulips, which cost during the first few years of +their introduction about $25.00 apiece. + +Horticultural mutations are as a rule very rare, especially in genera or +species which have not yet been brought to a high degree of variability. +In these the wide range of varieties and the large scale in which they +are multiplied of course give a greater chance for new varieties. But +then the possibilities of crossing are likewise much larger, and +apparent changes due to this cause may easily be taken for original +mutations. + +The rarity of the mutations is often proved by the lapse of time between +the introduction of a species and its first sport. Some instances may be +given. They afford a proof of the length of the period during which the +species remained unaltered, although some of these alterations may be +due to a cross with an allied form. _Erythrina Crista-galli_ was +introduced about 1770, and produced its first sport in 1884, after more +than a century of cultivation. _Begonia semperflorens_ has been +cultivated since 1829, and for half a century before it commenced +sporting. The same length of time has elapsed [621] between the first +culture and the first variation of _Crambe maritima_. Other cases are on +record in which the variability exhibited itself much sooner, perhaps +within a few years after the original discovery of the species. But such +instances seem, as a rule, to be subject to doubt as to the concurrence +of hybridization. So for instance the _Iris lortetii_, introduced in the +year 1895 from the Lebanon, which produced a white variety from its very +first seeds. If by chance the introduced plants were natural hybrids +between the species and the white variety, this apparent and rather +improbable mutation would find a very simple explanation. The length of +the period preceding the first signs of variability is largely, of +course, due to divergent methods of culture. Such species as +_Erythrina_, which are perennial and only sown on a small scale, should +not be expected to show varieties very soon. Annual species, which are +cultivated yearly in thousands or even hundreds of thousands of +individuals, have a much better chance. Perhaps the observed differences +are largely due to this cause. + +Monstrosities have, from time to time, given rise to cultivated races. +The cockscomb or _Celosia_ is one of the most notorious instances. +Cauliflowers, turnips and varieties of cabbages are recorded by De +Candolle to have arisen in [622] culture, more than a century ago, as +isolated monstrous individuals. They come true from seed, but show +deviations from time to time which seem to be intimately linked with +their abnormal characters. Apetalous flowers may be considered as +another form of monstrosity, and in _Salpiglossis sinuata_ such a +variety without a corolla made its appearance in the year 1892 in the +nursery of Vilmorin. It appeared suddenly, yielded a good crop of seed +and was constant from the outset, without any sign of vicinism or +impurity. + +In several cases the origin of a variety is obscure, while the +subsequent historical evidence is such as to make an original sudden +appearance quite probable. Although these instances offer but indirect +evidence, and will sooner or later lose their importance, it seems +desirable to lay some stress on them here, because most of these cases +are very obvious and more striking than purely historical facts. Sterile +varieties belong to this heading. Sometimes they bear fruit without +kernels, sometimes flowers without sexual organs, or even no flowers at +all. Instances have been given in the lecture on retrograde varieties; +they are ordinarily assumed to have originated by a leap, because it is +not quite clear how a loss of the capacity for the formation of seeds +could have been slowly accumulated [623] in preceding generations. An +interesting case is afforded by a sterile variety of corn, which +originated some time ago in my own pedigree-cultures made for another +purpose, and which had begun with an ear of 1886. The first generation +from the original seeds showed nothing particular, but the second at +once produced quite a number of sterile plants. The sterility was caused +by the total lack of branches, including those bearing the pistillate +flowers. The terminal spikes themselves were reduced to naked spindles, +without branches, without flowers and even almost without bracts. + +In some individuals, however, this negative character was seen to give +way at the tip, showing a few small naked branches. Of course it was +impossible to propagate this curious form, but my observations showed +that it sprang into existence from known ancestors by a single step or +sudden leap. This leap, however, was not confined to a single specimen; +on the contrary it affected 40 plants out of a culture of 340 +individuals. The same phenomenon was repeated from the seeds of the +normal plants in the following year, but afterwards the monstrosity +disappeared. + +The Italian poplar affords another instance. It is considered by some +authors as a distinct species, _Populus italica_, and by others as a +[624] broom-like variety of the _Populus nigra_, from which it is +distinguished by its erect branches and other characters of minor +importance. It is often called the pyramidal or fastigiate poplar. Its +origin is absolutely unknown and it occurs only in the cultivated state. +In Italy it seems to have been cultivated from the earliest historical +times, but it was not introduced into other countries till the +eighteenth century. In 1749 it was brought into France, and in 1758 into +England, and to day it may be seen along roads throughout central Europe +and in a large part of Asia. But the most curious fact is that it is +only observed in staminate specimens; pistillate trees have not been +found, although often sought for. This circumstance makes it very +probable that the origin of the broom-like poplar was a sudden mutation, +producing only one individual. This being staminate, it has been +propagated exclusively by cuttings. It is to be admitted, however, that +no material evidence is at hand to prove that it is not an original wild +species, the pistillate form of which has been lost by vegetative +multiplication. One form only of many dioecious plants is to be found in +cultivation, as, for instance some South American species of _Ribes_. + +Total lack of historical evidence concerning [625] the origin of a +variety has sometimes been considered as sufficient proof of a sudden +origin. The best known instance is that of the renowned cactus-dahlia +with its recurved instead of incurved ray-florets. It was introduced +from Mexico into the Netherlands by Van den Berg of Jutphaas, under the +following remarkable circumstances. In the autumn of 1872 one of his +friends had sent him a small case, containing seeds, bulbs and roots +from Mexico. From one of these roots a _Dahlia_ shoot developed. It was +cultivated with great care and bloomed next year. It surprised all who +saw it by the unexpected peculiarity of its large rich crimson flowers, +the rays of which were reversed tubular. The margins of the narrow rays +were curved backwards, showing the bright color of the upper surface. It +was a very showy novelty, rapidly multiplied by cuttings, and was soon +introduced into commerce. It has since been crossed with nearly all +other available varieties of the _Dahlia_, giving a large and rich group +of forms, bound together by the curious curling of the petals. It has +never been observed to grow in Mexico, either wild or in gardens, and +thus the introduced individual has come to be considered as the first of +its race. + +I have already mentioned that the rapid production of large numbers of +new varieties, by [626] means of the crossing of the offspring of a +single mutant with previously existing sorts, is a very common feature +in horticultural practice. It warns us that only a small part of the +novelties introduced yearly are due to real mutations. Further instances +of novelties with such a common origin are the purple-leaved dahlias, +the gooseberries without prickles, the double petunias, erect gloxinias +and many others. Accumulation of characters, acquired in different races +of a species, may easily be effected in this way; in fact it is one of +the important factors in the breeding of horticultural novelties. + +I have alluded more than once in this lecture to the question, whether +it is probable that mutations occur in one individual or in more. The +common belief among horticulturists is that, as a rule, they appear in a +single plant. This belief is so widespread that whenever a novelty is +seen for the first time in two or more specimens it is at once suggested +that it might have originated and been overlooked in a previous +generation. Not caring to confess a lack of close observation, the +number of mutants in such cases is usually kept secret. At least this +statement has been made to me by some of the horticulturists at Erfurt, +whom I visited some years ago in order to learn as much as [627] +possible about the methods of production of their novelties. Hence it is +simply impossible to decide the question on the basis of the experience +of the breeders. Even in the case of the same novelty arising in sundry +varieties of the same species, the question as to common origin, by +means of crossing, is often hard to decide, as for instance in +moss-roses and nectarines. On the other hand, instances are on record +where the same novelty has appeared at different times, often at long +intervals. Such is the case with the butterfly-cyclamen, a form with +wide-spreading petals which originated in Martin's nursery in England. +The first time it was seen it was thought to be of no value, and was +thrown away, but when appearing for a second time it was multiplied and +eventually placed on the market. Other varieties of _Cyclamen_, as for +instance the crested forms, are also known to have originated +repeatedly. + +In concluding this series of examples of horticultural mutations, I +might mention two cases, which have occurred in my own experimental +garden. The first refers to a tubular _Dahlia_. It has ray-florets, the +ligules of which have their margins grown together so as to form tubes, +with the outer surface corresponding to the pale under-surface of the +corolla. + +This novelty originated in a single plant in a [628] culture from the +seed of the dwarf variety "Jules Chretien." The seeds were taken from +introduced plants in my garden, and as the sport has no ornamental value +it is uncertain whether this was the first instance or whether it had +previously occurred in the nursery at Lyons, from whence the bulbs were +secured. Afterwards it proved true from seed, but was very variable, +exhibiting rather the features of an ever-sporting variety. + +Another novelty was seen the first time in several individuals. It was a +pink sport of the European cranesbill, _Geranium pratense_. It arose +quite unexpectedly in the summer of 1902 from a striped variety of the +blue species. It was seen in seven specimens out of a lot of about a +hundred plants. This strain was introduced into my garden in 1897, when +I bought two plants under the name of _Geranium pratense album_, which +however proved to belong to the striped variety. From their seeds I +sowed in 1898 a first generation, of which a hundred plants flowered the +next year, and from their seeds I sowed in 1900 the lot which produced +the sport. Neither the introduced plants nor their offspring had +exhibited the least sign of a color-variation, besides the blue and +white stripes. Hence it is very probable that my novelty was a true +first mutation, the more probably [629] so since a pink variety would +without doubt have a certain horticultural value and would have been +preserved if it had occurred. But as far as I have been able to +ascertain, it is as yet unknown, nor has it been described until today. + +Summing up the results of this long, though very incomplete, list of +horticultural novelties with a more or less well-known origin, we see +that sudden appearances are the rule. Having once sprung into existence +the new varieties are ordinarily constant, except as affected by +vicinism. Details concerning the process are mostly unavailable or at +least are of very doubtful value. And to this it should be added that +really progressive mutations have hardly been observed in horticulture. +Hence the theoretical value of the facts is far less than might have +been expected. + + +[630] +LECTURE XXII + +SYSTEMATIC ATAVISM + +The steady cooperation of progression and retrogression is one of the +important principles of organic evolution. I have dwelt upon this point +more than once in previous lectures. I have tried to show that both in +the more important lines of the general pedigree of the vegetable +kingdom, and in the numerous lateral branches ending in the genera and +species within the families, progression and retrogression are nearly +always at work together. Your attention has been directed to the +monocotyledons as an example, where retrogression is everywhere so +active that it can almost be said to be the prevailing movement. +Reduction in the vegetative and generative organs, in the anatomical +structure and growth of the stems, and in sundry other ways is the +method by which the monocotyledons have originated as a group from their +supposed ancestors among the lower dicotyledonous families. +Retrogression is the leading idea in the larger families of the group, +[631] as for instance in the aroids and the grasses. Retrograde +evolution is also typical in the highest and most highly differentiated +family of the monocotyledons, the orchids, which have but one or two +stamens. In the second place I have had occasion more than once to +assert that retrogression, though seemingly consisting in the +disappearance of some quality, need not, as a rule, be considered as a +complete loss. Quite on the contrary, it is very probable that real +losses are extremely rare, if not wholly lacking. Ordinarily the loss is +only apparent, the capacity becomes inactive only, but is not destroyed. +The character has become latent, as it is commonly stated, and therefore +may return to activity and to the full display of its peculiarity, +whenever occasion offers. + +Such a return to activity was formerly called atavism. But as we have +seen, when dealing with the phenomena of latency at large, sundry cases +of latency are to be distinguished, in order to get a clear insight into +these difficult processes. + +So it is with atavism, too. If any plant reverts to a known ancestor, we +have a positive and simple case. But ancestors with alternate specific +marks are as a rule neither historically nor experimentally manifest. +They are only reputed to be such, and the presumption rests [632] upon +the systematic affinity between the derivative species and its nearest +probable allies. Such reversions are now to be examined at some length +and may be adequately treated under the head of systematic atavism. To +this form of atavism pertain, on the basis of our definition, those +phenomena by which species assume one or more characters of allies, from +which they are understood to have descended by the loss of the character +under discussion. The phenomena themselves consist in the production of +anomalies and varieties, and as the genetic relation of the latter is +often hardly beyond doubt, the anomalies seem to afford the best +instances for the study of systematic atavism. This study has for its +chief aim the demonstration of the presence of the latent characters, +and to show that they return to activity suddenly and not by a slow and +gradual recovery of the former features. It supports the assertion that +the visible elementary characters are essentially an external display of +qualities carried by the bearers of heredity, and that these bearers are +separate entities, which may be mingled together, but are not fused into +a chaotic primitive life-substance. Systematic atavism by this means +leads us to a closer examination of the internal and concealed causes, +which rule the affinities and divergencies of [633] allied species. It +brings before us and emphasizes the importance of the conception of the +so-called unit-characters. + +The primrose will serve as an example. In the second lecture we have +seen that the old species of Linnaeus, the _Primula veris_, was split up +by Jacquin into three smaller ones, which are called _P. officinalis_, +_P. elatior_ and _P. acaulis_. From this systematic treatment we can +infer that these three forms are assumed to be derived from a common +ancestor. Now two of them bear their flowers in bracted whorls, +condensed into umbels at the summits of a scape. The scapes themselves +are inserted in the axils of the basal leaves, and produce the flowers +above them. In the third species, _Primula acaulis_, this scape is +lacking and the flowers are inserted singly in the axils on long slender +stalks. For this reason the species is called acaulescent, indicating +that it has no other stem than the subterranean rootstock. But on closer +inspection we observe that the flower stalks are combined into little +groups, each group occupying the aril of one of the basal leaves. This +fact at once points to an analogy with the umbellate allies, and induces +us to examine the insertion of the flowers more critically. In doing so +we find that they are united at their base so as to constitute a sessile +umbel. [634] The scapes are not absolutely lacking, but only reduced to +almost invisible rudiments. + +Relying upon this conclusion we infer that all of the three elementary +species have umbels, some pedunculate and the others not. On this point +they agree with the majority of the allied species in the genus and in +other genera, as for instance in _Androsace_. Hence the conclusion that +the common ancestors were perennial plants with a rootstock bearing +their flowers in umbels or whorls on scapes. Lacking in the _Primula +veris_, these scapes must obviously have been lost at the time of the +evolution of this form. + +Proceeding on this line of speculation we at once see that a very +adequate opportunity for systematic atavism is offered here. According +to our general conception the apparent loss of a scape is no proof of a +corresponding internal loss, but might as well be caused simply by the +reduction of the scape-growing capacity to a latent or inactive state. +It might be awakened afterwards by some unknown agency, and return to +activity. + +Now this is exactly what happens from time to time. In Holland the +acaulescent primrose is quite a common plant, filling the woods in the +spring with thousands of clusters of bright yellow flowers. It is a very +uniform type, but in [635] some years it is seen to return to atavistic +conditions in some rare individuals. More than once I have observed such +cases myself, and found that the variation is only a partial one, +producing one or rarely two umbels on the same plant, and liable to fail +of repetition when the varying specimens are transplanted into the +garden for further observation. But the fact remains that scapes occur. +The scapes themselves are of varying length, often very short, and +seldom long, and their umbels display the involucre of bracts in a +manner quite analogous to that of the _Primula officinalis_ and _P. +elatior_. To my mind this curious anomaly strongly supports the view of +the latent condition of the scape in the acaulescent species, and that +such a dormant character must be due to a descent from ancestors with +active scapes, seems to be in no need of further reiteration. Returning +to activity the scapes at once show a full development, in no way +inferior to that of the allied forms, and only unstable in respect to +their length. + +A second example is afforded by the bracts of the crucifers. This group +is easily distinguished by its cruciform petals and the grouping of the +flowers into long racemes. In other families each flower of such an +inflorescence would be subtended by a bract, according to the [636] +general rule that in the higher plants side branches are situated in the +arils of leaves. Bracts are reduced leaves, but the spikes of the +cruciferous plants are generally devoid of them. The flower-stalks, with +naked bases, seem to arise from the common axis at indefinite points. + +Hence the inference that crucifers are an exception to a general rule, +and that they must have originated from other types which did comply +with this rule, and accordingly were in the possession of floral bracts. +Or, in other words, that the bracts must have been lost during the +original evolution of the whole family. This conclusion being accepted, +the accidental re-apparition of bracts within the family must be +considered as a case of systematic atavism, quite analogous to the +re-appearance of the scapes in the acaulescent primrose. The systematic +importance of this phenomenon, however, is far greater than in the first +case, in which we had only to deal with a specific character, while the +abolition of the bracts has become a feature of a whole family. + +This reversion is observed to take place according to two widely +different principles. On one hand, bracts may be met with in a few stray +species, assuming the rank of a specific character. On the other hand +they may be seen [637] to occur as an anomaly, incompletely developed, +often very rare and with all the appearance of an accidental variation, +but sometimes so common as to seem nearly normal. + +Coming now to particular instances, we may turn our attention in the +first place to the genus _Sisymbrium_. This is a group of about 50 +species, of wide geographic distribution, among which the hedge mustard +(_S. officinalis_) is perhaps the most common of weeds. Two species are +reputed to have bracts, _Sisymbrium hirsutum_ and _S. supinum_. Each +flower-stalk of their long racemes is situated in the aril of such a +bract, and the peculiarity is quite a natural one, corresponding exactly +to what is seen in the inflorescence of other families. Besides the +_Sisymbrium some six other genera afford similar structures. + +_Erucastrum pollichii_ has been already alluded to in a former lecture +when dealing with the same problem from another point of view. As +previously stated, it is one of the most manifest and most easily +accessible examples of a latent character becoming active through +systematic atavism. In fact, its bracts are found so often as to be +considered by some authors as of quite normal occurrence. Contrasted +with those of the above mentioned species of _Sisymbrium_, they are not +seen at the base of all the flower [638] stalks, but are limited to the +lowermost part of the raceme, adorning a few, often ten or twelve, and +rarely more flower-stalks. Moreover they exhibit a feature which is +indicative of the presence of an abnormality. They are not all of the +same size, but decrease in length from the base of the raceme upward, +and finally slowly disappear. + +Besides these rare cases there are quite a number of cruciferous species +on record, which have been observed to bear bracts. Penzig in his +valuable work on teratology gives a list of 33 such genera, many of them +repeating the anomaly in more than one species. Ordinary cabbages are +perhaps the best known instance, and any unusual abundance of +nourishment, or anomalous cause of growth seems to be liable to incite +the development of bracts. The hedge garlic or garlic mustard +(_Alliaria_), the shepherd's purse, the wormseed or _Erysimum +cheiranthoides_ and many others afford instances. In my cultures of +Heeger's shepherd's purse, the new species derived at Landau in Germany +from the common shepherd's purse, the anomaly was observed to occur more +than once, showing that the mutation, which changed the fruits, had not +in the least affected this subordinate anomalous peculiarity. In all +these cases the bracts behave as with the Erucastrum, [639] being +limited to the base of the spike, and decreasing in size from the lower +flowers upward. Connected with these atavistic bracts is a feature of +minor importance, which however, by its almost universal accompaniment +of the bracts, deserves our attention, as it is indicative of another +latent character. As a rule, the bracts are grown together with their +axillary flower-stalk. This cohesion is not complete, nor is it always +developed in the same degree. Sometimes it extends over a large part of +the two organs, leaving only their tips free, but on other occasions it +is limited to a small part of the base. But it is very interesting that +this same cohesion is to be seen in the shepherd's purse, in the +wormseed and in the cabbage, as well as in the case of the _Erucastrum_ +and most of the other observed cases of atavistic bracts. This fact +suggests the idea of a common origin for these anomalies, and would lead +to the hypothesis that the original ancestors of the whole family, +before losing the bracts, exhibited this peculiar mode of cohesion. + +Bracts and analogous organs afford similar cases of systematic atavism +in quite a number of other families. Aroids sometimes produce long +bracts from various places on their spadix, as may be seen in the +cultivated greenhouse species, _Anthurium scherzerianum_. [640] Poppies +have been recorded to bear bracts analogous to the little scales on the +flower-stalks of the pansies, on the middle of their flower stalks. A +similar case is shown by the yellow foxglove or _Digitalis parviflora_. +The foxgloves as a rule have naked flower-stalks, without the two little +opposite leafy organs seen in so many other instances. The yellow +species, however, has been seen to produce such scales from time to +time. The honeysuckle genus is, as a rule, devoid of the stipules at the +base of the petiole, but _Lonicera etrusca_ has been observed to develop +such organs, which were seen to be free in some, but in other specimens +were adnate to the base of the leaf, and even connate with those of the +opposite leaf. + +Other instances could be given proving that bracts and stipules, when +systematically lacking, are liable to reappear as anomalies. In doing +so, they generally assume the peculiar characters that would be expected +of them by comparison with allied genera in which they are of normal +occurrence. There can be no doubt that their absence is due to an +apparent loss, resulting from the reduction of a formerly active quality +to inactivity. Resuming this effective state, the case attains the value +and significance accorded to systematic atavism. + +A very curious instance of reduced bracts, developing [641] to unusual +size, is afforded by a variety of corn, which is called _Zea Mays +cryptosperma_, or _Zea Mays tunicata_. In ordinary corn the kernels are +surrounded by small and thin, inconspicuous and membranaceous scales. +Invisible on the integrate spikes, when ripe, they are easily detected +by pulling the kernels out. In _cryptosperma_ they are so strongly +developed as to completely hide the kernels. Obviously they constitute a +case of reversion to the characters of some unknown ancestor, since the +corn is the only member of the grass-family with naked kernels. The var. +_tunicata_, for this same reason, has been considered to be the original +wild form, from which the other varieties of corn have originated. But +as no historical evidence on this point is at hand, we must leave it as +it is, notwithstanding the high degree of attractiveness attached to the +suggestion. + +The horsetail-family may be taken as a further support of our assertion. +Some species have stems of two kinds, the fertile being brownish and +appearing in early spring before the green or sterile ones. In others +the stems are all alike, green and crowned with a conelike spike of +sporangia-bearing scales. Manifestly the dimorphous cases are to be +considered as the younger ones, partly because they are obvious +exceptions to the common rule, and [642] partly because the division of +labor is indicative of a higher degree of evolution. But sometimes these +dimorphic species are seen to revert to the primary condition, +developing a fertile cone at the summit of the green summer-stem. I have +had the opportunity of collecting an instance of this anomaly on the +tall _Equisetum telmateja_ in Switzerland, and other cases are on record +in teratological literature. It is an obvious example of systematic +atavism, occurring suddenly and with the full development of all the +qualities needed for the normal production of sporangia and spores. All +of these must be concealed in a latent condition within the young +tissues of the green stems. + +More than once I have had occasion to deal with the phenomenon of +torsions, as exhibited by the teasels and some other plants. This +anomaly has been shown to be analogous to the cases described as double +adaptations. The capacity of evolving antagonistic characters is +prominent in both. The antagonists are assumed to lie quietly together +while inactive. But as soon as evolution calls them into activity they +become mutually exclusive, because only one of them can come to full +display in the same organ. External influences decide which of the two +becomes dominant and which remains dormant. This decision must take +place separately [643] for each stem and each branch, but as a rule, the +stronger ages are more liable to furnish anomalies than the weaker. + +Exactly the same thing is true of double adaptations. Every bud of the +water-persicaria may develop either into an erect or into a floating +stem, according as it is surrounded by water or by relatively dry soil. +In other cases utility is often less manifest, but some use may either +be proved, or shown to be very probable. At all events the term +adaptation includes the idea of utility, and obviously useless +contrivances could hardly be brought under the same head. + +We have also dealt with the question of heredity. It is obvious that +from the flowers of the floating and erect stems of the water-persicaria +seeds will result, each capable of yielding both forms. Quite the same +thing was the case with the teasels. Some 40% of the progeny produce +beautifully twisted stems, but whether the seed was saved from the most +completely twisted specimens or from the straight plants of the race was +of no importance. + +This phenomenon of twisting may now be considered from quite another +point of view. It is a case of systematic atavism, or of the +reacquirement of some ancient and long-lost quality. This quality is the +alternate position of [644] the leaves, which has been replaced in the +teasel family by a grouping in pairs. In order to prove the validity of +this assertion, it will be necessary to discuss two points separately, +viz.: relative positions of the leaves, and the manner in which the +alternate position causes the stems to become twisted. + +Leaves are affixed to their stems and branches in various ways. Among +them one is of wide occurrence throughout the whole realm of the higher +plants, while all the others are more rare. Moreover these subordinate +arrangements are, as a rule, confined to definite systematic groups. +Such groups may be large, as for instance, the monocotyledons, that have +their leaves arranged in two opposite rows in many families, or small, +as genera or subdivisions of genera. Apart from these special cases the +main stem and the greater part of the branches of the pedigree of the +higher plants exhibit a spiral condition or a screw arrangement, all +leaves being inserted at different points and on different sides of the +stem. This condition is assumed to be the original one, from which the +more specialized types have been derived. As is usual with characters in +general, it is seen to vary around an average, the spiral becoming +narrower and looser. A narrow spiral condenses the leaves, while a [645] +loose one disperses them. According to such fluctuating deviations the +number of leaves, inserted upon a given number of spiral circuits, is +different in different species. In a vast majority of cases 13 leaves +are found on 5 circuits, and as we have only to deal with this +proportion in the teasels we will not consider others. + +In the teasels this screw-arrangement has disappeared, and has been +replaced by a decussate grouping. The leaves are combined into pairs, +each pair occupying the opposite sides of one node. The succeeding pairs +alternate with one another, so as to place their leaves at right angles. +The leaves are thus arranged on the whole stem in four equidistant rows. + +On the normal stem of a teasel the two members of a pair are tied to one +another in a comparatively complicated way. The leaves are broadly +sessile and their bases are united so as to constitute a sort of cup. +The margins of these cups are bent upward, thereby enabling them to hold +water, and after a rainfall they may be seen filled to the brim. It is +believed that these little reservoirs are useful to the plant during the +flowering period, because they keep the ants away from the honey. +Considering the internal structure of the stem at the base of these cups +we find that the vascular bundles of the two opposite leaves are +strongly connected [646] with one another, constituting a ring which +narrowly surrounds the stem, and which would impede an increase in +thickness, if such were in the nature of the plant. But since the stems +end their existence during the summer of their development, this +structure is of no real harm. + +The grouping of the leaves in alternate pairs may be seen within the bud +as well as on the adult stems. In order to do this, it is necessary to +make transverse sections through the heart of the rosette of the leaves +of the first year. If cut through the base, the pair exhibit connate +wings, corresponding to the water-cups; if cut above these, the leaves +seem to be free from one another. + +In order to compare the position of leaves of the twisted plants with +this normal arrangement, the best way is to make a corresponding section +through the heart of the rosette of the first year. It is not necessary +to make a microscopic preparation. In the fall the changed disposition +may at once be seen to affect the central leaves of the group. All the +rosettes of the whole race commence with opposite leaves; those that are +to produce straight stems remain in this condition, but the preparation +for twisting begins at the end of the first year as shown by a special +arrangement of the leaves. This [647] disposition may then be seen to +extend to the very center of the rosette, by use of microscopical +sections. Examining sections made in the spring, the original +arrangement of the leaves of the stem is observed to continue until the +beginning of the growth of the shoot. It is easy to estimate the number +of leaves corresponding to a given number of spiral circuits in these +sections and the proportion is found to indicate 13 leaves on 5 turns. +These figures are the same as those given above for the ordinary +arrangement of alternate leaves in the main lines of the pedigree of the +vegetable kingdom. + +Leaving aside for the moment the subsequent changes of this spiral +arrangement, it becomes at once clear that here we have a case of +systematic atavism. The twisted teasels lose their decussation, but in +doing so the leaves are not left in a disorderly dispersion, but a +distinct new arrangement takes its place, which is to be assumed as the +normal one for the ancestors of the teasel family. The case is to be +considered as one of atavism. Obviously no other explanation is +possible, than the supposition that the 5-13 spiral is still latent, +though not displayed by the teasels. But in the very moment when the +faculty of decussation disappears, it resumes its place, and becomes +[648] as prominent as it must once have been in the ancestors, and is +still in that part of their offspring, which has not become changed in +this respect. Thus the proof of our assertion of systematic atavism is, +in this case, not obtained by the inspection of the adult, but by the +investigation of the conditions in an early stage. It remains to be +explained how the twisting may finally be caused by this incipient +grouping of the leaves. Before doing so, it may be as well to state that +the case of the teasel is not an isolated one, and that the same +conclusions are supported by the valerian, and a large number of other +examples. In early spring some rosettes show a special condition of the +leaves, indicating thereby at once their atavism and their tendency to +become twisted as soon as they begin to expand. The Sweet William or +_Dianthus barbatus_ affords another instance; it is very interesting +because a twisted race is available, which may produce thousands of +instances developed in all imaginable degrees, in a single lot of +plants. _Viscaria oculata_ is another instance belonging to the same +family. + +The bedstraw (_Galium_) also includes many species which from time to +time produce twisted stems. I have found them myself in Holland on +_Galium verum_ and _G. Aparine_. Both seem [649] to be of rare +occurrence, as I have not succeeded in getting any repetition by +prolonged culture. + +Species, which generally bear their leaves in whorls, are also subjected +to casual atavisms of this kind, as for instance the tall European +horsetail, _Equisetum Telmateja_, which occasionally bears cones on its +green summer stems. Its whorls are changed on the twisted parts into +clearly visible spirals. The ironwood or _Casuarina quadrivalvis_ is +sometimes observed to produce the same anomaly on its smaller lateral +branches. + +Coming now to the discussion of the way in which the twisting is the +result of the spiral disposition of the leaves, we may consider this +arrangement on stems in the adult state. These at once show the spiral +line and it is easy to follow this line from the base up to the apex. In +the most marked cases it continues without interruption, not rarely +however, ending in a whorl of three leaves and a subsequent straight +internode, of which there may even be two or three. The spiral exhibits +the basal parts of the leaves, with the axillary lateral branches. The +direction of the screw is opposed to that of the twisting, and the +spiral ribs are seen to cross the line of insertion of the leaves at +nearly right angles. On this line the leaves are nearer [650] to one +another than would correspond to the original proportion of 5 turns for +13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on a +single turn. Or the twist may become so strong locally as to change the +spiral into a longitudinal line. On this line all inserted leaves extend +themselves in the same direction, resembling an extended flag. + +The spiral on the stem is simply the continuation of the spiral line +from within the rosettes of the first year. Accordingly it is seen to +become gradually less steep at the base. For this reason it must be one +and the same with this line, and in extreme youth it must have produced +its leaves at the same mutual distances as this line. Transverse +sections of the growing summits of the stems support this conclusion. + +From these several facts we may infer that the steepness of the spiral +line increases on the stem, as it is gradually changed into a screw. +Originally 5 turns were needed for 13 leaves, but this number diminishes +and 4 or 3 or even 2 turns may take the same number of foliar organs, +until the screw itself is changed into a straight line. + +This change consists in an unwinding of the whole spiral, and in order +to effect this the stem must become wound up in the opposite direction. +The winding of the foliar screw must [651] curve the longitudinal ribs. +The straighter and steeper the screw becomes, the more the ribs will +become twisted. That this happens in the opposite direction is obvious, +without further discussion. The twisting is the inevitable consequence +of the reversal of the screw. + +Two points remain to be dealt with. One is the direct proof of the +reversal of the screw, the other the discussion of its cause. The first +may be observed by a simple experiment. Of course it proceeds only +slowly, but all that is necessary is to mark the position of one of the +younger leaves of a growing stem of a twisting individual and to observe +the change in its position in a few hours. It will be seen to have +turned some way around the stem, and finally may be seen to make a +complete revolution in the direction opposite to the screw, and thereby +demonstrating the fact of its uncurling. + +The cause of this phenomenon is to be sought in the intimate connection +of the basal parts of the leaves, which we have detailed above. The +fibrovascular strands constitute a strong rope, which is twisted around +the stem along the line on which the leaves are inserted. The +strengthening of the internodes may stretch this rope to some extent, +but it is too strong to be rent asunder. Hence it opposes the normal +growth, and the only manner in which the internodes [652] may adjust +themselves to the forces which tend to cause their expansion is by +straightening the rope. In doing so they may find the required space, by +growing out in an unusual direction, bending their axes and twisting the +ribs. + +To prove the validity of this explanation, a simple experiment may be +given. If the fibrovascular rope is the mechanical impediment which +hinders the normal growth, we may try the effect of cutting through this +rope. By this means the hindrance may at least locally be removed. Now, +of course, the operation must be made in an early stage before, or at +the beginning of the period of growth, in every case before the +uncurling of the rope begins. Wounds made at this time are apt to give +rise to malformations, but notwithstanding this difficulty I have +succeeded in giving the necessary proof. Stems operated upon become +straight where the rope is cut through, though above and under the +wounded part they go on twisting in the usual way. + +Sometimes the plants themselves succeed in tearing the rope asunder, and +long straight internodes divide the twisted stems in two or more parts +in a very striking manner. A line of torn leaf-bases connects the two +parts of the screw and gives testimony of what has passed within [653] +the tissues. At other times the straightening may have taken place +directly internal to a leaf, and it is torn and may be seen to be +attached to the stem by two distinct bases. + +Summing up this description of the hereditary qualities of our twisted +teasels and of their mechanical consequences, we may say that the loss +of the normal decussation is the cause of all the observed changes. This +special adaptation, which places the leaves in alternating pairs, +replaced and concealed the old and universal arrangement on a screw +line. In disappearing, it leaves the latter free, and according to the +rule of systematic atavism, this now becomes active and takes its place. +If the fibrovascular connection of the leaf-bases were lost at the same +time the stems would grow and become straight and tall. This change +however, does not occur, and the bases of the leaves now constitute a +continuous rope instead of separate rings, and thereby impede the +stretching of the internodes. These in their turn avoid the difficulty +by twisting themselves in a direction opposite to that of the spiral of +the leaves. + +As a last example of systematic atavism I will refer to the reversionary +changes, afforded by the tomatoes. Though the culture of this plant is a +recent one, it seems to be at present in a state of mutability, +producing new strains, or [654] assuming the features of their +presumable ancestors. In his work "The Survival of the Unlike," Bailey +has given a detailed description of these various types. Moreover, he +has closely studied the causes of the changes, and shown the great +tendency of the tomatoes to vicinism. By far the larger part of the +observed cases of running out of varieties are caused by accidental +crosses through the agency of insects. Even improvements are not rarely +due to this cause. Besides these common and often unavoidable changes, +others of greater importance occur from time to time. Two of them +deserve to be mentioned. They are called the "Upright" and the "Mikado" +types, and differ as much or even more from their parents than the +latter do from any one of their wild congeners. Their characters come +true from seed. The "Mikado" race or the _Lycopersicum grandifolium_ +(_L. latifolium_) has larger and fewer leaflets than the slender and +somewhat flimsy foliage of the common form. Flat or plane blades with +decurrent margins constitute another character. This variety, however, +does not concern our present discussion. The upright type has stiff and +self-sustaining stems and branches, resembling rather a potato-plant +than a tomato. Hence the name _Lycopersicum solanopsis_ or _L. validum_, +under which it is usually described. [655] The foliage of the plant is +so distinct as to yield botanical characters of sufficient importance to +justify this specific designation. The leaflets are reduced in numbers +and greatly modified, and the flowers in the inflorescence are reduced +to two or three. This curious race came in suddenly, without any +premonition, and the locality and date of its mutation are still on +record. Until some years ago it had not made its appearance for a second +time. Obviously it is to be considered as a reversionary form. The limp +stems of the common tomatoes are in all respects indicative of the +cultivated condition. They cannot hold themselves erect, but must be +tied up to supports. The color of the leaves is a paler green than +should be expected from a wild plant. Considering other species of the +genus _Solanum_, of which the _Lycopersicum_ is a subdivision, the stems +are as a rule erect and self-supporting, with some few exceptions. +These, however, are special adaptations as shown by the winding stems of +the bitter-sweet. + +From this discussion we seem justified in concluding that the original +appearance of the upright type was of the nature of systematic atavism. +It differs however, from the already detailed cases in that it is not a +monstrosity, nor an ever-sporting race, but is as constant a form [656] +as the best variety or species. Even on this ground it must be +considered as a representative of a separate group of instances of the +universal rule of systematic reversions. + +Of late the same mutation has occurred in the garden of C.A. White at +Washington. The parent form in this case was the "Acme," of the ordinary +weak and spreading habit of growth. It is known as one of the best and +most stable of the varieties and was grown by Mr. White for many years, +and had not given any sign of a tendency towards change. Seeds from some +of the best plants in 1899 were sown the following spring, and the young +seedlings unexpectedly exhibited a marked difference from their parents. +From the very outset they were more strong and erect, more compact and +of a darker green than the "Acme." When they reached the fruiting stage +they had developed into typical representatives of the _Lycopersicum +solanopsis_ or upright division. The whole lot of plants comprised only +some 30 specimens, and this number, of course, is too small to base +far-reaching conclusions upon. But all of the lot showed this type, no +true "Acme" being seen among them. The fruit differed in flavor, +consistency and color from that of the parent, and it also ripened +earlier than the latter. No seed was saved from [657] these plants, but +the following year the "Acme" was sown again and found true to its type. +Seeds saved from this generation in 1900 have, however, repeated the +mutation, giving rise to exactly the same new upright form in 1901. This +was called by its originator "The Washington." Seeds from this second +mutation were kindly sent to me by Mr. White, and proved true to their +type when sown in my garden. + +Obviously it is to be assumed in the case of the tomatoes as well as in +instances from other genera cited, that characters of ancestors, which +are not displayed in their progeny, have not been entirely lost, but are +still present, though in a latent condition. They may resume their +activity unexpectedly, and at once develop all the features which they +formerly had borne. + +Latency, from this point of view, must be one of the most common things +in nature. All organisms are to be considered as internally formed of a +host of units, partly active and partly inactive. Extremely minute and +almost inconceivably numerous, these units must have their material +representatives within the most intimate parts of the cells. + + +[658] +LECTURE XXIII + +TAXONOMIC ANOMALIES + +The theory of descent is founded mainly on comparative studies, which +have the advantage of affording a broad base and the convincing effect +of concurrent evidence brought together from widely different sources. +The theory of mutation on the other hand rests directly upon +experimental investigations, and facts concerning the actual descent of +one form from another are as yet exceedingly rare. It is always +difficult to estimate the validity of conclusions drawn from isolated +instances selected from the whole range of contingent phenomena, and +this is especially true of the present case. Systematic and physiologic +facts seem to indicate the existence of universal laws, and it is not +probable that the process of production of new species would be +different in the various parts of the animal and vegetable kingdoms. +Moreover the principle of unit-characters, the preeminent significance +of which has come to be more fully recognized of late, is in full +harmony [659] with the theory of sudden mutations. Together these two +conceptions go to strengthen the probability of the sudden origin of all +specific characters. + +Experimental researches are limited in their extent, and the number of +cases of direct observation of the process of mutation will probably +never become large enough to cover the whole field of the theory of +descent. Therefore it will always be necessary to show that the +similarity between observed and other cases is such as to lift above all +doubt the assertion of their resulting from the same causes. + +Besides the direct comparison of the mutations described in our former +lectures, with the analogous cases of the horticultural and natural +production of species and varieties at large, another way is open to +obtain the required proof. It is the study of the phenomena, designated +by Casimir de Candolle by the name of taxonomic anomalies. It is the +assertion that characters, which are specific in one case, may be +observed to arise as anomalies or as varieties in other instances. If +they can be shown to be identical or nearly so in both, it is obviously +allowable to assume the same origin for the specific character and for +the anomaly. In other terms, the specific marks may be considered as +having originated according to the laws [660] that govern the production +of anomalies, and we may assume them to lie within reach of our +experiments. The experimental treatment of the origin of species may +also be looked upon as a method within our grasp. + +The validity and the significance of these considerations will at once +become clear, if we choose a definite example. The broadest and most +convincing one appears to me to be afforded by the cohesion of the +petals in gamopetalous flowers. According to the current views the +families with the petals of their flowers united are regarded as one or +two main branches of the whole pedigree of the vegetable kingdom. +Eichler and others assume them to constitute one branch, and therefore +one large subdivision of the system. Bessey, on the other hand, has +shown the probability of a separate origin for those groups which have +inferior ovaries. Apart from such divergencies the connation of the +petals is universally recognized as one of the most important systematic +characters. + +How may this character have originated? The heath-family or the +Ericaceae and their nearest allies are usually considered to be the +lowest of the gamopetalous plants. In them the cohesion of the petals is +still subject to reversionary exceptions. Such cases of atavism may +[661] be observed either as specific marks, or in the way of anomalies. +_Ledum_, _Monotropa_ and _Pyrola_, or the Labrador tea, the Indian pipe +and wintergreen are instances of reversionary gamopetalism with free +petals. In heaths (_Erica Tetralix_) and in rhododendrons the same +deviation is observed to occur from time to time as an anomaly, and even +the common _Rhododendron ponticum_ of our gardens has a variety in which +the corolla is more or less split. Sometimes it exhibits five free +petals, while at other times only one or two are entirely free, the +remaining four being incompletely loosened. + +Such cases of atavism make it probable that the coherence of the petals +has originally arisen by the same method, but by action in the opposite +direction. The direct proof of this conclusion is afforded by a curious +observation, made by Vilmorin upon the bright and large-flowered +garden-poppy, _Papaver bracteatum_. Like all poppies it has four petals, +which are free from one another. In the fields of Messrs. Vilmorin, +where it is largely cultivated for its seeds, individuals occur from +time to time which are anomalous in this respect. They exhibit a +tendency to produce connate petals. Their flowers become monopetalous, +and the whole strain is designated by the name of _Papaver_ [662] +_bracteatum monopetalum_. Henry de Vilmorin had the kindness to send me +some of these plants, and they have flowered in my garden during several +years. The anomaly is highly variable. Some flowers are quite normal, +exhibiting no sign of connation; others are wholly gamopetalous, the +four petals being united from their base to the very margin of the cup +formed. In consequence of the broadness of the petals however, this cup +is so wide as to be very shallow. + +Intermediate states occur, and not infrequently. Sometimes only two or +three petals are united, or the connation does not extend the entire +length of the petals. These cases are quite analogous to the imperfect +splitting of the corolla of the rhododendron. Giving free rein to our +imagination, we may for a moment assume the possibility of a new +subdivision of the vegetable kingdom, arising from Vilmorin's poppy and +having gamopetalous flowers for its chief character. If the character +became fixed, so as to lose its present state of variability, such a +group of supposititious gamopetalous plants might be quite analogous to +the corresponding real gamopetalous families. Hence there can be no +objection to the view, that the heaths have arisen in an analogous +manner from their polypetalous ancestors. Other species of [663] the +same genus have also been recorded to produce gamopetalous flowers, as +for instance, _Papaver hybridum_, by Hoffmann. Poppies are not the sole +example of accidental gamopetaly. Linnaeus observed the same deviation +long ago for _Saponaria officinalis_, and since, it has been seen in +_Clematis Vitalba_ by Jaeger, in _Peltaria alliacea_ by Schimper, in +_Silene annulata_ by Boreau and in other instances. No doubt it is not +at all of rare occurrence, and the origin of the present gamopetalous +families is to be considered as nothing extraordinary. It is, as a +matter of fact, remarkable that it has not taken place in more numerous +instances, and the mallows show that such opportunities have been +available at least more than once. + +Other instances of taxonomic anomalies are afforded by leaves. Many +genera, the species of which mainly bear pinnate or palmate leaves, have +stray types with undivided leaves. Among the brambles, _Rubus odoratus_ +and _R. flexuosus_ may be cited, among the aralias, _Aralia crassifolia_ +and _A. papyrifera_, and among the jasmines, the deliciously scented +sambac (_Jasminum Sambac_). But the most curious instance is that of the +telegraph-plant, or _Desmodium gyrans_, each complete leaf of which +consists of a large terminal leaflet and two little lateral ones. These +latter keep up, [664] night and day, an irregular jerking movement, +which has been compared to the movements of a semaphore. _Desmodium_ is +a papilionaceous plant and closely allied to the genus _Hedysarum_, +which has pinnate leaves with numerous pairs of leaflets. Its place in +the system leaves no doubt concerning its origin from pinnate-leaved +ancestors. At the time of its origination its leaves must have become +reduced as to the number of the blades, while the size of the terminal +leaflet was correspondingly increased. + +It might seem difficult to imagine this great change taking place +suddenly. However, we are compelled to familiarize ourselves with such +hypothetical assumptions. Strange as they may seem to those who are +accustomed to the conception of continuous slow improvements, they are +nevertheless in complete agreement with what really occurs. Fortunately +the direct proof of this assertion can be given, and in a case which is +narrowly related, and quite parallel to that of the _Desmodium_, since +it affects a plant of the same family. It is the case of the +monophyllous variety of the bastard-acacia or _Robinia Pseud-Acacia_. In +a previous lecture we have seen that it originated suddenly in a French +nursery in the year 1855. It can be propagated by seed, and exhibits a +curious degree [665] of variability of its leaves. In some instances +these are one-bladed, the blade reaching a length of 15 cm., and hardly +resembling those of the common bastard-acacia. Other leaves produce one +or two small leaflets at the base of the large terminal one, and by this +contrivance are seen to be very similar to those of the _Desmodium_, +repeating its chief characters nearly exactly, and only differing +somewhat in the relative size of the various parts. Lastly real +intermediates are seen between the monophyllous and the pinnate types. +As far as I have been able to ascertain, these are produced on weak +twigs under unfavorable conditions; the size of the terminal leaflet +decreases and the number of the lateral blades increases, showing +thereby the presence of the original pinnate type in a latent condition. + +The sudden origin of this "one-leaved" acacia in a nursery may be taken +as a prototype of the ancient origin of _Desmodium_. Of course the +comparison only relates to a single character, and the movements of the +leaflets are not affected by it. But the monophylly, or rather the size +of the terminal blade and the reduction of the lateral ones, may be held +to be sufficiently illustrated by the bastard-acacia. It is worth while +to state, that analogous varieties have also arisen in other genera. The +"one-leaved" [666] strawberry has already been referred to. It +originated from the ordinary type in Norway and at Paris. The walnut +likewise, has its monophyllous variety. It was mentioned for the first +time as a cultivated tree about 1864, but its origin is unknown. A +similar variety of the walnut, with "one-bladed" leaves but of varying +shapes, was found wild in a forest near Dieppe in France some years ago, +and appeared to be due to a sudden mutation. + +Something more is known concerning the "one-bladed" ashes, varieties of +which are often seen in our parks and gardens. The common form has broad +and deeply serrate leaves, which are far more rounded than the leaflets +of the ordinary ash. The majority of the leaves are simple, but some +produce one or two smaller leaflets at their base, closely corresponding +in this respect to the variations of the "one-bladed" bastard-acacia, +and evidently indicating the same latent and atavistic character. In +some instances this analogy goes still further, and incompletely pinnate +leaves are produced with two or more pairs of leaflets. Besides this +variable type another has been described by Willdenow. It has single +leaves exclusively, never producing smaller lateral leaflets, and it is +said to be absolutely constant from seed, while the more variable types +[667] seem to be also more inconstant when propagated sexually. The +difference is so striking and affords such a reliable feature that Koch +proposed to make two distinct varieties of them, calling the pure type +_Fraxinus excelsior monophylla_, and the varying trees _F. excelsior +exheterophylla_. Some writers, and among them Willdenow, have preferred +to separate the "one-leaved" forms from the species, and to call them +_Fraxinus simplicifolia_. + +According to Smith and to Loudon, the "one-leaved" ashes are found wild +in different districts in-England. Intermediate forms have not been +recorded from these localities. This mode of origin is that already +detailed for the laciniate varieties of alders and so many other trees. +Hence it may be assumed that the "one-leaved" ashes have sprung suddenly +but frequently from the original pinnate species. The pure type of +Willdenow should, in this case, be considered as due to a slightly +different mutation, perhaps as a pure retrograde variety, while the +varying strains may only be eversporting forms. This would likewise +explain part of their observed inconstancy. + +In this respect the historic dates, as collected by Korshinsky, are not +very convincing. Vicinism has of course, almost never been excluded, and +part of the multiformity of the offspring [668] must obviously be due to +this most universal agency. Indirect vicinism also plays some part, and +probably affords the explanation of some reputed mutative productions of +the variety. So, for instance, in the case of Sinning, who after sowing +the seeds of the common ash, got as large a proportion as 2% of +monophyllous trees in a culture of some thousand plants. It is probable +that his seeds were taken partly from normal plants, and partly from +hybrids between the normal and the "one-bladed" type, assuming that +these hybrids have pinnate leaves like their specific parent, and bear +the characters of the other parent only in a latent condition. + +Our third example relates to peltate leaves. They have the stalk +inserted in the middle of the blade, a contrivance produced by the +connation of the two basal lobes. The water-lilies are a well known +instance, exhibiting sagittate leaves in the juvenile stage and changing +in many species, into nearly circular peltate forms, of which _Victoria +regia_ is a very good example, although its younger stages do not always +excite all the interest they deserve. The Indian cress (_Tropaeolum_), +the marsh pennywort or _Hydrocotyle_, and many other instances could be +quoted. Sometimes the peltate leaves are not at all orbicular, but are +elongated, oblong or elliptic, and with only the lobes [669] at the base +united. The lemon-scented _Eucalyptus citriodora_ is one of the most +widely known cases. In other instances the peltate leaves become more or +less hollow, constituting broad ascidia as in the case of the +crassulaceous genus _Umbilicus_. + +This connation of the basal lobes is universally considered as a good +and normal specific character. Nevertheless it has its manifest analogy +in the realm of the anomalies. This is the pitcher or ascidium. On some +trees it is of quite common occurrence, as on the lime-tree (_Tilia +parvifolia_) and the magnolia (_Magnolia obovata_ and its hybrids). It +is probable that both these forms have varieties with, and others +without, ascidia. Of the lime-tree, instances are known of single trees +which produce hundreds of such anomalous leaves yearly, and one such a +tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I +have alluded to these cases more than once, but on this occasion a +closer inspection of the structure of the ascidium is required. For this +purpose we may take the lime-tree as an example. Take the shape of the +normal leaves in the first place. These are cordate at their base and +mainly inequilateral, but the general shape varies to a considerable +extent. This variation is closely related to the position of the leaves +on the twigs, and shows [670] distinct indications of complying with the +general law of periodicity. The first leaves are smaller, with more +rounded lobes, the subsequent leaves attain a larger size, and their +lobes slightly change their forms. In the first leaves the lobes are so +broad as to touch one another along a large part of their margins, but +in organs formed later this contact gradually diminishes and the typical +leaves have the lobes widely separated. Now it is easily understood that +the contact or the separation of the lobes must play a part in the +construction of the ascidia, as soon as the margins grow together. +Leaves which touch one-another, may be affected by the connation without +any further malformation. They remain flat, become peltate and exhibit a +shape which in some way holds a middle position between the pennyworts +and the lemon-scented eucalyptus. Here we have the repetition of the +specific characters of these plants by the anomaly of another. Whenever +the margins are not in contact, and become connate, notwithstanding +their separation, the blade must be folded together in some slight +degree, in order to produce the required contact. This is the origin of +the ascidium. It is quite superfluous to insist upon the fact that their +width or narrowness must depend upon the corresponding normal form. The +more distant the [671] lobes, the deeper the ascidium will become. It +should be added that this explanation of the different shapes of ascidia +is of general validity. + +Ascidia of the snake-plantain or _Plantago lanceolata_ are narrow tubes, +because the leaves are oblong or lanceolate, while those of the broad +leaved species of arrowhead, as for instance, the _Sagittaria japonica_, +are of a conical shape. + +From the evidence of the lime-tree we may conclude that normal peltate +leaves may have originated in the same way. And from the fact that +pitchers are one of the most frequent anomalies, we may conclude that +the chance of producing peltate leaves must have been a very great one, +and wholly sufficient to account for all observed cases. In every +instance the previously existing shape of the leaf must have decided +whether peltate or pitcher-like leaves would be formed. As far as we can +judge peltate anomalies are quite uninjurious, while ascidia are forms +which must impede the effect of the light on the leaf, as they conceal +quite an important part of the upper surface. In this way it is easily +conceivable that peltate leaves are a frequent specific character, while +ascidia are not, as they only appear in the special cases of limited +adaptation, as in the instances of the so called pitcher-plants. The +genera _Nepenthes_, [672] _Sarracenia_ and some others are very well +known and perhaps even the bladderworts or _Utricularia_ might be +included here. + +The reproduction of specific characters by anomalous ascidia is not at +all limited to the general case as described above. More minute details +may be seen to be duplicated in the same way. Proofs are afforded on one +side by incomplete ascidia, and on the other by the double cups. + +Incomplete ascidia are those of the _Nepenthes_. The leaf is divided +into three parts, a blade, a tendril and the pitcher. Or in other words, +the limb produces a tendril at its summit, by means of which the plant +is enabled to fasten itself to surrounding shrubs and to climb between +their branches. But the end of this tendril bears a well-formed urn, +which however, is produced only after the revolving and grasping +movements of the tendril have been made. Some species have more rounded +and some more elongated ascidia and often the shape is seen to change +with the development of the stem. The mouth of the urn is strengthened +by a thick rim and covered with a lid. Numerous curious contrivances in +these structures to catch ants and other insects have been described, +but as they have no relation to our present discussion, we shall abstain +from dealing with them. [673] Likewise we must refrain from a +consideration of the physiologic qualities of the tendril, and confine +our attention to the combination of a limb, a naked midvein and an +ascidium. This combination is to be the basis of our discussion. It is +liable to be produced all of a sudden. This assertion is proved by its +occurrence as a varietal mark in one of our most ordinary cultivated +plants. It is the group known as _Croton_, belonging to the genus +_Codiaeum_. A variety is called _interruptum_ and another +_appendiculatum_, and these names both relate to the interruption of the +leaves by a naked midvein. The leaves are seen to be built up of three +parts. The lower half retains the aspect of a limb; it is crowned by a +vein without lateral nerves or blade-like expansions, and this stalk in +its turn bears a short limb on its summit. The base of this apical limb +exhibits two connate lobes, forming together a wide cup or ascidium. It +should be stated that these _interruptum_ varieties are highly variable, +especially in the relative size of the three principal parts of the +leaf. Though it is of course conceded that the ascidium of _Nepenthes_ +has many secondary devices which are lacking in _Croton_, it seems +hardly allowable to deny the possibility of an analogous origin for +both. Those of the _Croton_, according to our knowledge regarding +similar cases, must [674] have arisen at once, and hence the conclusion +that the ascidia of _Nepenthes_ are also originally due to a sudden +mutation. Interrupted leaves, with an ascidium on a naked prolongation +of the midvein, are by no means limited to the _Croton_ varieties. As +stray anomalies they have often been observed, and I myself had the +opportunity of collecting them on magnolia, on clover and on some other +species. They are additional evidence in support of the explanation +given above. + +In the same way double ascidia may be made use of to explain the foliar +cups of the teasels and some other plants, as for instance, some +European snakeroots (_Eryngium maritimum_ and _E. campestre_), or the +floral leaves of the honeysuckle. The leaves on the stems of the teasels +are disposed in pairs, and the bases of the two leaves of each pair are +connate so as to constitute large cups. We have already mentioned these +cups, and recall them in the present connection to use them as a +prototype of the double ascidia. These are constituted of two opposite +leaves, accidentally connated at their base or along some part of their +margins. If the leaves are sessile, the analogy with the teasels is +complete, as shown, for instance, in a case of _Cotyledon_, a +crassulaceous plant which is [675] known to produce such cups from time +to time. They are narrower than those of the teasel, but this depends, +as we have seen for the "one-leaved" ascidium, on the shape of the +original leaf. In other respects they exactly imitate the teasel cups +showing thereby how these cups may probably have originated. + +In numerous cases of anomalies some accidental structures are parallel +to specific characters, while others are not, being obviously injurious +to their bearers. So it is also with the double ascidia. In the case of +stalked leaves the two opposite stalks must, of course, constitute a +long and very narrow tube, when growing together. This tube must bear at +its summit the conical ascidium produced by the two connate limbs. At +its base however, it includes the terminal bud of the stem, and +frequently the tube is so narrow as to impede its further development. +By this contrivance the double ascidium assumes a terminal position. +Instances have been observed on magnolia, in _Boehmeria_ and in other +cases. + +Flowers on leaves are of rare occurrence. Notwithstanding this, they +constitute specific characters in some instances, accidental anomalies +in others. _Helwingia rusciflora flora_ is the most curious and best +known instance. It is a little shrub, belonging to the Cornaceae, and +[676] has broad elliptical undivided leaves. On the middle of the +midvein these leaves are seen to bear small clusters of flowers; indeed +this is the only place where flowers are produced. Each cluster has from +13-15 flowers, of which some are staminate and borne on stalks, while +others are pistillate and nearly sessile. These flowers are small and of +a pale greenish color and yield small stone-fruits, with a thin coating +of pulpy tissue. As the name indicates, this mode of flowering is +closely similar to that of _Ruscus_, which however, does not bear its +flowers and berries on real leaves, but on leaflike expansions of the +twigs. _Phyllonoma ruscifolia_, a saxifragaceous plant, bears the same +specific name, indicating a similar origin of the flowers. Other +instances have been collected by Casimir de Candolle, but their number +is very small. + +As a varietal mark, flowers on leaves likewise rarely occur. One +instance however, is very remarkable, and we have already dealt with it, +when treating of constant varieties, and of the lack of vicinism in the +case of species with exclusive self-fertilization. + +It is the "Nepaul-barley" or _Hordeum trifurcatum_. The leaves, which in +this case bear the adventitious flowers, are the inner scales of the +spikelets, and not on green leaves as in the [677] cases already alluded +to. But this of course makes no real difference. The character is +variable to a high degree, and this fact indicates its varietal nature, +though it should be recalled that at least with the _Helwingia_, the +majority of the leaves are destitute of flowers, and that in this way +some degree of variability is present in this normal case too. + +All in all there are three sorts of "Nepaul-barley." They have the same +varietal mark, but belong to different species of barley. These are +differentiated according to the number of the rows in which the grains +are seen on the spikes. These numbers may be two, four or six, giving +rise to the specific names of _Hordeum distichum_, _tetrastichum_ and +_hexastichum_. Whether these three varieties are of independent, but +parallel origin, or are to be considered as due to a single mutation and +subsequent crosses is not known, all of them being of ancient origin. +Historic evidence concerning their birth is wholly wanting. From analogy +it would seem probable that the character had arisen by a mutation in +one of the three named species, and had been transferred to others by +means of accidental crosses, even as it has been artificially +transmitted of late to quite a number of other sorts. But however +admissible this conception may seem, there is of course no real +objection [678] to the assumption of independent and parallel mutations. + +For the purpose of a comparison with the _Helwingia_ type we are +however, not at all concerned with the species to which the +_trifurcatum_ variety belongs, but only with the varietal mark itself. +The spikelets may be one-, two- or three-flowered, according to the +species. If we choose for further consideration the _hexastichum_ type, +each spikelet produces three normal flowers and afterwards three normal +grains. Morphologically however, the spikelet is not homologous to those +parts of other grasses which have the same name. It is constituted of +three real spikelets, and thus deserves the name of a triple +construction. Each of these three little organs has its normal pair of +outer scales or glumae. These are linear and short, ending in a long and +narrow spine. Those of the middle-most spikelets stand on its outer +side, while those of the lateral part are placed transversely. In this +way they form a kind of involucre around the central parts. The latter +consist of the inner and outer palets or scales, each two of which +include one of the flowers. The outer palet is to be considered as the +metamorphosed leaf, in the aril of which the flower is produced. In the +common sorts of barley it bears a long awn, giving thereby its typical +aspect to the [679] whole spike. The axillary flower is protected on the +opposite side by a two-keeled inner palet. Each flower exhibits three +stamens and an ovary. In the six-rowed barley all the three flowers of a +triple spikelet are fertile, and each of them has a long awn on the top +of the outer palet. But in the two-rowed species only the middle-most +flower is normal and has an awn, the two remaining being sterile and +more or less rudimentary and with only very short awns. From this +description it is easily seen that the species of barley may be +distinguished from one another, even at a casual glance, by the number +of the rows of the awns, and therefore by the shape of the entire +spikes. This striking feature, however, does not exist in the +"Nepaul-barley." The awns are replaced by curiously shaped appendices, +which are three-lobed. The central lobe is oblong and hollow, and forms +a kind of hood, which covers a small supernumerary floret. The two +lateral lobes are narrower, often linear and extended into a smaller or +longer awn. These awns are mostly turned away from the center of the +spike. The central lobe may sometimes bear two small florets, but +ordinarily only one is to be found, and this is often incomplete, having +only one or two stamens, or is different in some other way. [680] These +narrow lateral lobes heighten the abnormal aspect of the whole spike. + +They are only produced at a somewhat advanced stage of the development +of the palet, are united to one another and to the central part by +strong veins, which form transversal anastomoses at their insertion. The +length of these awns is very variable, and this quality is perhaps the +most striking of the whole variety. Often they reach only 1-2 mm., or +the majority may become longer and attain even 1 cm., while here and +there, between them, longer ones are inserted, extending in some +instances even as far as 3 cm. from the spike. Their transverse position +in such cases is strikingly contrasted with the ordinary erect type of +the awns. + +These lateral lobes are to be regarded, from the morphologic point of +view, as differentiated parts of the blade of the leaf. Before they are +formed, or coincidently with the beginning of their development, the +summit of the central lobe becomes hollow, and the development of the +supernumerary flower commences. In different varieties, and especially +in the most recent crosses of them, this development is excessively +variable. + +The accidental flower arises at some distance beneath the summit of the +scale, on its middle [681] vein. The development begins with the +protrusion of a little scale, and the flower itself is situated beneath +this scale, and is to be protected by it and by the primary scale, but +is turned upside down at the same time. Opposite to this organ, which +represents the outer palet of the adventitious flower, two little +swollen bodies are evolved. In the normal flowers of barley and other +grains and grasses their function is to open the flowers by swelling, +and afterwards collapse and allow them to close. + +In the adventitious flowers of the "Nepaul-barley," however, this +function is quite superfluous. The stamens occur in varying numbers; +typically there are three, but not rarely less, or more, are seen. In +some instances the complete double whorl of six, corresponding to the +ancestral monocotyledonous type, has been found. This is a very curious +case of systematic atavism, quite analogous to the _Iris pallida +abavia_, previously alluded to, which likewise has six stamens, and to +the cases given in a previous lecture. But for our present discussion it +is of no further interest. The ovary is situated in the middle of the +flower, and in some instances two have been observed. This is also to be +considered as a case of atavism. + +All these parts of the adventitious flower are more or less subject to +arrest of development, [682] in a later stage. They may even sometimes +become abnormal. Stamens may unite into pairs, or carpels bear four +stigmas. The pollen-sacs are as a rule barren, the mother-cells +undergoing atrophy, while normal grains are seen but rarely. Likewise +the ovaries are rudimentary, but Wittmack has observed the occasional +production of ripe grains from these abnormal florets. + +The scale is seldom seen to extend any farther upwards than the +supernumerary flower. But in the rare instances where it does prolong +its growth, it may repeat the abnormality and bear a second floret above +the first. This of course is generally much weaker, and more +rudimentary. + +Raciborsky, who has lately given a full and very accurate description of +this anomaly, lays great stress upon the fact that it is quite useless. +It is perhaps the most obviously useless structure in the whole +vegetable kingdom. Notwithstanding this, it has come to be as completely +hereditary as any of the most beautiful adaptations in nature. Therefore +it is one of the most serious objections to the hypothesis of slow and +gradual improvements on the sole ground of their usefulness. The +struggle for life and natural selection are manifestly inadequate to +give even the slightest indication of [683] an explanation of this case. +It is simply impossible to imagine the causes that might have produced +such a character. The only way out of this difficulty is to assume that +it has arisen at once, in its present apparently differentiated and very +variable condition, and that, being quite uninjurious and since it does +not decrease the fertility of the race, it has never been subjected to +natural selection, and so has saved itself from destruction. + +But if we once grant the probability of the origin of the +"Nepaul-barley" by a sudden mutation, we obviously must assume the same +in the case of the _Helwingia_ and other normal instances. In this way +we gain a further support for our assertion, that even the strangest +specific characters may have arisen suddenly. + +After having detailed at some length those proofs which seem to be the +most striking, and which had not been previously described with +sufficient detail, we may now take a hasty survey of other contingent +cases. In the first place the cruciate flowers of some onagraceous +plants should be remembered. Small linear petals occur as a specific +character in _Oenothera cruciata_ of the Adirondacks, but have been seen +to arise as sudden mutations in the common evening-primrose (_O. +biennis_) in Holland, and in the willow-herb (_Epilobium hirsutum_) in +England. [684] Leaves placed in whorls of three are very rare. The +oleander, juniper and some few other plants have ternate whorls as a +specific character. As an anomaly, ternate whorls are far more common, +and perhaps any plant with opposite leaves may from time to time produce +them. Races rich in this abnormality are found in the wild state in the +yellow loosestrife or _Lysimachia vulgaris_, in which it is a very +variable specific character, the whorls varying from two to four leaves. +In the cultivated state it is met with in the myrtle or _Myrtus +communis_, where it has come to be of some importance in Israelitic +ritual. Crisped leaves are known in a mallow, _Malva crispa_, and as a +variety in cabbages, parsley, lettuce and others. The orbicular fruits +of Heeger's shepherd's purse (_Capsella heegeri_) recall similar fruits +of other cruciferous genera, as for instance, _Camelina_. Screw-like +stems with wide spirals are specific in the flower-stalks of _Cyclamen_ +and _Vallisneria_, varietal in _Juncus effusus spiralis_ and accidental +in _Scirpus lacustris_. Dormant buds or small bulbs in inflorescences +are normal for wild onions, _Polygonum viviparum_ and others, varietal +in _Poa alpina vivipara_ and perhaps in _Agave vivipara_, and accidental +in plantains (_Plantago lanceolata_), _Saxifraga umbrosa_ and others. +[685] Cleft leaves, one of the most general anomalies, are typical in +_Boehmeria biloba_. The adnation of the peduncles of the inflorescences +to the stem is typical in _Solanum_ and accidental in many other cases. + +It seems quite superfluous to add further proof. It is a very general +phenomenon that specific characters occur in other genera as anomalies, +and under such circumstances that the idea of a slow evolution on the +ground of utility is absolutely excluded. No other explanation remains +than that of a sudden mutation, and once granted for the abnormal cases, +this explanation must obviously likewise be granted for the analogous +specific characters. + +Our whole discussion shows that mutations, once observed in definite +instances, afford the most probable basis for the explanation of +specific characters at large. + + +[686] +LECTURE XXIV + +THE HYPOTHESIS OF PERIODIC MUTATIONS + +The prevailing belief that slow and gradual, nearly invisible changes +constitute the process of evolution in the animal and vegetable kingdom, +did not offer a strong stimulus for experimental research. No +appreciable response to any external agency was of course to be +expected. Responses were supposed to be produced, but the corresponding +outward changes would be too small to betray themselves to the +investigator. + +The direct observation of the mutations of the evening-primrose has +changed the whole aspect of the problem at once. It is no longer a +matter dealing with purely hypothetical conditions. Instead of the vague +notions, uncertain hopes, and a priori conceptions, that have hitherto +confused the investigator, methods of observation have been formulated, +suitable for the attainment of definite results, the general nature of +which is already known. + +To my mind the real value of the discovery [687] of the mutability of +the evening-primrose lies in its usefulness as a guide for further work. +The view that it might be an isolated case, lying outside of the usual +procedure of nature, can hardly be sustained. On such a supposition it +would be far too rare to be disclosed by the investigation of a small +number of plants from a limited area. Its appearance within the limited +field of inquiry of a single man would have been almost a miracle. + +The assumption seems justified that analogous cases will be met with, +perhaps even in larger numbers, when similar methods of observation are +used in the investigation of plants of other regions. The mutable +condition may not be predicated of the evening-primroses alone. It must +be a universal phenomenon, although affecting a small proportion of the +inhabitants of any region at one time: perhaps not more than one in a +hundred species, or perhaps not more than one in a thousand, or even +fewer may be expected to exhibit it. The exact proportion is immaterial, +because the number of mutable instances among the many thousands of +species in existence must be far too large for all of them to be +submitted to close scrutiny. + +It is evident from the above discussion that next in importance to the +discovery of the prototype of mutation is the formulation of methods +[688] for bringing additional instances to light. These methods may +direct effort toward two different modes of investigation. We may search +for mutable plants in nature, or we may hope to induce species to become +mutable by artificial methods. The first promises to yield results most +quickly, but the scope of the second is much greater and it may yield +results of far more importance. Indeed, if it should once become +possible to bring plants to mutate at our will and perhaps even in +arbitrarily chosen directions, there is no limit to the power we may +finally hope to gain over nature. + +What is to guide us in this new line of work? Is it the minute +inspection of the features of the process in the case of the +evening-primroses? Or are we to base our hopes and our methods on +broader conceptions of nature's laws? Is it the systematic study of +species and varieties, and the biologic inquiry into their real +hereditary units? Or is the theory of descent to be our starting-point? +Are we to rest our conceptions on the experience of the breeder, or is +perhaps the geologic pedigree of all organic life to open to us better +prospects of success? + +The answer to all such questions is a very simple one. All possibilities +must be considered, and no line of investigation ignored. For myself I +have based my field-researches and my [689] testing of native plants on +the hypothesis of unit-characters as deduced from Darwin's _Pangenesis_. +This conception led to the expectation of two different kinds of +variability, one slow and one sudden. The sudden ones known at the time +were considered as sports, and seemed limited to retrograde changes, or +to cases of minor importance. The idea that sudden steps might be taken +as the principal method of evolution could be derived from the +hypothesis of unit characters, but the evidence might be too remote for +a starting point for experimental investigation. + +The success of my test has given proof to the contrary. Hence the +assertion that no evidence is to be considered as inadequate for the +purpose under discussion. Sometime a method of discovering, or of +producing, mutable plants may be found, but until this is done, all +facts of whatever nature or direction must be made use of. A very slight +indication may change forever the whole aspect of the problem. + +The probabilities are now greatly in favor of our finding out the causes +of evolution by a close scrutiny of what really happens in nature. A +persistent study of the physiologic factors of this evolution is the +chief condition of success. To this study field-observations may +contribute as well as direct experiments, [690] microscopical +investigations as well as extended pedigree-cultures. The cooperation of +many workers is required to cover the field. Somewhere no doubt the +desired principle lies hidden, but until it is discovered, all methods +must be tried. + +With this conception as the best starting point for further +investigation, we may now make a brief survey of the other phase of the +problem. We shall try to connect our observations on the +evening-primroses with the theory of descent at large. + +We start with two main facts. One is the mutability of Lamarck's +primrose, and the second is the immutable condition of quite a number of +other species. Among them are some of its near allies, the common and +the small flowered evening-primrose, or _Oenothera biennis_ and _O. +muricata_. + +From these facts, a very important question arises in connection with +the theory of descent. Is the mutability of our evening-primroses +temporary, or is it a permanent condition? A discussion of this problem +will give us the means of reaching a definite idea as to the scope of +our inquiries. + +Let us consider the present first. If mutability is a permanent +condition, it has of course no beginning, and moreover is not due to the +[691] agency of external circumstances. Should this be granted for the +evening-primrose, it would have to be predicated for other species found +in a mutable state. Then, of course, it would be useless to investigate +the causes of mutability at large, and we should have to limit ourselves +to the testing of large numbers of plants in order to ascertain which +are mutable and which not. + +If, on the other hand, mutability is not a permanent feature, it must +once have had a beginning, and this beginning itself must have had an +external cause. The amount of mutability and its possible directions may +be assumed to be due to internal causes. The determination of the moment +at which they will become active can never be the result of internal +causes. It must be assigned to some external factor, and as soon as this +is discovered the way for experimental investigation is open. + +In the second place we must consider the past. On the supposition of +permanency all the ancestors of the evening-primrose must have been +mutable. By the alternative view mutability must have been a periodic +phenomenon, producing at times new qualities, and at other times leaving +the plants unchanged during long successions of generations. The present +mutable state must then have been preceded by an immutable [692] +condition, but of course thousands of mutations must have been required +to produce the evening-primroses from their most remote ancestors. + +If we take the species into consideration that are not mutable at +present, we may ask how we are to harmonize them with each of the two +theories proposed. If mutability is permanent, it is manifest that the +whole pedigree of the animal and vegetable kingdom is to be considered +as built up of main mutable lines, and that the thousands of constant +species can only be taken to represent lateral branches of the +genealogic tree. + +These lateral branches would have lost the capacity of mutating, +possessed by all their ancestors. And as the principle of the hypothesis +under discussion does not allow a resumption of this habit, they would +be doomed to eternal constancy until they finally die out. Loss of +mutability, under this conception, means loss of the capacity for all +further development. Only those lines of the main pedigree which have +retained this capacity would have a future; all others would die out +without any chance of progression. + +If, on the other hand, mutability is not permanent, but a periodic +condition, all lines of the genealogic tree must be assumed to show +alternatively [693] mutating and constant species. Some lines may be +mutating at the present moment; others may momentarily be constant. The +mutating lines will probably sooner or later revert to the inactive +state, while the powers of development now dormant may then become +awakened on other branches. + +The view of permanency represents life as being surrounded with +unavoidable death, the principle of periodicity, on the contrary, +follows the idea of resurrection, granting the possibility of future +progression for all living beings. At the same time it yields a more +hopeful prospect for experimental inquiry. + +Experience must decide between the two main theories. It demonstrates +the existence of polymorphous genera, such as _Draba_ and _Viola_ and +hundreds of others. They clearly indicate a previous state of +mutability. Their systematic relation is exactly what would be expected, +if they were the result of such a period. Perhaps mutability has not +wholly ceased in them, but, might be found to survive in some of their +members. Such very rich genera however, are not the rule, but are +exceptional cases, indicating the rarity of powerful mutative changes. + +On the other hand, species may remain in a state of constancy during +long, apparently during indefinite, ages. + +[694] Many facts plead in favor of the constancy of species. This +principle has always been recognized by systematists. Temporarily the +current form of the theory of natural selection has assumed species to +be inconstant, ever changing and continuously being improved and adapted +to the requirements of the life-conditions. The followers of the theory +of descent believed that this conclusion was unavoidable, and were +induced to deny the manifest fact that species are constant entities. +The mutation theory gives a clew to the final combination of the two +contending ideas. Reducing the changeability of the species to distinct +and probably short periods, it at once explains how the stability of +species perfectly agrees with the principle of descent through +modification. + +On the other hand, the hypothesis of mutative periods is by no means +irreconcilable with the observed facts of constancy. Such casual changes +can be proved by observations such as those upon the evening-primrose, +but it is obvious that a disproof can never be given. The principle +grants the present constancy of the vast majority of living forms, and +only claims the exceptional occurrence of definite changes. + +Proofs of the constancy of species have been given in different ways. +The high degree of similarity of the individuals of most of our [695] +species has never been denied. It is observed throughout extended +localities, and during long series of years. Other proofs are afforded +by those plants which have been transported to distant localities some +time since, but do not exhibit any change as a result of this migration. +Widely dispersed plants remain the same throughout their range, provided +that they belong to a single elementary species. Many species have been +introduced from America into Europe and have spread rapidly and widely. +The Canadian horsetail (_Erigeron canadensis_), the evening-primrose and +many other instances could be given. They have not developed any special +European features after their introduction. Though exposed to other +environmental conditions and to competition with other species, they +have not succeeded in developing a new character. Such species as proved +adequate to the new environment have succeeded, while those which did +not have succumbed. + +Much farther back is the separation of the species which now live both +in arctic regions and on the summits of our highest mountaintops. If we +compare the alpine flora with the arctic plants, a high degree of +similarity at once strikes us. Some forms are quite identical; others +are slightly different, manifestly representing elementary species of +the same systematic [696] type. Still others are more distant or even +belong to different genera. The latter, and even the diverging, though +nearly allied, elementary species, do not yield adequate evidence in any +direction. + +They may as well have lived together in the long ages before the +separation of the now widely distant floras, or have sprung from a +common ancestor living at that time, and subsequently have changed their +habits. After excluding these unreliable instances, a good number of +species remain, which are quite the same in the arctic and alpine +regions and on the summits of distant mountain ranges. As no +transportation over such large distances can have brought them from one +locality to the other, no other explanation is left than that they have +been wholly constant and unchanged ever since the glacial period which +separated them. Obviously they must have been subjected to widely +changing conditions. The fact of their stability through all these +outward changes is the best proof that the ordinary external conditions +do not necessarily have an influence on specific evolution. They may +have such a result in some instances, in others they obviously have not. +Many arctic forms bearing the specific name of _alpinus_ justify this +conclusion. _Astragalus alpinus_, _Phleum alpinum_, _Hieracium alpinum_ +and [697] others from the northern parts of Norway may be cited as +examples. + +Thus Primula imperialis has been found in the Himalayas, and many other +plants of the high mountains of Java, Ceylon and northern India are +identical forms. Some species from the Cameroons and from Abyssinia have +been found on the mountains of Madagascar. Some peculiar Australian +types are represented on the summit of Kini Balu in Borneo. None of +these species, of course, are found in the intervening lowlands, and the +only possible explanation of their identity is the conception of a +common post-glacial origin, coupled with complete stability. This +stability is all the more remarkable as nearly allied but slightly +divergent forms have also been reported from almost all of these +localities. Other evidence is obtained by the comparison of ancient +plants with their living representatives. The remains in tombs of +ancient Egypt have always afforded strong support of the views of the +adherents of the theory of stability, and to my mind they still do so. +The cereals and fruits and even the flowers and leaves in the funeral +wreaths of Rameses and Amen-Hotep are the same that are still now +cultivated in Egypt. Nearly a hundred or more species have been +identified. Flowers of _Acacia_, leaves of _Mimusops_, [698] petals of +_Nymphaea_ may be cited as instances, and they are as perfectly +preserved as the best herbarium-specimens of the present time. The +petals and stamens retain their original colors, displaying them as +brightly as is consistent with their dry state. + +Paleontologic evidence points to the same conclusion. Of course the +remains are incomplete, and rarely adequate for a close comparison. The +range of fluctuating variability should be examined first, but the test +of elementary species given by their constancy from seed cannot, of +course, be applied. Apart from these difficulties, paleontologists agree +in recognizing the very great age of large numbers of species. It would +require a too close survey of geologic facts to go into details on this +point. Suffice it to say that in more recent Tertiary deposits many +species have been identified with living forms. In the Miocene period +especially, the similarity of the types of phanerogamic plants with +their present offspring, becomes so striking that in a large number of +cases specific distinctions rest in greater part on theoretical +conceptions rather than on real facts. For a long time the idea +prevailed that the same species could not have existed through more than +one geologic period. Many distinctions founded on this belief have since +had to be abandoned. [699] Species of algae belonging to the +well-preserved group of the diatoms, are said to have remained unchanged +from the Carboniferous period up to the present time. + +Summing up the results of this very hasty survey, we may assert that +species remain unchanged for indefinite periods, while at times they are +in the alternative condition. Then at once they produce new forms often +in large numbers, giving rise to swarms of subspecies. All facts point +to the conclusion that these periods of stability and mutability +alternate more or less regularly with one another. Of course a direct +proof of this view cannot, as yet, be given, but this conclusion is +forced upon us by a consideration of known facts bearing on the +principle of constancy and evolution. + +If we are right in this general conception, we may ask further, what is +to be the exact place of our group of new evening-primroses in this +theory? In order to give an adequate answer, we must consider the whole +range of the observations from a broader point of view. First of all it +is evident that the real mutating period must be assumed to be much +longer than the time covered by my observations. Neither the beginning +nor the end have been seen. It is quite obvious that _Oenothera +lamarckiana_ was in a mutating condition when I first [700] saw it, +seventeen years ago. How long had it been so? Had it commenced to mutate +after its introduction into Europe, some time ago, or was it already +previously in this state? It is as yet impossible to decide this point. +Perhaps the mutable state is very old, and dates from the time of the +first importation of the species into Europe. + +Apart from all such considerations the period of the direct +observations, and the possible duration of the mutability through even +more than a century, would constitute only a moment, if compared with +the whole geologic time. Starting from this conception the pedigree of +our mutations must be considered as only one small group. Instead of +figuring a fan of mutants for each year, we must condense all the +succeeding swarms into one single fan, as might be done also for _Draba +verna_ and other polymorphous species. In _Oenothera_ the main stem is +prolonged upwards beyond the fan; in the others the main stem is lacking +or at least undiscernable, but this feature manifestly is only of +secondary importance. We might even prefer the image of a fan, adjusted +laterally to a stem, which itself is not interrupted by this branch. + +On this principle two further considerations are to be discussed. First +the structure of the [701] fan itself, and secondly the combination of +succeeding fans into a common genealogic tree. + +The composition of the fan as a whole includes more than is directly +indicated by the facts concerning the birth of new species. They arise +in considerable quantities, and each of them in large numbers of +individuals, either in the same or in succeeding years. This multiple +origin must obviously have the effect of strengthening the new types, +and of heightening their chances in the struggle for life. Arising in a +single specimen they would have little chance of success, since in the +field among thousands of seeds perhaps one only survives and attains +complete development. Thousands or at least hundreds of mutated seeds +are thus required to produce one mutated individual, and then, how small +are its chances of surviving! The mutations proceed in all directions, +as I have pointed out in a former lecture. Some are useful, others might +become so if the circumstances were accidentally changed in definite +directions, or if a migration from the original locality might take +place. Many others are without any real worth, or even injurious. +Harmless or even slightly useless ones have been seen to maintain +themselves in the field during the seventeen years of my research, as +proved by _Oenothera laevifolia_ and _Oenothera_ [702] _brevistylis_. +Most of the others quickly disappear. + +This failure of a large part of the productions of nature deserves to be +considered at some length. It may be elevated to a principle, and may be +made use of to explain many difficult points of the theory of descent. +If, in order to secure one good novelty, nature must produce ten or +twenty or perhaps more bad ones at the same time, the possibility of +improvements coming by pure chance must be granted at once. All +hypotheses concerning the direct causes of adaptation at once become +superfluous, and the great principle enunciated by Darwin once more +reigns supreme. + +In this way too, the mutation-period of the evening-primrose is to be +considered as a prototype. Assuming it as such provisionally, it may aid +us in arranging the facts of descent so as to allow of a deeper insight +and a closer scrutiny. All swarms of elementary species are the remains +of far larger initial groups. All species containing only a few +subspecies may be supposed to have thrown off at the outset far more +numerous lateral branches, out of which however, the greater part have +been lost, being unfit for the surrounding conditions. It is the +principle of the struggle for life between elementary species, followed +by the survival of the [703] fittest, the law of the selection of +species, which we have already laid stress upon more than once. + +Our second consideration is also based upon the frequent repetition of +the several mutations. Obviously a common cause must prevail. The +faculty of producing _nanella_ or _lata_ remains the same through all +the years. This faculty must be one and the same for all the hundreds of +mutative productions of the same form. When and how did it originate? At +the outset it must have been produced in a latent condition, and even +yet it must be assumed to be continuously present in this state, and +only to become active at distant intervals. But it is manifest that the +original production of the characters of _Oenothera gigas_ was a +phenomenon of far greater importance than the subsequent accidental +transition of this quality into the active state. Hence the conclusion +that at the beginning of each series of analogous mutations there must +have been one greater and more intrinsic mutation, which opened the +possibility to all its successors. This was the origination of the new +character itself, and it is easily seen that this incipient change is to +be considered as the real one. All others are only its visible +expressions. + +Considering the mutative period of our evening-primrose [704] as one +unit-stride section in the great genealogic tree, this period includes +two nearly related, but not identical changes. One is the production of +new specific characters in the latent condition, and the other is the +bringing of them to light and putting them into active existence. These +two main factors are consequently to be assumed in all hypothetic +conceptions of previous mutative periods. + +Are all mutations to be considered as limited to such periods? Of course +not. Stray mutations may occur as well. Our knowledge concerning this +point is inadequate for any definite statement. Swarms of variable +species are easily recognized, if the remnants are not too few. But if +only one or two new species have survived, how can we tell whether they +have originated-alone or together with others. This difficulty is still +more pronounced in regard to paleontologic facts, as the remains of +geologic swarms are often found, but the absence of numerous mutations +can hardly be proved in any case. + +I have more than once found occasion to lay stress on the importance of +a distinction between progressive and retrograde mutations in previous +lectures. All improvement is, of course, by the first of these modes of +evolution, but apparent losses of organs or qualities are [705] perhaps +of still more universal occurrence. Progression and regression are seen +to go hand in hand everywhere. No large group and probably even no genus +or large species has been evolved without the joint agency of these two +great principles. In the mutation-period of the evening-primroses the +observed facts give direct support to this conclusion, since some of the +new species proved, on closer inspection, to be retrograde varieties, +while others manifestly owe their origin to progressive steps. Such +steps may be small and in a wrong direction; notwithstanding this they +may be due to the acquisition of a wholly new character and therefore +belong to the process of progression at large. + +Between them however, there is a definite contrast, which possibly is in +intimate connection with the question of periodic and stray mutations. +Obviously each progressive change is dependent upon the production of a +new character, for whenever this is lacking, no such mutation is +possible. Retrograde changes, on the other hand, do not require such +elaborate preliminary work. Each character may be converted into the +latent condition, and for all we know, a special preparation for this +purpose is not at all necessary. It is readily granted that such special +preparation may occur, because the [706] great numbers in which our +dwarf variety of the _Oenothera_ are yearly produced are suggestive of +such a condition. On the other hand, the _laevifolia_ and _brevistylis_ +mutations have not been repeated, at least not in a visible way. + +From this discussion we may infer that it is quite possible that a large +part of the progressive changes, and a smaller part of the retrograde +mutations, are combined into groups, owing their origin to common +external agencies. The periods in which such groups occur would +constitute the mutative periods. Besides them the majority of the +retrograde changes and some progressive steps might occur separately, +each being due to some special cause. Degressive mutations, or those +which arise by the return of latent qualities to activity, would of +course belong with the latter group. + +This assumption of a stray and isolated production of varieties is to a +large degree supported by experience in horticulture. Here there are no +real swarms of mutations. Sudden leaps in variability are not rare, but +then they are due to hybridization. Apart from this mixture of +characters, varieties as a rule appear separately, often with intervals +of dozens of years, and without the least suggestion of a common cause. +It is quite superfluous to go into details, as we have dealt with the +horticultural [707] mutations at sufficient length on a previous +occasion. Only the instance of the peloric toadflax might be recalled +here, because the historic and geographic evidence, combined with the +results of our pedigree-experiment, plainly show that peloric mutations +are quite independent of any periodic condition. They may occur anywhere +in the wide range of the toad-flax, and the capacity of repeatedly +producing them has lasted some centuries at least, and is perhaps even +as old as the species itself. + +Leaving aside such stray mutations, we may now consider the probable +constitution of the great lines of the genealogic tree of the evening +primroses, and of the whole vegetable and animal kingdom at large. The +idea of drawing up a pedigree for the chief groups of living organisms +is originally due to Haeckel, who used this graphic method to support +the Darwinian theory of descent. Of course, Haeckel's genealogic trees +are of a purely hypothetic nature, and have no other purpose than to +convey a clear conception of the notion of descent, and of the great +lines of evolution at large. Obviously all details are subject to doubt, +and many have accordingly been changed by his successors. These changes +may be considered as partial improvements, and the somewhat picturesque +form of Haeckel's pedigree might well be replaced by [708] more simple +plans. But the changes have by no means removed the doubts, nor have +they been able to supplant the general impression of distinct groups, +united by broad lines. This feature is very essential, and it is easily +seen to correspond with the conception of swarms, as we have deduced it +from the study of the lesser groups. + +Genealogic trees are the result of comparative studies; they are far +removed from the results of experimental inquiry concerning the origin +of species. What are the links which bind them together? Obviously they +must be sought in the mutative periods, which have immediately preceded +the present one. In the case of the evening-primrose the systematic +arrangement of the allied species readily guides us in the delimitations +of such periods. For manifestly the species of the large genus of +_Oenothera_ are grouped in swarms, the youngest or most recent of which +we have under observation. Its immediate predecessor must have been the +subgenus _Onagra_, which is considered by some authors as consisting of +a single systematic species, _Oenothera biennis_. Its multifarious forms +point to a common origin, not only morphologically but also +historically. Following this line backward or downward we reach another +apparent mutation-period, which includes the origin of [709] the group +called _Oenothera_, with a large number of species of the same general +type as the _Onagra-forms, Still farther downward comes the old genus +_Oenothera_ itself, with numerous subgenera diverging in sundry +characters and directions. + +Proceeding still farther we might easily construct a main stem with +numerous succeeding fans of lateral branches, and thus reach, from our +new empirical point of view, the theoretical conclusion already +formulated. + +Paleontologic facts readily agree with this conception. The swarms of +species and varieties are found to succeed one another like so many +stories. The same images are repeated, and the single stories seem to be +connected by the main stems, which in each tier produce the whole number +of allied forms. Only a few prevailing lines are prolonged through +numerous geologic periods; the vast majority of the lateral branches are +limited each to its own storey. It is simply the extension of the +pedigree of the evening-primroses backward through ages, with the same +construction and the same leading features. There can be no doubt that +we are quite justified in assuming that evolution has followed the same +general laws through the whole duration of life on earth. Only a moment +of their lifetime is disclosed to us, but it [710] is quite sufficient +to enable us to discern the laws and to conjecture the outlines of the +whole scheme of evolution. + +A grave objection which has, often, and from the very outset, been urged +against Darwin's conception of very slow and nearly imperceptible +changes, is the enormously long time required. If evolution does not +proceed any faster than what we can see at present, and if the process +must be assumed to have gone on in the same slow manner always, +thousands of millions of years would have been needed to develop the +higher types of animals and plants from their earliest ancestors. + +Now it is not at all probable that the duration of life on earth +includes such an incredibly long time. Quite on the contrary the +lifetime of the earth seems to be limited to a few millions of years. +The researches of Lord Kelvin and other eminent physicists seem to leave +no doubt on this point. Of course all estimates of this kind are only +vague and approximate, but for our present purposes they may be +considered as sufficiently exact. + +In a paper published in 1862 Sir William Thomson (now Lord Kelvin) first +endeavored to show that great limitation had to be put upon the enormous +demand for time made by Lyell, Darwin and other biologists. From a +consideration [711] of the secular cooling of the earth, as deduced from +the increasing temperature in deep mines, he concluded that the entire +age of the earth must have been more than twenty and less than forty +millions of years, and probably much nearer twenty than forty. His views +have been much criticised by other physicists, but in the main they have +gained an ever-increasing support in the way of evidence. New mines of +greater depth have been bored, and their temperatures have proved that +the figures of Lord Kelvin are strikingly near the truth. George Darwin +has calculated that the separation of the moon from the earth must have +taken place some fifty-six millions of years ago. Geikie has estimated +the existence of the solid crust of the earth at the most as a hundred +million years. The first appearance of the crust must soon have been +succeeded by the formation of the seas, and a long time does not seem to +have been required to cool the seas to such a degree that life became +possible. It is very probable that life originally commenced in the +great seas, and that the forms which are now usually included in the +plankton or floating-life included the very first living beings. +According to Brooks, life must have existed in this floating condition +during long primeval epochs, and evolved nearly all the main branches of +the animal and vegetable kingdom [712] before sinking to the bottom of +the sea, and later producing the vast number of diverse forms which now +adorn the sea and land. + +All these evolutions, however, must have been very rapid, especially at +the beginning, and together cannot have taken more time than the figures +given above. + +The agency of the larger streams, and the deposits which they bring into +the seas, afford further evidence. The amount of dissolved salts, +especially of sodium chloride, has been made the subject of a +calculation by Joly, and the amount of lime has been estimated by Eugene +Dubois. Joly found fifty-five and Dubois thirty-six millions of years as +the probable duration of the age of the rivers, and both figures +correspond to the above dates as closely as might be expected from the +discussion of evidence so very incomplete and limited. + +All in all it seems evident that the duration of life does not comply +with the demands of the conception of very slow and continuous +evolution. Now it is easily seen, that the idea of successive mutations +is quite independent of this difficulty. Even assuming that some +thousands of characters must have been acquired in order to produce the +higher animals and plants of the present time, no valid objection is +raised. The demands of the biologists and the results of [713] the +physicists are harmonized on the ground of the theory of mutation. + +The steps may be surmised to have never been essentially larger than in +the mutations now going on under our eyes, and some thousands of them +may be estimated as sufficient to account for the entire organization of +the higher forms. Granting between twenty and forty millions of years +since the beginning of life, the intervals between two successive +mutations may have been centuries and even thousands of years. As yet +there has been no objection cited against this assumption, and hence we +see that the lack of harmony between the demands of biologists and the +results of the physicists disappears in the light of the theory of +mutation. + +Summing up the results of this discussion, we may justifiably assert +that the conclusions derived from the observations and experiments made +with evening-primroses and other plants in the main agree satisfactorily +with the inferences drawn from paleontologic, geologic and systematic +evidence. Obviously these experiments are wonderfully supported by the +whole of our knowledge concerning evolution. For this reason the laws +discovered in the experimental garden may be considered of great +importance, and they may guide us in our further inquiries. Without +doubt many minor [714] points are in need of correction and elaboration, +but such improvements of our knowledge will gradually increase our means +of discovering new instances and, new proofs. + +The conception of mutation periods producing swarms of species from time +to time, among which only a few have a chance of survival, promises to +become the basis for speculative pedigree-diagrams, as well as for +experimental investigations. + + +[715] + +LECTURE XXV + +GENERAL LAWS OF FLUCTUATION + +The principle of unit-characters and of elementary species leads at once +to the recognition of two kinds of variability. The changes of wider +amplitude consist of the acquisition of new units, or the loss of +already existing ones. The lesser variations are due to the degree of +activity of the units themselves. + +Facts illustrative of these distinctions were almost wholly lacking at +the time of the first publication of Darwin's theories. It was a bold +conception to point out the necessity for such distinction on purely +theoretical grounds. Of course some sports were well known and +fluctuations were evident, but no exact analysis of the details was +possible, a fact that was of great importance in the demonstration of +the theory of descent. The lack of more definite knowledge upon this +matter was keenly felt by Darwin, [716] and exercised much influence +upon his views at various times. + +Quetelet's famous discovery of the law of fluctuating variability +changed the entire situation and cleared up many difficulties. While a +clear conception of fluctuations was thus gained, mutations were +excluded from consideration, being considered as very rare, or +non-existent. They seemed wholly superfluous for the theory of descent, +and very little importance was attached to their study. Current +scientific belief in the matter has changed only in recent years. +Mendel's law of varietal hybrids is based upon the principle of +unit-characters, and the validity of this conception has thus been +brought home to many investigators. + +A study of fluctuating or individual variability, as it was formerly +called, is now carried on chiefly by mathematical methods. It is not my +purpose to go into details, as it would require a separate course of +lectures. I shall consider the limits between fluctuation and mutation +only, and attempt to set forth an adequate idea of the principles of the +first as far as they touch these limits. The mathematical treatment of +the facts is no doubt of very great value, but the violent discussions +now going on between mathematicians such as Pearson, Kapteyn and others +should warn biologists to abstain [717] from the use of methods which +are not necessary for the furtherance of experimental work. + +Fortunately, Quetelet's law is a very clear and simple one, and quite +sufficient for our considerations. It claims that for biologic phenomena +the deviations from the average comply with the same laws as the +deviations from the average in any other case, if ruled by chance only. +The meaning of this assertion will become clear by a further discussion +of the facts. First of all, fluctuating variability is an almost +universal phenomenon. Every organ and every quality may exhibit it. Some +are very variable, while others seem quite constant. Shape and size vary +almost indefinitely, and the chemical composition is subject to the same +law, as is well known for the amount of sugar in sugar-beets. Numbers +are of course less liable to changes, but the numbers of the rays of +umbels, or ray-florets in the composites, of pairs of blades in pinnate +leaves, and even of stamens and carpels are known to be often +exceedingly variable. The smaller numbers however, are more constant, +and deviations from the quinate structure of flowers are rare. +Complicated structures are generally capable of only slight deviations. + +From a broad point of view, fluctuating variability [718] falls under +two heads. They obey quite the same laws and are therefore easily +confused, but with respect to questions of heredity they should be +carefully separated. They are designated by the terms individual and +partial fluctuation. Individual variability indicates the differences +between individuals, while partial variability is limited to the +deviations shown by the parts of one organism from the average +structure. The same qualities in some cases vary individually and in +others partially. Even stature, which is as markedly individual for +annual and biennial plants as it is for man, becomes partially variant +in the case of perennial herbs with numbers of stems. Often a character +is only developed once in the whole course of evolution, as for +instance, the degree of connation of the seed-leaves in tricotyls and in +numerous cases it is impossible to tell whether a character is +individual or partial. Consequently such minute details are generally +considered to have no real importance for the hereditary transmission of +the character under discussion. + +Fluctuations are observed to take place only in two directions. The +quality may increase or decrease, but is not seen to vary in any other +way. This rule is now widely established by numerous investigations, and +is fundamental to [719] the whole method of statistical investigation. +It is equally important for the discussion of the contrast between +fluctuations and mutations, and for the appreciation of their part in +the general progress of organization. Mutations are going on in all +directions, producing, if they are progressive, something quite new +every time. Fluctuations are limited to increase and decrease of what is +already available. They may produce plants with higher stems, more +petals in the flowers, larger and more palatable fruits, but obviously +the first petal and the first berry, cannot have originated by the +simple increase of some older quality. Intermediates may be found, and +they may mark the limit, but the demonstration of the absence of a limit +is quite another question. It would require the two extremes to be shown +to belong to one unit, complying with the simple law of Quetelet. + +Nourishment is the potent factor of fluctuating variability. Of course +in thousands of cases our knowledge is not sufficient to allow us to +analyze this relation, and a number of phases of the phenomenon have +been discovered only quite recently. But the fact itself is thoroughly +manifest, and its appreciation is as old as horticultural science. +Knight, who lived at the beginning of the last century, has laid great +stress upon it, and it has since influenced practice in a [720] large +measure. Moreover, Knight pointed out more than once that it is the +amount of nourishment, not the quality of the various factors, that +exercises the determinative influence. Nourishment is to be taken in the +widest sense of the word, including all favorable and injurious +elements. Light and temperature, soil and space, water and salts are +equally active, and it is the harmonious cooperation of them all that +rules growth. + +We treated this important question at some length, when dealing with the +anomalies of the opium-poppies, consisting of the conversion of stamens +into supernumerary pistils. The dependency upon external influences +which this change exhibited is quite the same as that shown by +fluctuating variability at large. We inquired into the influence of good +and bad soil, of sunlight and moisture and of other concurrent factors. +Especial emphasis was laid upon the great differences to which the +various individuals of the same lot may be exposed, if moisture and +manure differ on different portions of the same bed in a way unavoidable +even by the most careful preparation. Some seeds germinate on moist and +rich spots, while their neighbors are impeded by local dryness, or by +distance from manure. Some come to light on a sunny day, and increase +their first leaves rapidly, while on [721] the following day the weather +may be unfavorable and greatly retard growth. The individual differences +seem to be due, at least in a very great measure, to such apparent +trifles. + +On the other hand partial differences are often manifestly due to +similar causes. Considering the various stems of plants, which multiply +themselves by runners or by buds on the roots, the assertion is in no +need of further proof. The same holds good for all cases of artificial +multiplication by cuttings, or by other vegetative methods. But even if +we limit ourselves to the leaves of a single tree, or the branches of a +shrub, or the flowers on a plant, the same rule prevails. The +development of the leaves is dependent on their position, whether +inserted on strong or weak branches, exposed to more or less light, or +nourished by strong or weak roots. The vigor of the axillary buds and of +the branches which they may produce is dependent upon the growth and +activity of the leaves to which the buds are axillary. + +This dependency on local nutrition leads to the general law of +periodicity, which, broadly speaking, governs the occurrence of the +fluctuating deviations of the organs. This law of periodicity involves +the general principle that every axis, as a rule, increases in strength +when [722] growing, but sooner or later reaches a maximum and may +afterwards decrease. + +This periodic augmentation and declination is often boldly manifest, +though in other cases it may be hidden by the effect of alternate +influences. Pinnate leaves generally have their lower blades smaller +than the upper ones, the longest being seen sometimes near the apex and +sometimes at a distance from it. Branches bearing their leaves in two +rows often afford quite as obvious examples, and shoots in general +comply with the same rule. Germinating plants are very easy of +observation on this point. When they are very weak they produce only +small leaves. But their strength gradually increases and the subsequent +organs reach fuller dimensions until the maximum is attained. The +phenomenon is so common that its importance is usually overlooked. It +should be considered as only one instance of a rule, which holds good +for all stems and all branches, and which is everywhere dependent on the +relation of growth to nutrition. + +The rule of periodicity not only affects the size of the organs, but +also their number, whenever these are largely variable. Umbellate plants +have numerous rays on the umbels of strong stems, but the number is seen +to decrease and to become very small on the weakest lateral [723] +branches. The same holds good for the number of ray-florets in the +flower-heads of the composites, even for the number of stigmas on the +ovaries of the poppies, which on weak branches may be reduced to as few +as three or four. Many other instances could be given. + +One of the best authenticated cases is the dependency of partial +fluctuation on the season and on the weather. Flowers decline when the +season comes to an end, become smaller and less brightly colored. The +number of ray-florets in the flower-heads is seen to decrease towards +the fall. Extremes become rarer, and often the deviations from the +average seem nearly to disappear. Double flowers comply with this rule +very closely, and many other cases will easily occur to any student of +nature. + +Of course, the relation to nourishment is different for individual and +partial fluctuations. Concerning the first, the period of development of +the germ within the seed is decisive. Even the sexual cells may be in +widely different conditions at the moment of fusion, and perhaps this +state of the sexual cells includes the whole matter of the decision for +the average characters of the new individual. Partial fluctuation +commences as soon as the leaves and buds begin to form, and all later +changes in nutrition can only cause partial differences. All leaves, +[724] buds, branches, and flowers must come under the influence of +external conditions during the juvenile period, and so are liable to +attain a development determined in part by the action of these factors. + +Before leaving these general considerations, we must direct our +attention to the question of utility. Obviously, fluctuating variability +is a very useful contrivance, in many cases at least. It appears all the +more so, as its relation to nutrition becomes manifest. Here two aspects +are intimately combined. More nutrient matter produces larger leaves and +these are in their turn more fit to profit by the abundance of +nourishment. So it is with the number of flowers and flower-groups, and +even with the numbers of their constituent organs. Better nourishment +produces more of them, and thereby makes the plant adequate to make a +fuller use of the available nutrient substances. Without fluctuation +such an adjustment would hardly be possible, and from all our notions of +usefulness in nature, we therefore must recognize the efficiency of this +form of variability. + +In other respects the fluctuations often strike us as quite useless or +even as injurious. The numbers of stamens, or of carpels are dependent +on nutrition, but their fluctuation is not known to have any attraction +for the visiting insects. + +[725] If the deviations become greater, they might even become +detrimental. The flowers of the St. Johnswort, or _Hypericum +perforatum_, usually have five petals, but the number varies from three +to eight or more. Bees could hardly be misled by such deviations. The +carpels of buttercups and columbines, the cells in the capsules of +cotton and many other plants are variable in number. The number of seeds +is thereby regulated in accordance with the available nourishment, but +whether any other useful purpose is served, remains an open question. +Variations in the honey-guides or in the pattern of color-designs might +easily become injurious by deceiving insects, and such instances as the +great variability of the spots on the corolla of some cultivated species +of monkey-flowers, for instance, the _Mimulus quinquevulnerus_, could +hardly be expected to occur in wild plants. For here the dark brown +spots vary between nearly complete deficiency up to such predominancy as +almost to hide the pale yellow ground-color. + +After this hasty survey of the causes of fluctuating variability, we now +come to a discussion of Quetelet's law. It asserts that the deviations +from the average obey the law of probability. They behave as if they +were dependent on chance only. + +Everyone knows that the law of Quetelet can [726] be demonstrated the +most readily by placing a sufficient number of adult men in a row, +arranging them according to their size. The line passing over their +heads proves to be identical with that given by the law of probability. +Quite in the same way, stems and branches, leaves and petals and even +fruits can be arranged, and they will in the main exhibit the same line +of variability. Such groups are very striking, and at the first glance +show that the large majority of the specimens deviate from the mean only +to a very small extent. Wider deviations are far more rare, and their +number lessens, the greater the deviation, as is shown by the curvature +of the line. It is almost straight and horizontal in the middle portion, +while at the ends it rapidly declines, going sharply downward at one +extreme and upward at the other. + +It is obvious however, that in these groups the leaves and other organs +could conveniently be replaced by simple lines, indicating their size. +The result would be quite the same, and the lines could be placed at +arbitrary, but equal distances. Or the sizes could be expressed by +figures, the compliance of which with the general law could be +demonstrated by simple methods of calculation. In this manner the +variability of different organs can easily be compared. Another method +of demonstration consists in [727] grouping the deviations into +previously fixed divisions. For this purpose the variations are measured +by standard units, and all the instances that fall between two limits +are considered to constitute one group. Seeds and small fruits, berries +and many other organs may conveniently be dealt with in this way. As an +example we take ordinary beans and select them according to their size. +This can be done in different ways. On a small piece of board a long +wedge-shaped slit is made, into which seeds are pushed as far as +possible. The margin of the wedge is calibrated in such a manner that +the figures indicate the width of the wedge at the corresponding place. +By this device the figure up to which a bean is pushed at once shows its +length. Fractions of millimeters are neglected, and the beans, after +having been measured, are thrown into cylindrical glasses of the same +width, each glass receiving only beans of equal length. It is clear that +by this method the height to which beans fill the glasses is +approximately a measure of their number. If now the glasses are put in a +row in the proper sequence, they at once exhibit the shape of a line +which corresponds to the law of chance. In this case however, the line +is drawn in a different manner from the first. It is to be pointed out +that the glasses may be replaced by lines indicating [728] the height of +their contents, and that, in order to reach a more easy and correct +statement, the length of the lines may simply be made proportionate to +the number of the beans in each glass. If such lines are erected on a +common base and at equal distances, the line which unites their upper +ends will be the expression of the fluctuating variability of the +character under discussion. + +The same inquiry may be made with other seeds, with fruits, or other +organs. It is quite superfluous to arrange the objects themselves, and +it is sufficient to arrange the figures indicating their value. In order +to do this a basal line is divided into equal parts, the demarcations +corresponding to the standard-units chosen for the test. The observed +values are then written above this line, each finding its place between +the two demarcations, which include its value. It is very interesting +and stimulating to construct such a group. The first figures may fall +here and there, but very soon the vertical rows on the middle part of +the basal line begin to increase. Sometimes ten or twenty measurements +will suffice to make the line of chance appear, but often indentations +will remain. With the increasing number of the observations the +irregularities gradually [729] disappear, and the line becomes smoother +and more uniformly curved. + +This method of arranging the figures directly on a basal line is very +convenient, whenever observations are made in the field or garden. Very +few instances need be recorded to obtain an appreciation of the mean +value, and to show what may be expected from a continuance of the test. +The method is so simple and so striking, and so wholly independent of +any mathematical development that it should be applied in all cases in +which it is desired to ascertain the average value of any organ, and the +measure of the attendant deviations. + +I cite an instance, secured by counting the ray-florets on the +flower-heads of the corn-marigold or _Chrysanthemum segetum_. It was +that, by which I was enabled to select the plant, which afterwards +showed the first signs of a double head. I noted them in this way; + + 47 + 47 52 + 41 54 68 + 44 50 62 75 + 36 45 58 65 72 __ 99 + +Of course the figures might be replaced in this work by equidistant dots +or by lines, but experience teaches that the chance of making mistakes +is noticeably lessened by writing down [730] the figures themselves. +Whenever decimals are made use of it is obviously the best plan to keep +the figures themselves. For afterwards it often becomes necessary to +arrange them according to a somewhat different standard. + +Uniting the heads of the vertical rows of figures by a line, the form +corresponding to Quetelet's law is easily seen. In the main it is always +the same as the line shown by the measurements of beans and seeds. It +proves a dense crowding of the single instances around the average, and +on both sides of the mass of the observations, a few wide deviations. +These become more rare in proportion to the amount of their divergency. +On both sides of the average the line begins by falling very rapidly, +but then bends slowly so as to assume a nearly horizontal direction. It +reaches the basal line only beyond the extreme instances. + +It is quite evident that all qualities, which can be expressed by +figures, may be treated in this way. First, of all the organs occurring +in varying numbers, as for instance the ray-florets of composites, the +rays of umbels, the blades of pinnate and palmate leaves, the numbers of +veins, etc., are easily shown to comply with the same general rule. +Likewise the amount of chemical substances can be expressed in +percentage numbers, as is done on a large [731] scale with sugar in +beets and sugar-cane, with starch in potatoes and in other instances. +These figures are also found to follow the same law. + +All qualities which are seen to increase and to decrease may be dealt +with in the same manner, if a standard unit for their measurement can be +fixed. Even the colors of flowers may not escape our inquiry. + +If we now compare the lines, compiled from the most divergent cases, +they will be found to exhibit the same features in the main. Ordinarily +the curve is symmetrical, the line sloping down on both sides after the +same manner. But it is not at all rare that the inclination is steep on +one side and gradual on the other. This is noticeably the case if the +observations relate to numbers, the average of which is near zero. Here +of course the allowance for variation is only small on one side, while +it may increase with out distinct limits on the alternate slope. So it +is for instance with the numbers of ray-florets in the example given on +p. 729. Such divergent cases, however, are to be considered as +exceptions to the rule, due to some unknown cause. + +Heretofore we have discussed the empirical side of the problem only. For +the purpose of experimental study of questions of heredity this is +ordinarily quite sufficient. The inquiry [732] into the phenomenon of +regression, or of the relation of the degree of deviation of the progeny +to that of their parents, and the selection of extreme instances for +multiplication are obviously independent of mathematical considerations. +On the other hand an important inquiry lies in the statistical treatment +of these phenomena, and such treatment requires the use of mathematical +methods. + +Statistics however, are not included in the object of these lectures, +and therefore I shall refrain from an explanation of the method of their +preparation and limit myself to a general comparison of the observed +lines with the law of chance. Before going into the details, it should +be repeated once more that the empirical result is quite the same for +individual and for partial fluctuations. As a rule, the latter occur in +far greater number, and are thus more easily investigated, but +individual or personal averages have also been studied. + +Newton discovered that the law of chance can be expressed by very simple +mathematical calculations. Without going into details, we may at once +state that these calculations are based upon his binomium. If the form +(a + b) is calculated for some value of the exponent, and if the values +of the coefficients after development are alone considered, they yield +the basis [733] for the construction of what is called the line or curve +of probability. For this construction the coefficients are used as +ordinates, the length of which is to be made proportionate to their +value. If this is done, and the ordinates are arranged at equal +distances, the line which unites their summits is the desired curve. At +first glance it exhibits a form quite analogous to the curves of +fluctuating variability, obtained by the measurements of beans and in +other instances. Both lines are symmetrical and slope rapidly down in +the region of the average, while with increasing distance they gradually +lose their steep inclination, becoming nearly parallel to the base at +their termination. + +This similarity between such empirical and theoretical lines is in +itself an empirical fact. The causes of chance are assumed to be +innumerable, and the whole calculation is based on this assumption. The +causes of the fluctuations of biological phenomena have not as yet been +critically examined to such an extent as to allow of definite +conceptions. The term nourishment manifestly includes quite a number of +separate factors, as light, space, temperature, moisture, the physical +and chemical conditions of the soil and the changes of the weather. +Without doubt the single factors are very numerous, but whether they are +numerous enough to be treated [734] as innumerable, and thereby to +explain the laws of fluctuations, remains uncertain. Of course the +easiest way is to assume that they combine in the same manner as the +causes of chance, and that this is the ground of the similarity of the +curves. On the other hand, it is manifestly of the highest importance to +inquire into the part the several factors play in the determination of +the curves. It is not at all improbable that some of them have a larger +influence on individual, and others on partial, fluctuations. If this +were the case, their importance with respect to questions of heredity +might be widely different. In the present state of our knowledge the +fluctuation-curves do not contribute in any large measure to an +elucidation of the causes. Where these are obvious, they are so without +statistics, exactly as they were, previous to Quetelet's discovery. + +In behalf of a large number of questions concerning heredity and +selection, it is very desirable to have a somewhat closer knowledge of +these curves. Therefore I shall try to point out their more essential +features, as far as this can be done without mathematical calculations. + +At a first glance three points strike us, the average or the summit of +the curve, and the extremes. If the general shape is once denoted by the +results of observations or by the coefficients [735] of the binomium, +all further details seem to depend upon them. In respect to the average +this is no doubt the case; it is an empirical value without need of any +further discussion. The more the number of the observations increases, +the more assured and the more correct is this mean value, but generally +it is the same for smaller and for larger groups of observations. + +This however, is not the case with the extremes. It is quite evident +that small groups have a chance of containing neither of them. The more +the number of the observations increases, the larger is the chance of +extremes. As a rule, and excluding exceptional cases, the extreme +deviations will increase in proportion to the number of cases examined. +In a hundred thousand beans the smallest one and the largest one may be +expected to differ more widely from one another than in a few hundred +beans of the same sample. Hence the conclusion that extremes are not a +safe criterion for the discussion of the curves, and not at all adequate +for calculations, which must be based upon more definite values. + +A real standard is afforded by the steepness of the slope. This may be +unequal on the two sides of one curve, and likewise it may differ for +different cases. This steepness is usually measured by means of a point +on the half curve and [736 ] for this purpose a point is chosen which +lies exactly half way between the average and the extreme. Not however +half way with respect to the amplitude of the extreme deviation, for on +this ground it would partake of the uncertainty of the extreme itself. +It is the point on the curve which is surpassed by half the number, and +not reached by the other half of the number of the observations included +in the half of the curve. This point corresponds to the important value +called the probable error, and was designated by Galton as the quartile. +For it is evident that the average and the two quartiles divide the +whole of the observations into four equal parts. + +Choosing the quartiles as the basis for calculations we are independent +of all the secondary causes of error, which necessarily are inherent in +the extremes. At a casual examination, or for demonstrative purposes, +the extremes may be prominent, but for all further considerations the +quartiles are the real values upon which to rest calculations. + +Moreover if the agreement with the law of probability is once conceded, +the whole curve is defined by the average and the quartiles, and the +result of hundreds of measurements or countings may be summed up in +three, or, in [737] the case of symmetrical curves, perhaps in two +figures. + +Also in comparing different curves with one another, the quartiles are +of great importance. Whenever an empirical fluctuation-curve is to be +compared with the theoretical form, or when two or more cases of +variability are to be considered under one head, the lines are to be +drawn on the same base. It is manifest that the averages must be brought +upon the same ordinate, but as to the steepness of the line, much +depends on the manner of plotting. Here we must remember that the mutual +distance of the ordinates has been a wholly arbitrary one in all our +previous considerations. And so it is, as long as only one curve is +considered at a time. But as soon as two are to be compared, it is +obvious that free choice is no longer allowed. The comparison must be +made on a common basis, and to this effect the quartiles must be brought +together. They are to lie on the same ordinates. If this is done, each +division of the base corresponds to the same proportionate number of +individuals, and a complete comparison is made possible. + +On the ground of such a comparison we may thus assert that, +fluctuations, however different the organs or qualities observed, are +the same whenever their curves are seen to overlap one [738] another. +Furthermore, whenever an empirical curve agrees in this manner with the +theoretical one, the fluctuation complies with Quetelet's law, and may +be ascribed to quite ordinary and universal causes. But if it seems to +diverge from this line, the cause of this divergence should be inquired +into. + +Such abnormal curves occur from time to time, but are rare. +Unsymmetrical instances have already been alluded to, and seem to be +quite frequent. Another deviation from the rule is the presence of more +than one summit. This case falls under two headings. If the ray florets +of a composite are counted, and the figures brought into a curve, a +prominent summit usually corresponds to the average. But next to this, +and on both sides, smaller summits are to be seen. On a close inspection +these summits are observed to fall on the same ordinates, on which, in +the case of allied species, the main apex lies. The specific character +of one form is thus repeated as a secondary character on an allied +species. Ludwig discovered that these secondary summits comply with the +rule discovered by Braun and Schimper, stating the relation of the +subsequent figures of the series. This series gives the terms of the +disposition of leaves in general, and of the bracts and flowers on the +composite flower [739] heads in our particular case. It is the series to +which we have already alluded when dealing with the arrangement of the +leaves on the twisted teasels. It commences with 1 and 2 and each +following figure is equal to the sum of its two precedents. The most +common figures are 3, 5, 8, 13, 18, 21, higher cases seldom coming under +observation. Now the secondary summits of the ray-curves of the +composites are seen to agree, as a rule, with these figures. Other +instances could readily be given. + +Our second heading includes those cases which exhibit two summits of +equal or nearly equal height. Such cases occur when different races are +mixed, each retaining its own average and its own curve-summit. We have +already demonstrated such a case when dealing with the origin of our +double corn-chrysanthemum. The wild species culminates with 13 rays, and +the grandiflorum variety with 21. Often the latter is found to be +impure, being mixed with the typical species to a varying extent. This +is not easily ascertained by a casual inspection of the cultures, but +the true condition will promptly betray itself, if curves are +constructed. In this way curves may in many instances be made use of to +discover mixed races. Double curves may also result from the +investigation [740] of true double races, or ever-sporting varieties. +The striped snapdragon shows a curve of its stripes with two summits, +one corresponding to the average striped flowers, and the other to the +pure red ones. Such cases may be discovered by means of curves, but the +constituents cannot be separated by culture-experiments. + +A curious peculiarity is afforded by half curves. The number of petals +is often seen to vary only in one direction from what should be expected +to be the mean condition. With buttercups and brambles and many others +there is only an increase above the typical five; quaternate flowers are +wanting or at least are very rare. With weigelias and many others the +number of the tips of the corolla varies downwards, going from five to +four and three. Hundreds of flowers show the typical five, and determine +the summit of the curve. This drops down on one side only, indicating +unilateral variability, which in many cases is due to a very intimate +connection of a concealed secondary summit and the main one. In the case +of the bulbous buttercup, _Ranunculus bulbosus_, I have succeeded in +isolating this secondary summit, although not in a separate variety, but +only in a form corresponding to the type of ever-sporting varieties. + +[741] Recapitulating the results of this too condensed discussion, we +may state that fluctuations are linear, being limited to an increase and +to a decrease of the characters. These changes are mainly due to +differences in nourishment, either of the whole organism or of its +parts. In the first case, the deviations from the mean are called +individual; they are of great importance for the hereditary characters +of the offspring. In the second case the deviations are far more +universal and far more striking, but of lesser importance. They are +called partial fluctuations. + +All these fluctuations comply, in the main, with the law of probability, +and behave as if their causes were influenced only by chance. + + +[742] + +LECTURE XXVI + +ASEXUAL MULTIPLICATION OF EXTREMES + +Fluctuating variability may be regarded from two different points of +view. The multiformity of a bed of flowers is often a desirable feature, +and all means which widen the range of fluctuation are therefore used to +enhance this feature, and variability affords specimens, which surpass +the average, by yielding a better or larger product. + +In the case of fruits and other cultivated forms, it is of course +profitable to propagate from the better specimens only, and if possible +only from the very best. Obviously the best are the extremes of the +whole range of diverging forms, and moreover the extremes on one side of +the group. Almost always the best for practical purposes is that in +which some quality is strengthened. Cases occur however, in which it is +desirable to diminish an injurious peculiarity as far as possible, and +in these instances the opposite extreme is the most profitable one. + +These considerations lead us to a discussion [743] of the results of the +choice of extremes, which it may be easily seen is a matter of the +greatest practical importance. This choice is generally designated as +selection, but as with most of the terms in the domain of variability, +the word selection has come to have more than one meaning. Facts have +accumulated enormously since the time of Darwin, a more thorough +knowledge has brought about distinctions, and divisions at a rapidly +increasing rate, with which terminology has not kept pace. Selection +includes all kinds of choice. Darwin distinguished between natural and +artificial selection, but proper subdivisions of these conceptions are +needed. + +In the fourth lecture we dealt with this same question, and saw that +selection must, in the first place, make a choice between the elementary +species of the same systematic form. This selection of species or +species-selection was the work of Le Couteur and Patrick Shirreff, and +is now in general use in practice where it has received the name of +variety-testing. This clear and unequivocal term however, can hardly be +included under the head of natural selection. The poetic terminology of +selection by nature has already brought about many difficulties that +should be avoided in the future. On the other hand, the designation of +the process as a natural [744] selection of species complies as closely +as possible with existing terminology, and does not seem liable to any +misunderstanding. + +It is a selection between species. Opposed to it is the selection within +the species. Manifestly the first should precede the second, and if this +sequence is not conscientiously followed it will result in confusion. +This is evident when it is considered that fluctuations can only appear +with their pure and normal type in pure strains, and that each admixture +of other units is liable to be shown by the form of the curves. More +over, selection chooses single individuals, and a single plant, if it is +not a hybrid, can scarcely pertain to two different species. The first +choice therefore is apt to make the strain pure. + +In contrasting selection between species with that within the species, +of course elementary species are meant, including varieties. The terms +would be of no consequence if only rightly understood. For the sake of +clearness we might designate the last named process with the term of +intra-specific selection, and it is obvious that this term is applicable +both to natural and to artificial selection. + +Having previously dealt with species-selection at sufficient length, we +may now confine ourselves to the consideration of the intra-specific +[745] selection process. In practice it is of secondary importance, and +in nature it takes a very subordinate position. For this reason it will +be best to confine further discussions to the experience of the +breeders. + +Two different ways are open to make fluctuating variability profitable. +Both consist in the multiplication of the chosen extremes, and this +increase may be attained in a vegetative manner, or by the use of seeds. +Asexual and sexual propagation are different in many respects, and so +they are also in the domain of variability. + +In order to obtain a clear comprehension of this difference, it is +necessary to start from the distinction between individual and partial +fluctuations, as given in the last lecture. This distinction may be +discussed more understandingly if the causes of the variability are +taken into consideration. We have dealt with them at some length, and +are now aware that inner conditions only, determine averages, while some +fluctuation around them is allowable, as influenced by external +conditions. These outward influences act throughout life. At the very +first they impress their stamp on the whole organism, and incite a +lasting change in distinct directions. This is the period of the +development of the germ within the seed; it begins with the fusion of +the sexual cells, and each of them may be influenced [746] to a +noticeable degree before this union. This is the period of the +determination of individual variability. As soon as ramifications begin, +the external conditions act separately on every part, influencing some +to a greater and others to a lesser degree. Here we have the beginning +of partial variability. At the outset all parts may be affected in the +same way and in the same measure, but the chances of such an agreement, +of course, rapidly diminish. This is partly due to differences in +exposure, but mainly to alterations of the sensibility of the organs +themselves. + +It is difficult to gain a clear conception of the contrast between +individual and partial variability, and neither is it easy to appreciate +their cooperation rightly. Perhaps the best way is to consider their +activity as a gradual narrowing of possibilities. At the outset the +plant may develop its qualities in any measure, nothing being as yet +fixed. Gradually however, the development takes a definite direction, +for better or for worse. Is a direction once taken, then it becomes the +average, around which the remaining possibilities are grouped. The plant +or the organ goes on in this way, until finally it reaches maturity with +one of the thousands of degrees of development, between which at the +beginning it had a free choice. + +[747] Putting this discussion in other terms, we find every individual +and every organ in the adult state corresponding with a single ordinate +of the curve. The curve indicates the range of possibilities, the +ordinate shows the choice that has been made. Now it is clear at once +that this choice has not been made suddenly but gradually. Halfway of +the development, the choice is halfway determined, but the other half is +still undefined. The first half is the same for all the organs of the +plant, and is therefore termed individual; the second differs in the +separate members, and consequently is known as partial. Which of the two +halves is the greater and which the lesser, of course depends on the +cases considered. + +Finally we may describe a single example, the length of the capsules of +the evening-primrose. This is highly variable, the longest reaching more +than twice the length of the smallest. Many capsules are borne on the +same spike, and they are easily seen to be of unequal size. They vary +according to their position, the size diminishing in the main from the +base upwards, especially on the higher parts. Likewise the fruits of +weaker lateral branches are smaller. Curves are easily made by measuring +a few hundred capsules from corresponding parts of different plants, or +even by limiting the [748] inquiry to a single individual. These curves +give the partial variability, and are found to comply with Quetelet's +law. + +Besides this limited study, we may compare the numerous individuals of +one locality or of a large plot of cultivated plants with one another. +In doing so, we are struck with the fact that some plants have large and +others small fruits. We now limit ourselves to the main spike of each +plant, and perhaps to its lower parts, so as to avoid as far as possible +the impression made by the partial fluctuations. The differences remain, +and are sufficient to furnish an easy comparison with the general law. +In order to do this, we take from each plant a definite number of +capsules and measure their average length. In some experiments I took +the twenty lowermost capsules of the main spikes. In this way one +average was obtained for each plant, and combining these into a curve, +it was found that these fluctuations also came under Quetelet's law. +Thus the individual averages, and the fluctuations around each of them, +follow the same rule. The first are a measure for the whole plant, the +second only for its parts. As a general resume we can assert that, as a +rule, a quality is determined in some degree during the earlier stages +of the organism, and that this determination is valid throughout its +[749] life. Afterwards only the minor points remain to be regulated. +This makes it at once clear that the range of individual and partial +variability together must be wider than that of either of them, taken +alone. Partial fluctuations cannot, of course, be excluded. Thus our +comparison is limited to individual and partial variability on one side, +and partial fluctuations alone on the other side. + +Intra-specific selection is thus seen to fall under two heads: a +selection between the individuals, and a choice within each of them. The +first affords a wider and the latter a narrower field. + +Individual variability, considered as the result of outward influences +operative during extreme youth, can be excluded in a very simple manner. +Obviously it suffices to exclude extreme youth, in other words, to +exclude the use of seeds. Multiplication in a vegetative way, by +grafting and budding, by runners or roots, or by simple division of +rootstocks and bulbs is the way in which to limit variability to the +partial half. This is all we may hope to attain, but experience shows +that it is a very efficient means of limitation. Partial fluctuations +are generally far smaller than individual and partial fluctuations +together. + +Individual variability in the vegetable kingdom [750] might be called +seed-variation, as opposed to partial or bud-fluctuation. And perhaps +these terms are more apt to convey a clear conception of the distinction +than any other. The germ within the unripe seed is easily understood to +be far more sensitive to external conditions than a bud. + +Multiplication of extremes by seed is thus always counteracted by +individual variability, which at once reopens all, or nearly all, the +initial possibilities. Multiplication by buds is exempt from this danger +and thus leads to a high degree of uniformity. And this uniformity is in +many cases exactly what the breeder endeavors to obtain. + +We will treat of this reopening of previous possibilities under the head +of regression in the next lecture. It is not at all absolute, at least +not in one generation. Part of the improvement remains, and favors the +next generation. This part may be estimated approximately as being about +one-third or one-half of the improvement attained. Hence the conclusion +that vegetative multiplication gives rise to varieties which are as a +rule twice or thrice as good as selected varieties of plants propagated +by seeds. Hence, likewise the inference that breeders generally prefer +vegetative multiplication of improved forms, and apply it in all +possible cases. [751] Of course the application is limited, and forage +crops and the greater number of vegetables will always necessarily be +propagated by seed. + +Nature ordinarily prefers the sexual way. Asexual multiplications, +although very common with perennial plants, appear not to offer +important material for selection. Hence it follows that in comparing the +work of nature with that of man, the results of selection followed by +vegetative propagation should always be carefully excluded. Our large +bulb-flowers and delicious fruits have nothing in common with natural +products, and do not yield a standard by which to judge nature's work. + +It is very difficult for a botanist to give a survey of what practice +has attained by the asexual multiplication of extremes. Nearly all of +the large and more palatable fruits are due to such efforts. Some +flowers and garden-plants afford further instances. By far the greatest +majority of improved asexual varieties, however, are not the result of +pure intra-specific selection. They are due largely to the choice of the +best existing elementary species, and to some extent to crosses between +them, or between distinct systematic species. In practice selection and +hybridization go hand in hand and it is often difficult to ascertain +what part of [752] the result is due to the one, and what to the other +factor. + +The scientist, on the contrary, has nothing to do with the industrial +product. His task is the analysis of the methods, in order to reach a +clear appreciation of the influence of all the competing factors. This +study of the working causes leads to a better understanding of the +practical processes, and may become the basis of improvement in methods. + +Starting from these considerations, we will now give some illustrative +examples, and for the first, choose one in which hybridization is almost +completely excluded. + +Sugar-canes have long been considered to be plants without seed. Their +numerous varieties are propagated only in a vegetative way. The stems +are cut into pieces, each bearing one or two or more nodes with their +buds. An entire variety, though it may be cultivated in large districts +and even in various countries, behaves with respect to variability as a +single individual. Its individual fluctuability has been limited to the +earliest period of its life, when it arose from an unknown seed. The +personal characters that have been stamped on this one seed, partly by +its descent, and partly in the development of its germ during the period +of ripening, have become the indelible characters [753] of the variety, +and only the partial fluctuability, due to the effect of later +influences, can now be studied statistically. + +This study has for its main object the production of sugar in the stems, +and the curves, which indicate the percentage of this important +substance in different stems of the same variety, comply with Quetelet's +law. Each variety has its own average, and around this the data of the +majority of the stems are densely crowded, while deviations on both +sides are rare and become the rarer the wider they are. The "Cheribon" +cane is the richest variety cultivated in Java, and has an average of +19% sugar, while it fluctuates between 11% and 28%. "Chunnic" averages +14%, "Black Manilla" 13% and "White Manilla" 10%; their highest and +lowest extremes diverge in the same manner, being for the last named +variety 1% and 15%. + +This partial variability is of high practical interest, because on it a +selection may be founded. According to the conceptions described in a +previous lecture, fluctuating variability is the result of those outward +factors that determine the strength of development of the plant or the +organ. The inconstancy of the degree of sensibility, combined with the +ever-varying weather conditions preclude any close proportionality, but +apart from this difficulty there is, in the [754] main, a distinct +relation between organic strength and the development of single +qualities. This correlation has not escaped observation in the case of +the sugar-cane, and it is known that the best grown stocks are generally +the richest in sugar. Now it is evident that the best grown and richest +stems will have the greater chance of transmitting these qualities to +the lateral-buds. This at once gives, a basis for vegetative selection, +upon which it is not necessary to choose a small number of very +excellent stems, but simply to avoid the planting of all those that are +below the average. By this means the yield of the cultures has often +noticeably been enhanced. + +As far as experience goes, this sort of selection, however profitable, +does not conduce to the production of improved races. Only temporary +ameliorations are obtained, and the selection must be made in the same +manner every year. Moreover the improvement is very limited and does not +give any promise of further increase. In order to reach this, one has to +recur to the individual fluctuability, and therefore to seed. + +Nearly half a century ago, Parris discovered, on the island of Barbados, +that seeds might occasionally be gathered from the canes. These, +however, yielded only grass-like plants of no real value. The same +observation was made [755] shortly afterwards in Java and in other sugar +producing countries. In the year 1885, Soltwedel, the director of one of +the experiment stations for the culture of sugar-cane in Java, conceived +the idea of making use of seedlings for the production of improved +races. This idea is a very practical one, precisely because of the +possibility of vegetative propagation. If individuals would show the +same range as that of partial fluctuability, then the choice of the +extremes would at once bring the average up to the richness of the best +stocks. Once attained, this average would be fixed, without further +efforts. + +Unfortunately there is one great drawback. This is the infertility of +the best variety, that of the "Cheribon" cane. It flowers abundantly in +some years, but it has never been known to produce ripe seeds. For this +reason Soltwedel had to start from the second best sort, and chose the +"Hawaii" cane. This variety usually yields about 14% sugar, and +Soltwedel found among his seedlings one that showed 15%. This fact was +quite unexpected at that time, and excited widespread interest in the +new method, and since then it has been applied to numerous varieties, +and many thousands of seedlings have been raised and tested as to their +sugar-production. + +[756] From a scientific point of view the results are quite striking. +From the practical standpoint, however, the question is, whether the +"Hawaii" and other fertile varieties are adequate to yield seedlings, +which will surpass the infertile "Cheribon" cane. Now "Hawaii" averages +14% and "Cheribon" 19%, and it is easily understood that a "Hawaii" +seedling with more than 19% can be expected only from very large +sowings. Hundreds of thousands of seedlings must be cultivated, and +their juice tested, before this improvement can be reached. Even then, +it may have no significance for practical purposes. Next to the amount +of sugar comes the resistance to the disease called "Sereh," and the new +race requires to be ameliorated in this important direction, too. Other +qualities must also be considered, and any casual deterioration in other +characters would make all progress illusory. For these reasons much time +is required to attain distinct improvements. + +These great difficulties in the way of selecting extremes for vegetative +propagation are of course met with everywhere. They impede the work of +the breeder to such a degree, that but few men are able to surmount +them. Breeding new varieties necessitates the bending of every effort to +this purpose, and a clear conception of [757] the manifold aspects of +this intricate problem. These fall under two heads, the exigencies of +practice, and the physiologic laws of variability. Of course, only the +latter heading comes within the limits of our discussion which includes +two main points. First comes the general law of fluctuation that, though +slight deviations from the average may be found by thousands, or rather +in nearly every individual, larger and therefore important deviations +are very rare. Thousands of seedlings must be examined carefully in +order to find one or two from which it might be profitable to start a +new race. This point is the same for practical and for scientific +investigation. In the second place however, a digression is met with. +The practical man must take into consideration all the varying qualities +of his improved strains. Some of them must be increased and others be +decreased, and their common dependency on external conditions often +makes it very difficult to discover the desired combinations. It is +obvious, however, that the neglect of one quality may make all +improvement of other characters wholly useless. No augmentation of +sugar-percentage, of size and flavor of fruits can counterbalance an +increase in sensitiveness to disease, and so it is with other qualities +also. + +[758] Improved races for scientific investigation can be kept free from +infection, and protected against numerous other injuries. In the +experimental garden they may find conditions which cannot be realized +elsewhere. They may show a luxuriant growth, and prove to be excellent +material for research, but have features which, having been overlooked +at the period of selection, would at once condemn them if left to +ordinary conditions, or to the competition of other species. + +Considering all these obstacles, it is only natural that breeders should +use every means to reach their goal. Only in very rare instances do they +follow methods analogous to scientific processes, which tend to simplify +the questions as much as possible. As a rule, the practical way is the +combination of as many causes of variability as possible. Now the three +great sources of variability are, as has been pointed out on several +occasions, the original multiformity of the species, fluctuating +variability, and hybridization. Hence, in practical experiments, all +three are combined. Together they yield results of the highest value, +and Burbank's improved fruits and flowers give testimony to the +practical significance of this combination. + +From a scientific point of view however, it is [759] ordinarily +difficult, if not impossible, to discern the part which each of the +three great branches of variability has taken in the origination of the +product. A full analysis is rarely possible, and the treatment of one of +the three factors must necessarily remain incomplete. + +Notwithstanding these considerations, I will now give some examples in +order to show that fluctuating variability plays a prominent part in +these improvements. Of course it is the third in importance in the +series. First comes the choice of the material from the assemblage of +species, elementary species and varieties. Hybridization comes next in +importance. But even the hybrids of the best parents may be improved, +because they are no less subject to Quetelet's law than any other +strain. Any large number of hybrids of the same ancestry will prove +this, and often the excellency of a hybrid variety depends chiefly, or +at least definitely, on the selection of the best individuals. Being +propagated only in a vegetative way, they retain their original good +qualities through all further culture and multiplication. + +As an illustrative example I will take the genus _Canna_. Originally +cultivated for its large and bright foliage only, it has since become a +flowering plant of value. Our garden strains have originated by the +crossing of [760] a number of introduced wild species, among which the +_Canna indica_ is the oldest, now giving its name to the whole group. It +has tall stems and spikes with rather inconspicuous flowers with narrow +petals. It has been crossed with _C. nepalensis_ and _C. warczewiczii_, +and the available historic evidence points to the year 1846 as that of +the first cross. This was made by Annee between the _indica_ and the +_nepalensis_; it took ten years to multiply them to the required degree +for introduction into commerce. These first hybrids had bright foliage +and were tall plants, but their flowers were by no means remarkable. + +Once begun, hybridization was widely practiced. About the year 1889 +Crozy exhibited at Paris the first beautifully flowering form, which he +named for his wife, "Madame Crozy." Since that time he and many others, +have improved the flowers in the shape and size, as well as in color and +its patterns. In the main, these ameliorations have been due to the +discovery and introduction of new wild species possessing the required +characters. This is illustrated by the following incident. In the year +1892 I visited Mr. Crozy at Lyons. He showed me his nursery and numerous +acquisitions, those of former years as well as those that were quite +new, and which were in the process of rapid [761] multiplication, +previous to being given to the trade. I wondered, and asked, why no pure +white variety was present. His answer was "Because no white species had +been found up to the present time, and there is no other means of +producing white varieties than by crossing the existing forms with a new +white type." + +Comparing the varieties produced in successive periods, it is very easy +to appreciate their gradual improvement. On most points this is not +readily put into words, but the size of the petals can be measured, and +the figures may convey at least some idea of the real state of things. +Leaving aside the types with small flowers and cultivated exclusively +for their foliage, the oldest flowers of _Canna_ had petals of 45 mm. +length and 13 mm. breadth. The ordinary types at the time of my visit +had reached 61 by 21 mm., and the "Madame Crozy" showed 66 by 30 mm. It +had however, already been surpassed by a few commercial varieties, which +had the same length but a breadth of 35 mm. And the latest production, +which required some years of propagation before being put on the market, +measured 83 by 43 mm. Thus in the lapse of some thirty years the length +had been doubled and the breadth tripled, giving flowers with broad +corollas and with petals [762] joined all around, resembling the best +types of lilies and _Amaryllis_. + +Striking as this result unquestionably is, it remains doubtful as to +what part of it is due to the discovery and introduction of new large +flowered species, and what to the selection of the extremes of +fluctuating variability. As far as I have been able to ascertain +however, and according to the evidence given to me by Mr. Crozy, +selection has had the largest part in regard to the size, while the +color-patterns are introduced qualities. + +The scientific analysis of other intricate examples is still more +difficult. To the practical breeder they often seem very simple, but the +student of heredity, who wishes to discern the different factors, is +often quite puzzled by this apparent simplicity. So it is in the case of +the double lilacs, a large number of varieties of which have recently +been originated and introduced into commerce by Lemoine of Nancy. In the +main they owe their origin to the crossing and recrossing of a single +plant of the old double variety with the numerous existing +single-flowered sorts. + +This double variety seems to be as old as the culture of the lilacs. It +was already known to Munting, who described it in the year 1671. Two +centuries afterwards, in 1870, a new description [763] was given by +Morren, and though more than one varietal name is recorded in his paper, +it appears from the facts given that even at that time only one variety +existed. It was commonly called _Syringa vulgaris azurea plena_, and +seems to have been very rare and without real ornamental value. + +Lemoine, however, conceived the desirability of a combination of the +doubling with the bright colors and large flower-racemes of other +lilacs, and performed a series of crosses. The "_azurea plena_" has no +stamens, and therefore must be used in all crosses as the pistil-parent; +its ovary is narrowly inclosed in the tube of the flower, and difficult +to fertilize. On the other hand, new crosses could be made every year, +and the total number of hybrids with different pollen-parents was +rapidly increased. After five years the hybrids began to flower and +could be used for new crosses, yielding a series of compound hybrids, +which however, were not kept separate from the products of the first +crosses. + +Gradually the number of the flowering specimens increased, and the +character of doubling was observed to be variable to a high degree. +Sometimes only one supernumerary petal was produced, sometimes a whole +new typical corolla was extruded from within the first. In the same +[764] way the color and the number of the flowers on each raceme were +seen to vary. Thousands of hybrids were produced, and only those which +exhibited real advantages were selected for trade. These were multiplied +by grafting, and each variety at present consists only of the buds of +one original individual and their products. No constancy from seed is +assumed, many varieties are even quite sterile. + +Of course, no description was given of the rejected forms. It is only +stated that many of them bore either single or poorly filled flowers, or +were objectionable in some other way. The range of variability, from +which the choices were made, is obscure and only the fact of the +selection is prominent. What part is due to the combination of the +parental features and what to the individual fluctuation of the hybrid +itself cannot be ascertained. + +So it is in numerous other instances. The dahlias have been derived from +three or more original species, and been subjected to cultivation and +hybridization in an ever-increasing scale for a century. The best +varieties are only propagated in the vegetative way, by the roots and +buds, or by grafting and cutting. Each of them is, with regard to its +hereditary qualities, only one individual, and the individual characters +were selected at the same time with the [765] varietal and hybrid +characters. Most of them are very inconstant from seed and as a rule, +only mixtures are offered for sale in seed-lists. Which of their +ornamental features are due to fluctuating deviation from an average is +of course unknown. _Amaryllis_ and _Gladiolus_ are surrounded with the +same scientific uncertainties. Eight or ten, or even more, species have +been combined into one large and multiform strain, each bringing its +peculiar qualities into the mixed mass. Every hybrid variety is one +individual, being propagated by bulbs only. Colors and color-patterns, +shape of petals and other marks, have been derived from the wild +ancestors, but the large size of many of the best varieties is probably +due to the selection of the extremes of fluctuating variability. So it +is with the begonias of our gardens, which are also composite hybrids, +but are usually sown on a very large scale. Flowers of 15 cm. diameter +are very showy, but there can be no doubt about the manner in which they +are produced, as the wild species fall far short of this size. + +Among vegetables the potatoes afford another instance. Originally quite +a number of good species were in culture, most of them having small +tubers. Our present varieties are due to hybridization and selection, +each of them being propagated only in the vegetative way. + +[766] Selection is founded upon different qualities, according to the +use to be made of the new sort. Potatoes for the factory have even been +selected for their amount of starch, and in this case at least, +fluctuating variability has played a very important part in the +improvement of the race. + +Vegetative propagation has the great advantage of exempting the +varieties from regression to mediocrity, which always follows +multiplication by seeds. It affords the possibility of keeping the +extremes constant, and this is not its only advantage. Another, likewise +highly interesting, side of the question is the uniformity of the whole +strain. This is especially important in the case of fruits, though +ordinarily it is regarded as a matter of course, but there are some +exceptions which give proof of the real importance of the usual +condition. For example, the walnut-tree. Thousands of acres of +walnut-orchards consist of seedling trees grown from nuts of unknown +parentage. The result is a great diversity in the types of trees and in +the size and shape of the nuts, and this diversity is an obvious +disadvantage to the industry. The cause lies in the enormous +difficulties attached to grafting or budding of these trees, which make +this method very expensive and to a high degree uncertain and +unsatisfactory. + +[767] After this hasty survey of the more reliable facts of the practice +of an asexual multiplication of the extremes of fluctuating variability, +we may now return to the previously mentioned theoretical +considerations. These are concerned with an estimation of the chances of +the occurrence of deviations, large enough to exhibit commercial value. +This chance may be calculated on the basis of Quetelet's law, whenever +the agreement of the fluctuation of the quality under consideration has +been empirically determined. In the discussion of the methods of +comparing two curves, we have pointed to the quartiles as the decisive +points, and to the necessity of drawing the curves so that these points +are made to overlie one another, on each side of the average. If now we +calculate the binomium of Newton for different values of the exponent, +the sum of the coefficients is doubled for each higher unit of the +exponent, and at the same time the extreme limit of the curve is +extended one step farther. Hence it is possible to calculate a relation +between the value of the extreme and the number of cases required. It +would take us too long to give this calculation in detail, but it is +easily seen that for each succeeding step the number of individuals must +be doubled, though the length of the steps, or the amount of increase of +the quality [768] remains the same. The result is that many thousands of +seedlings are required to go beyond the ordinary range of variations, +and that every further improvement requires the doubling of the whole +culture. If ten thousand do not give a profitable deviation, the next +step requires twenty thousand, the following forty thousand, and so on. +And all this work would be necessary for the improvement of a single +quality, while practice requires the examination and amelioration of +nearly all the variable characters of the strain. + +Hence the rule that great results can only be obtained by the use of +large numbers, but it is of no avail to state this conclusion from a +scientific point of view. Scientific experimenters will rarely be able +to sacrifice fifty thousand plants to a single selection. The problem is +to introduce the principle into practice and to prove its direct +usefulness and reliability. It is to Luther Burbank that we owe this +great achievement. His principles are in full harmony with the teachings +of science. His methods are hybridization and selection in the broadest +sense and on the largest scale. One very illustrative example of his +methods must suffice to convey an idea of the work necessary to produce +a new race of superlative excellency. Forty thousand blackberry and +raspberry [769] hybrids were produced and grown until the fruit matured. +Then from the whole lot a single variety was chosen as the best. It is +now known under the name of "Paradox." All others were uprooted with +their crop of ripening berries, heaped up into a pile twelve feet wide, +fourteen feet high and twenty-two feet long, and burned. Nothing +remained of that expensive and lengthy experiment, except the one +parent-plant of the new variety. Similar selections and similar amount +of work have produced the famous plums, the brambles and the +blackberries, the Shasta daisy, the peach almond, the improved +blueberries, the hybrid lilies, and the many other valuable fruits and +garden-flowers that have made the fame of Burbank and the glory of +horticultural California. + + + +[770] + +LECTURE XXVII + +INCONSTANCY OF IMPROVED RACES + +The greater advantages of the asexual multiplication of extremes are of +course restricted to perennial and woody plants. Annual and biennial +species cannot as a rule, be propagated in this way, and even with some +perennials horticulturists prefer the sale of seeds to that of roots and +bulbs. In all these cases it is clear that the exclusion of the +individual variability, which was shown to be an important point in the +last lecture, must be sacrificed. + +Seed-propagation is subject to individual as well as to fluctuating +variability. The first could perhaps be designated by another term, +embryonic variability, since it indicates the fluctuations occurring +during the period of development of the germ. This period begins with +the fusion of the male and female elements and is largely dependent upon +the vigor of these cells at the moment, and on the varying qualities +they may have acquired. It comprises in the main the time of the +ripening of the seed, and [771] might perhaps best be considered to end +with the beginning of the resting stage of the ripe seed. Hence it is +clear that the variability of seed-propagated annual races has a wider +range than that of perennials, shrubs and trees. At present it is +difficult to discern exactly the part each of these two main factors +plays in the process. Many indications are found however, that make it +probable that embryonic variability is wider, and perhaps of far greater +importance than the subsequent partial fluctuations. The high degree of +similarity between the single specimens of a vegetative variety, and the +large amount of variability in seed-races strongly supports this view. +The propagation and multiplication of the extremes of fluctuating +variability by means of seeds requires a close consideration of the +relation between seedling and parent. The easiest way to get a clear +conception of this relation is to make use of the ideas concerning the +dependency of variability upon nourishment. Assuming these to be correct +in the main, and leaving aside all minor questions, we may conclude that +the chosen extreme individual is one of the best nourished and +intrinsically most vigorous of the whole culture. On account of these +very qualities it is capable of nourishing all of its organs better and +also its seeds. In other words, the seeds [772] of the extreme +individuals have exceptional chances of becoming better nourished than +the average of the seeds of the race. Applying the same rule to them, it +is easily understood that they will vary, by reason of this better +nourishment, in a direction corresponding to that of their parent. + +This discussion gives a very simple explanation of the acknowledged fact +that the seeds of the extremes are in the main the best for the +propagation of the race. It does not include however, all the causes for +this preferment. Some are of older date and due to previous influences. + +A second point in our discussion is the appreciation of the fact that a +single individual may be chosen to gather the seed from, and that these +seeds, and the young plants they yield, are as a rule, numerous. Hence +it follows that we are to compare their average and their extremes with +the qualities of the parents. Both are of practical as well as of +theoretical interest. The average of the progeny is to be considered as +the chief result of the selection in the previous generation, while the +extremes, at least those which depart in the same direction, are +obviously the means of further improvement of the race. + +Thus our discussion should be divided into [773] two heads. One of these +comprises the relation of the average of the progeny to the exceptional +qualities of the chosen parent, and the other the relation of +exceptional offspring to the exceptional parents. + +Let us consider the averages first. Are they to be expected to be equal +to the unique quality of the parent, or perhaps to be the same as the +average of the whole unselected race? Neither of these cases occur. +Experience is clear and definite on this important point. Vilmorin, when +making the first selections to improve the amount of sugar in beets, was +struck with the fact that the average of the progeny lies between that +of the original strain and the quality of the chosen parent. He +expressed his observation by stating that the progeny are grouped around +and diverge in all directions from some point, placed on the line which +unites their parent with the type from which it sprang. All breeders +agree on this point, and in scientific experiments it has often been +confirmed. We shall take up some illustrative examples presently, but in +order to make them clear, it is necessary to give a closer consideration +to the results of Vilmorin. + +From his experience it follows that the average of the progeny is higher +than that of the race at large, but lower than the chosen parent. [774] +In other words, there is a progression and a regression. A progression +in relation to the whole race, and a regression in comparison with the +parent. The significance of this becomes clear at once, if we recall the +constancy of the variety which could be obtained from the selected +extreme in the case of vegetative multiplication. The progression is +what the breeder wants, the regression what he detests. Regression is +the permanency of part of the mediocrity which the selection was invoked +to overcome. Manifestly it is of the highest interest that the +progression should be as large, and the regression as small as possible. +In order to attain this goal the first question is to know the exact +measure of progression and regression as they are exhibiting themselves +in the given cases, and the second is to inquire into the influences, on +which this proportion may be incumbent. + +At present our notions concerning the first point are still very limited +and those concerning the second extremely vague. Statistical inquiries +have led to some definite ideas about the importance of regression, and +these furnish a basis for experimental researches concerning the causes +of the phenomenon. Very advantageous material for the study of +progression and regression in the realm of fluctuating variability is +afforded by the [775] ears of corn or maize. The kernels are arranged in +longitudinal rows, and these rows are observed to occur in varying, but +always even, numbers. This latter circumstance is due to the fact that +each two neighboring rows contain the lateral branches of a single row +of spikelets, the ages of which however, are included in the fleshy body +of the ear. The variation of the number of the rows is easily seen to +comply with Quetelet's law, and often 30 or 40 ears suffice to give a +trustworthy curve. Fritz Muller made some experiments upon the +inheritance of the number of the rows, in Brazil. He chose a race which +averaged 12 rows, selected ears with 14, 16 and 18 rows, etc., and sowed +their kernels separately. In each of-these cultures he counted the rows +of the seeds on the ears of all the plants when ripe, and calculated +their average. This average, of course, does not necessarily correspond +to a whole number, and fractions should not be neglected. + +According to Vilmorin's rule he always found some progression of the +average and some regression. Both were the larger, the more the +parent-ear differed from the general average, but the proportion between +both remained the same, and seems independent of the amount of the +deviation. Putting the deviation at 5, the progression calculated from +his figures is [776] 2 and the regression 3. In other words the average +of the progeny has gained over the average of the original variety +slightly more than one-third, and slightly less than one-half of the +parental deviation. I have repeated this experiment of Fritz Miller's +and obtained nearly the same regression of three-fifths, though working +with another variety, and under widely different climatic conditions. + +The figures of Fritz Muller were, as given below, in one experiment. In +the last column I put the improvement calculated for a proportion of +two-fifths above the initial average of 12. + + Rows on Average of rows 12 + 2/5 of + parent ears of progeny Difference + 14 12.6 12.8 + 16 14.1 13.6 + 18 15.2 14.4 + 20 15.8 15.2 + 22 16.1 16.0 + +Galton, in his work on natural inheritance, describes an experiment with +the seeds of the sweet pea or _Lathyrus odoratus_. He determined the +average size in a lot of purchased seeds, and selected groups of seeds +of different, but for each group constant, sizes. These were sown, and +the average of the seeds was determined anew in the subsequent harvest +they yielded. These figures agreed with the rule of Vilmorin and were +calculated in the manner [777] given for the test of the corn. The +progression and regression were found to be proportionate to the amount +of the deviation. The progression of the average was one-third, and the +regression in consequence two-thirds of the total deviation. The +amelioration is thus seen to be nearly, though not exactly, the same as +in the previous case. + +From the evidence of the other corresponding experiments, and from +various statistical inquiries it seems that the value of the progression +is nearly the same in most cases, irrespective of the species used and +the quality considered. It may be said to be from one-third to one-half +of the parental deviation, and in this form the statement is obviously +of wide and easy applicability. + +Our figures also demonstrate the great preeminence of vegetative +varieties above the improved strains multiplied by seeds. They have a +definite relation. Asexually multiplied strains may be said to be +generally two times or even three times superior to the common +offspring. This is a difference of great practical importance, and +should never be lost sight of in theoretical considerations of the +productive capacity of selection. Multiplication by seed however, has +one great advantage over the asexual method; it may be repeated. The +[778] selection is not limited to a single choice, but may be applied in +two or more succeeding generations. Obviously such a repetition affords +a better chance of increasing the progression of the average and of +ameliorating the race to a greater degree than would be possible by a +single choice. This principle of repeated selection is at present the +prominent feature of race improvement. Next to variety-testing and +hybridizing it is the great source of the steady progression of +agricultural crops. From a practical standpoint the method is clear and +as perfect as might be expected, but this is not the side of the problem +with which we are concerned here. The theoretical analysis and +explanation of the results obtained, however, is subject to much doubt, +and to a great divergence of conceptions. So it is also with the +application of the practical processes to those occurring in nature. +Some assume that here repeated selection is only of subordinate +importance, while others declare that the whole process of evolution is +due to this agency. This very important point however, will be reserved +for the next lecture, and only the facts available at present will be +considered here. + +As a first example we may take the ray-florets of the composites. On a +former occasion we have dealt with their fluctuation in number and [779] +found that it is highly variable and complies in the main with +Quetelet's law. _Madia elegans_, a garden species, has on the average 21 +rays on each head, fluctuating between 16 and 25 or more. I saved the +seeds of a plant with only 17 rays on the terminal head, and got from +them a culture which averaged 19 rays, which is the mean between 21 and +17. In this second generation I observed the extremes to be 22 and 12, +and selected a plant with 13 rays as the parent for a continuation of +the experiment. The plants, which I got from its seeds, averaged 18 and +showed 22 and 13 as extremes. The total progression of the average was +thus, in two generations, from 21 to 18, and the total regression from +13 to 18, and the proportion is thus seen to diminish by the repetition +rather than to increase. + +This experiment, however, is of course too imperfect upon which to found +general conclusions. It only proves the important fact that the improved +average of the second generation is not the starting-point for the +further improvement. But the second generation allows a choice of an +extreme, which diverges noticeably more from the mean than any +individual of the first culture, and thereby gives a larger amount of +absolute progression, even if the proportion between progression and +regression remains [780] the same. The repetition is only an easy method +of getting more widely deviating extremes; whether it has, besides this, +another effect, remains doubtful. In order to be able to decide this +question, it is necessary to repeat the selection during a series of +generations. In this way the individual faults may be removed as far as +possible. I chose an experiment of Fritz Muller, relating to the number +of rows of grains on the ears exactly as in the case above referred to, +and which I have repeated in my experimental garden at Amsterdam. + +I started from a variety known to fructify fairly regularly in our +climate, and exhibiting in the mean 12-14 rows, but varying between 8 +and 20 as exceptional cases. I chose an ear with 16 rows and sowed its +seeds in 1887. A number of plants were obtained, from each of which, one +ear was chosen in order to count its rows. An average of 15 rows was +found with variations complying with Quetelet's law. One ear reached 22 +rows, but had not been fertilized, some others had 20 rows, and the best +of these was chosen for the continuation of the experiment. I repeated +the sowing during 6 subsequent generations in the same way, choosing +each time the most beautiful ear from among those with the greatest +number of rows. Unfortunately with the increase of the number the [781] +size of the grains decreases, the total amount of nourishment available +for all of them remaining about the same. Thus the kernels and +consequently the new plants became smaller and weaker, and the chance of +fertilization was diminished in the ears with the highest number of +rows. Consequently the choice was limited, and after having twice chosen +a spike with 20 and once one with 24 rows, I finally preferred those +with the intermediate number of 22. + +This repeated choice has brought the average of my race up from 13 to +20, and thus to the extreme limit of the original variety. Seven years +were required to attain this result, or on an average the progression +was one row in a year. This augmentation was accompanied by an +accompanying movement of the whole group in the same direction. The +extreme on the side of the small numbers came up from 8 to 12 rows, and +cobs with 8 or 10 rows did not appear in my race later than the third +generation. On the other side the extreme reached 28, a figure never +reached by the original variety as cultivated with us, and ears with 24 +and 26 rows have been seen during the four last generations in +increasing numbers. + +This slow and gradual amelioration was partly due to the mode of +pollination of the corn. [782] The pollen falls from the male spikes on +the ears of the same plant, but also is easily blown on surrounding +spikes. In order to get the required amount of seed it is necessary in +our climate to encroach as little as possible upon free pollination, +aiding the self-pollination, but taking no precautions against +intercrossing. It is assumed that the choice of the best ears indicates +the plants which have had the best pollen-parents as well as the best +pistil parents, and that selection here, as in other cases, corrects the +faults of free intercrossing. But it is granted that this correction is +only a slow one, and accounts in a great degree for the slowness of the +progression. Under better climatic conditions and with a more entire +isolation of the individuals, it seems very probable that the same +result could have been reached in fewer generations. + +However this may be, the fact is that by repeated selection the strain +can be ameliorated to a greater extent than by a single choice. This +result completely agrees with the general experience of breeders and the +example given is only an instance of a universal rule. It has the +advantage of being capable of being recorded in a numerical way, and of +allowing a detailed and definite description of all the succeeding +generations. The entire harvest of all [783] of them has been counted +and the figures combined into curves, which at once show the whole +course of the pedigree-experiment. These curves have in the main taken +the same shape, and have only gradually been moved in the chosen +direction. + +Three points are now to be considered in connection with this +experiment. The first is the size of the cultures required for the +resulting amelioration. In other words, would it have been possible to +attain an average of 20 rows in a single experiment? This is a matter of +calculation, and the calculation must be based upon the experience +related above, that the progression in the case of maize is equal to +two-fifths of the parental deviation. A cob with 20 rows means a +deviation of 7 from the average of 13, the incipient value of my race. +To reach such an average at once, an ear would be required with 7 x 5/2 += 17-1/2 rows above the average, or an ear with 30-32 rows. These never +occur, but the rule given in a previous lecture gives a method of +calculating the probability of their occurrence, or in other words, the +number of ears required to give a chance of finding such an ear. It +would take too long to give this calculation here, but I find that +approximately 12,000 ears would be required to give one with 28 rows, +which was the highest number attained in [784] my experiment, while +100,000 ears would afford a chance of one with 32 rows*. Had I been able +to secure and inspect this number of ears, perhaps I would have needed +only a year to get an average of 20 rows. This however, not being the +case, I have worked for seven years, but on the other hand have +cultivated all in all only about one thousand individuals for the entire +experiment. + +Obviously this reduction of the size of the experiment is of importance. +One hundred thousand ears of corn could of course, be secured directly +from trade or from some industrial culture, but corn is cultivated only +to a small extent in Holland, and in most cases the requisite number of +individuals would be larger than that afforded by any single plantation. + +Repeated selection is thereby seen to be the means of reducing the size +of the required cultures to possible measures, not only in the +experimental-garden, but also for industrial purposes. A selection from +among 60,000-100,000 individuals may be within reach of Burbank, but of +few others. As a rule they prefer a longer time with a smaller lot of +plants. This + + + * On about 200 ears the variability ranges from 8-22 rows, and + this leads approximately to one row more by each doubling of + the numbers of instances. One ear with 22 rows in 200 would + thus lead to the expectation of one ear with 32 rows in + 100,000 ears. + +[785] is exactly what is gained by repeated selections. To my mind this +reduction of the size of the cultures is probably the sole effect of the +repetition. But experience is lacking on this point, and exact +comparisons should be made whenever possible, between the descendants of +a unique but extreme choice, and a repeated but smaller selection. The +effect of the repetition on the nourishment of the chosen +representatives should be studied, for it is clear that a plant with 22 +rows, the parents and grandparents of which had the same number, +indicates a better condition of internal qualities than one with the +same number of rows, produced accidentally from the common race. In this +way it may perhaps be possible to explain, why in my experiment an ear +with 22 rows gave an average offspring with 20, while the calculation, +founded on the regression alone would require a parental ear with 32 +rows. + +However, as already stated, this discussion is only intended to convey +some general idea as to the reduction of the cultures by means of +repeated selections, as the material at hand is wholly inadequate for +any closer calculation. This important point of the reduction may be +illustrated in still another manner. + +The sowing of very large numbers is only required because it is +impossible to tell from the [786] inspection of the seeds which of them +will yield the desired individual. But what is impossible in the +inspection of the seeds may be feasible, at least in important measure, +in the inspection of the plants which bear the seeds. Whenever such an +inspection demonstrates differences, in manifest connection with the +quality under consideration, any one will readily grant that it would be +useless to sow the seeds of the worst plants, and that even the whole +average might be thrown over, if it were only possible to point out a +number of the best. But it is clear that by this inspection of the +parent plants the principle of repeated selection is introduced for two +succeeding generations, and that its application to a larger series of +generations is only a question of secondary importance. + +Summing up our discussion of this first point we may assert that +repeated selection is only selection on a small and practical scale, +while a single choice would require numbers of individuals higher than +are ordinarily available. + +A second discussion in connection with our pedigree-culture of corn is +the question whether the amelioration obtained was of a durable nature, +or only temporary. In other words, whether the progeny of the race would +remain constant, if cultivated after cessation of the selection. In +order to ascertain this, [787] I continued the culture during several +generations, choosing ears with less than the average number of rows. +The excellence of the race at once disappeared, and the ordinary average +of the variety from which I had started seven years before, returned +within two or three seasons. This shows that the attained improvement is +neither fixed nor assured and is dependent on continued selection. This +result only confirms the universal experience of breeders, which teaches +the general dependency of improved races on continued selection. Here a +striking contrast with elementary species or true varieties is obvious. +The strains which nature affords are true to their type; their average +condition remains the same during all the succeeding generations, and +even if it should be slightly altered by changes in the external +conditions, it returns to the type, as soon as these changes come to an +end. It is a real average, being the sum of the contribution of all the +members of the strain. Improved races have only an apparent average, +which is in fact biased by the exclusion of whole groups of individuals. +If left to themselves, their appearance changes, and the real average +soon returns. This is the common experience of breeders. + +A third point is to be discussed in connection [788] with the detailed +pedigree-cultures. It is the question as to what might be expected from +a continuation of improvement selection. Would it be possible to obtain +any imaginable deviation from the original type, and to reach +independency from further selection? This point has not until now +attracted any practical interest, and from a practical point of view and +within the limits of ordinary cultures, it seems impossible to obtain a +positive answer. But in the theoretical discussion of the problems of +descent it has become of the highest importance, and therefore requires +a separate treatment, which will be reserved for the next lecture. + +Here we come upon another equally difficult problem. It relates to the +proportion of embryonic or individual fluctuation, to partial variation +as involved in the process of selection. Probably all qualities which +may be subjected to selection vary according to both principles, the +embryonic decision giving only a more definite average, around which the +parts of the individual are still allowed to oscillate. It is so with +the corn, and whenever two or more ears are ripening or even only +flowering on the same plant, differences of a partial nature may be seen +in the number of their rows. These fluctuations are only small however, +ordinarily not exceeding two and rarely four [789] rows. Choosing always +the principal ear, the figures may be taken to indicate the degree of +personal deviation from the average of the race. But whenever we make a +mistake, and perchance sow from an ear, the deviation of which was +largely due to partial variation, the regression should be expected to +become considerably larger. Hence it must be conceded that exact +calculations of the phenomena of inheritance are subject to much +uncertainty, resulting from our very imperfect knowledge concerning the +real proportion of the contributing factors, and the difficulty of +ascertaining their influence in any given case. Here also we encounter +more doubts than real facts, and much remains to be done before exact +calculations may become of real scientific value. + +Returning to the question of the effects of selection in the long run, +two essentially different cases are to be considered. Extremes may be +selected from among the variants of ordinary fluctuating variability, or +from ever-sporting varieties. These last we have shown to be double +races. Their peculiar and wide range of variability is due to the +substitution of two characters, which exclude one another, or if +combined, are diminished in various degrees. Striped flowers and stocks, +"five-leaved" clover, pistilloid opium-poppies and numerous other [790] +monstrosities have been dealt with as instances of such ever-sporting +varieties. + +Now the question may be put, what would be the effect of selection if in +long series of years one of the two characters of such a double race +were preferred continuously, to the complete exclusion of the other. +Would the race become changed thereby? Could it be affected to such a +degree as to gradually lose the inactive quality, and cease to be a +double race? + +Here manifestly we have a means by which to determine what selection is +able to accomplish. Physiologic experiments may be said to be too short +to give any definite evidence. But cases may be cited where nature has +selected during long centuries and with absolute constancy in her +choice. Moreover unconscious selections by man have often worked in an +analogous manner, and many cultivated plants may be put to the test +concerning the evidence they might give on this point. Stating +beforehand the result of this inquiry, we may assert that long-continued +selection has absolutely no appreciable effect. Of course I do not deny +the splendid results of selection during the first few years, nor the +necessity of continued selection to keep the improved races to the +height of their ameliorated qualities. I only wish to state that the +work [791] of selection here finds its limit and that centuries and +perhaps geologic periods of continued effort in the same direction are +not capable of adding anything more to the initial effect. Some +illustrative examples may suffice to prove the validity of this +assertion. Every botanist who has studied the agricultural practice of +plant-breeding, or the causes of the geographic distribution of plants, +will easily recall to his mind numerous similar cases. Perhaps the most +striking instance is afforded by cultivated biennial plants. The most +important of them are forage-beets and sugar-beets. They are, of course, +cultivated only as biennials, but some annual specimens may be seen each +year and in nearly every field. They arise from the same seed as the +normal individuals, and their number is obviously dependent on external +conditions, and especially on the time of sowing. Ordinary cultures +often show as much as 1% of these useless plants, but the exigencies of +time and available labor often compel the cultivator to have a large +part of his fields sown before spring. In central Europe, where the +climate is unfavorable at this season, the beets respond by the +production of far larger proportions of annual specimens, their number +coming often up to 20% or more, thus constituting noticeable losses in +the product [792] of the whole field. Rimpau, who has made a thorough +study of this evil and has shown its dependency on various external +conditions, has also tried to find methods of selection with the aim of +overcoming it, or at least of reducing it to uninjurious proportions. +But in these efforts he has reached no practical result. The annuals are +simply inexterminable. + +Coming to the alternative side of the problem it is clear that annuals +have always been excluded in the selection. Their seeds cannot be mixed +with the good harvest, not even accidentally, since they have ripened in +a previous year. In order to bear seeds in the second year beets must be +taken from the field, and kept free from frost through the winter. The +following spring they are planted out, and it is obvious that even the +most careless farmer is not liable to mix them with annual specimens. +Hence we may conclude that a strict and unexcelled process of selection +has been applied to the destruction of this tendency, not only for +sugar-beets, since Vilmorin's time, when selection had become a well +understood process, but also for forage-beets since the beginning of +beet culture. Although unconscious, the selection of biennials must have +been uninterrupted and strict throughout many centuries. + +It has had no effect at all. Annuals are seen [793] to return every +year. They are ineradicable. Every individual is in the possession of +this latent quality and liable to convert it into activity as soon as +the circumstances provoke its appearance, as proved by the increase of +annuals in the early sowings. Hence the conclusion that selection in the +long run is not adequate to deliver plants from injurious qualities. +Other proofs could be given by other biennials, and among them the stray +annual plants of common carrots are perhaps the most notorious. In my +own cultures of evening-primroses I have preferred the annuals and +excluded the biennials, but without being able to produce a pure annual +race. As soon as circumstances are favorable, the biennials return in +large numbers. Cereals give analogous proofs. Summer and winter +varieties have been cultivated separately for centuries, but in trials +it is often easy to convert the one into the other. No real and definite +isolation has resulted from the effect of the long continued unconscious +selection. + +Striped flowers, striped fruits, and especially striped radishes afford +further examples. It would be quite superfluous to dwell upon them. +Selection always tends to exclude the monochromatic specimens, but does +not prevent their return in every generation. Numerous [794] rare +monstrosities are in the same category, especially when they are of so +rare occurrence as not to give any noticeable contribution to the +seed-production, or even if they render their bearers incapable of +reproduction. In such cases the selection of normal plants is very +severe or even absolute, but the anomalies are by no means exterminated. +Any favorable circumstances, or experimental selection in their behalf +shows them to be still capable of full development. Numerous cases of +such subordinate hereditary characters constitute the greater part of +the science of vegetable teratology. + +If it should be objected that all these cases cover too short a time to +be decisive, or at least fail in giving evidence relative to former +times, alpine plants afford a proof which one can hardly expect to be +surpassed. During the whole present geologic epoch they have been +subjected to the never failing selection of their climate and other +external conditions. They exhibit a full and striking adaptation to +these conditions, but also possess the latent capacity for assuming +lowland characters as soon as they are transported into such +environment. Obviously this capacity never becomes active on the +mountains, and is always counteracted by selection. This agency is +evidently without any effect, for as we have seen when dealing [795] +with the experiments of Nageli, Bonnier and others, each single +individual may change its habits and its aspect in response to +transplantation. The climate has an exceedingly great influence on each +individual, but the continuance of this influence is without permanent +result. + +So much concerning ever-sporting varieties and double adaptations. We +now come to the effects of a continuous selection of simple characters. + +Here the sugar-beets stand preeminent. Since Vilmorin's time they have +been selected according to the amount of sugar in their roots, and the +result has been the most striking that has ever been attained, if +considered from the standpoint of practice. But if critically examined, +with no other aim than a scientific appreciation of the improvement in +comparison with other processes of selection, the support of the +evidence for the theory of accumulative influence proves to be very +small. + +The amount of sugar is expressed by percentage-figures. These however, +are dependent on various causes, besides the real quantity of sugar +produced. One of these causes is the quantity of watery fluid in the +tissues, and this in its turn is dependent on the culture in dryer or +moister soil, and on the amount of moisture in the air, and the same +variety of sugar-beets [796] yields higher percentage-figures in a dry +region than in a wet one. This is seen when comparing, for instance, the +results of the analyses from the sandy provinces of Holland with those +from the clay-meadows, and it is very well known that Californian beets +average as high as 26% or more, while the best European beets remain at +about 20%. As far as I have been able to ascertain, these figures +however, are not indicative of any difference of race, but simply direct +responses to the conditions of climate and of soil. + +Apart from these considerations the improvement reached in half a +century or in about twenty to thirty generations is not suggestive of +anything absolute. Everything is fluctuating now, even as it was at the +outset, and equally dependent on continual care. Vilmorin has given some +figures for the beets of the first generations from which he started his +race. He quotes 14% as a recommendable amount, and 7 and 21 as the +extreme instances of his analyses. However incorrect these figures may +be, they coincide to a striking degree with the present condition of the +best European races. Of course minor values are excluded each year by +the selection, and in consequence the average value has increased. For +the year 1874 we find a standard of 10-14% considered as normal, [797] +bad years giving 10%, good years from 12% to 14% in the average. Extreme +instances exceeded 17%. From that time the practice of the polarization +of the juice for the estimate of the sugar has rapidly spread throughout +Europe, and a definite increase of the average value soon resulted. This +however, often does not exceed 14%, and beets selected in the field for +the purpose of polarization come up to an average of 15 to 16%, varying +downward to less than 10% and upward to 20 and 21%. In the main the +figures are the same as those of Vilmorin, the range of variability has +not been reduced, and higher extremes are not reached. An average +increase of 1% is of great practical importance, and nothing can excel +the industry and care displayed in the improvement of the beet-races. +Notwithstanding this a lasting influence has not been exercised; the +methods of selection have been improved, and the number of polarized +beets has been brought up to some hundreds of thousands in single +factories, but the improvement is still as dependent upon continuous +selection as it was half a century ago. + +The process is practically very successful, but the support afforded by +it to the selection theory vanishes on critical examination. + + +[798] + +LECTURE XXVIII + +ARTIFICIAL AND NATURAL SELECTION + +The comparison of artificial and natural selection has furnished +material support for the theory of descent, and in turn been the object +of constant criticism since the time of Darwin. The criticisms, in +greater part, have arisen chiefly from an imperfect knowledge of both +processes. By the aid of distinctions recently made possible, the +contrast between elementary species and improved races has become much +more vivid, and promises to yield better results on which to base +comparisons of artificial and natural selection. + +Elementary species, as we have seen in earlier lectures, occur in wild +and in cultivated plants. In older genera and systematic species they +are often present in small numbers only, but many of the more recent +wild types and also many of the cultivated forms are very rich in this +respect. In agriculture the choice of the most adequate elementary forms +for any special purpose is acknowledged [799] as the first step in the +way of selection, and is designated by the name of variety-testing, +applying the term variety to all the subdivisions of systematic species +indiscriminately. In natural processes it bears the title of survival of +species. The fact that recent types show large numbers, and in some +instances even hundreds of minor constant forms, while the older genera +are considerably reduced in this respect, is commonly explained by the +assumption of extinction of species on a correspondingly large scale. +This extinction is considered to affect the unfit in a higher measure +than the fit. Consequently the former vanish, often without leaving any +trace of their existence, and only those that prove to be sufficiently +adapted to the surrounding external conditions, resist and survive. + +This selection exhibits far-reaching analogies between the artificial +and the natural processes, and is in both cases of the very highest +importance. In nature the dying out of unfit mutations is the result of +the great struggle for life. In a previous lecture we have compared its +agency with that of a sieve. All elements which are too small or too +weak fall through, and only those are preserved which resist the sifting +process. Reduced in number they thrive and multiply and are thus enabled +to [800] strike out new mutative changes. These are again submitted to +the sifting tests, and the frequent repetition of this process is +considered to give a good explanation of the manifold, highly +complicated, and admirable structures which strike the beginner as the +only real adaptations in nature. + +Exactly in the same way artificial selection isolates and preserves some +elementary species, while it destroys others. Of course the time is not +sufficient to secure new mutations, or at least these are only rare at +present, and their occurrence is doubtful in historic periods. Apart +from this unavoidable difference the analogy between natural and +artificial selection appears to me to be very striking. + +This form of selection may be termed selection between species. Opposed +to it stands the selection within the elementary species or variety. It +has of late, alone come to be known as selection, though in reality it +does not deserve this distinction. I have already detailed the +historical evidence which gives preference to selection between species. +The process can best be designated by the name of intraspecific +selection, if it is understood that the term intraspecific is meant to +apply to the conception of small or elementary species. + +I do not wish to propose new terms, but [801] I think that the principal +differences might better become understood by the introduction of the +word election into the discussion of questions of heredity. Election +meant formerly the preferential choice of single individuals, while the +derivation of the word selection points to a segregation of assemblies +into their larger parts. Or to state it in a shorter way, individual +selection is exactly what is usually termed election. Choosing one man +from among thousands is to elect him, but a select party is a group of +chosen persons. There would be no great difficulty in the introduction +of the word election, as breeders are already in the habit of calling +their choice individuals "elite," at least in the case of beets and of +cereals. + +This intraspecific selection affords a second point for the comparison +between natural and artificial processes. This case is readily granted +to be more difficult than the first, but there can be no doubt that the +similarity is due to strictly comparable causes. In practice this +process is scarcely second in importance to the selection between +species, and in numerous cases it rests upon it, and crowns it, bringing +the isolated forms up to their highest possible degree of usefulness. In +nature it does quite the same, adapting strains of individuals to the +local conditions of their environment. Improved [802] races do not +generally last very long in practice; sooner or later they are surpassed +by new selections. Exactly so we may imagine the agency of natural +intraspecific selection. It produces the local races, the marks of which +disappear as soon as the special external conditions cease to act. It is +responsible only for the smallest lateral branches of the pedigree, but +has nothing in common with the evolution on the main stems. It is of +very subordinate importance. + +These assertions of course, are directly opposed to the current run of +scientific belief, but they are supported by facts. A considerable part +of the evidence has already been dealt with and for our closing +discussion only an exact comparison remains to be made between the two +detailed types of intraspecific selection. In coming to this I will +first dwell upon some intermediate types and conclude with a critical +discussion of the features of artificial selection, which to my mind +prove the invalidity of the conclusions drawn from it in behalf of an +explanation of the processes of nature. + +Natural selection occurs not only in the wild state, but is also active +in cultivated fields. Here it regulates the struggle of the selected +varieties and improved races with the older types, and even with the +wild species. In a previous [803] lecture I have detailed the rapid +increase of the wild oats in certain years, and described the +experiments of Risler and Rimpau in the running out of select varieties. +The agency is always the same. The preferred forms, which give a larger +harvest, are generally more sensitive to injurious influences, more +dependent on rich manure and on adequate treatment. The native varieties +have therefore the advantage, when climatic or cultural conditions are +unfavorable for the fields at large. They suffer in a minor degree, and +are thereby enabled to propagate themselves afterwards more rapidly and +to defeat the finer types. This struggle for life is a constant one, and +can easily be followed, whenever the composition of a strain is noted in +successive years. It is well appreciated by breeders and farmers, +because it is always liable to counteract their endeavors and to claim +their utmost efforts to keep their races pure. There can be no doubt +that exactly the same struggle exempt from man's intrusion is fought out +in the wild state. + +Local races of wild plants have not been the object for field +observations recently. Some facts however, are known concerning them. On +the East Friesian Islands in the North Sea the flowers are strikingly +larger and brighter colored than those of the same species on the [804] +neighboring continent. This local difference is ascribed by Behrens to a +more severe selection by the pollinating insects in consequence of their +lesser frequency on these very windy isles. Seeds of the pines from the +Himalayas yield cold-resisting young plants if gathered from trees in a +high altitude, while the seeds of the same species from lower regions +yield more sensitive seedlings. Similar instances are afforded by +_Rhododendron_ and other mountain species. According to Cieslar +corresponding differences are shown by seeds of firs and larches from +alpine and lowland provinces. + +Such changes are directly dependent on external influences. This is +especially manifest in experiments extending the cultures in higher or +in more northern regions. The shorter summer is a natural agent of +selection; it excludes all individuals which cannot ripen their seeds +during so short a period. Only the short lived ones survive. Schubeler +made very striking experiments with corn and other different cereals, +and has succeeded in making their culture possible in regions of Norway +where it formerly failed. In the district of Christiania, corn had +within some few years reduced its lifetime from 123 to 90 days, yielding +smaller stems and fewer kernels, but still sufficient to make its +culture profitable under the existing conditions. [805] This change was +not permanent, but was observed to diminish rapidly and to disappear +entirely, whenever the Norwegian strain was cultivated in the southern +part of Germany. It was a typical improved race, dependent on continual +selection by the short summers which had produced it. Similar results +have been reached by Von Wettstein in the comparison of kinds of flax +from different countries. The analogy between such cultivated local +races and the local races of nature is quite striking. The practice of +seed exchange rests for a large part on the experience that the +characters, acquired under the definite climatic and cultural conditions +of some select regions, hold good for one or two, and sometimes even +more generations, before they decrease to practical uselessness. The +Probstei, the Hanna and other districts owe their wealth to this +temporary superiority of their wheat and other cereals. + +Leaving these intermediate forms of selection, we now come to our +principal point. It has already been discussed at some length in the +previous lecture, but needs further consideration. It is the question +whether intraspecific selection may be regarded as a cause of lasting +and ever-increasing improvement. This is assumed by biologists who +consider fluctuating variability as the main source of progression [806] +in the organic world. But the experience of the breeders does not +support this view, since the results of practice prove that selection +according to a constant standard soon reaches a limit which it is not +capable of transgressing. In order to attain further improvements the +method of selection itself must be improved. A better and sharper method +assures the choice of more valuable representatives of the race, even if +these must be sought for in far larger numbers of individuals, as is +indicated by the law of Quetelet. + +Continuous or even prolonged improvement of a cultivated race is not the +result of frequently repeated selection, but of the improvement of the +standard of appreciation. Nature, as far as we know, changes her +standard from time to time only in consequence of the migrations of the +species, or of local changes of climate. Afterwards the new standard +remains unchanged for centuries. + +Selection, according to a constant standard, reaches its results in few +generations. The experience of Van Mons and other breeders of apples +shows that the limit of size and lusciousness may be soon attained. +Vilmorin's experiments with wild carrots and those of Carriere with +radishes lead to the same conclusion as regards roots. Improvements of +flowers in [807] size and color are usually easy and rapid in the +beginning, but an impassable limit is soon reached. Numerous other +instances could be given. + +Contrasted with these simple cases is the method of selecting sugar +beets. More than once I have alluded to this splendid example of the +influence of man upon domestic races, and tried to point out how little +support it affords to the current scientific opinion concerning the +power of natural selection. For this reason it is interesting to see how +a gradual development of the methods of selection has been, from the +very outset, one of the chief aims of the breeders. None of them doubts +that an improvement of the method alone is adequate to obtain results. +This result, in the main, is the securing of a few percent more of +sugar, a change hardly comparable with that progress in evolution, which +our theories are destined to explain. + +Vilmorin's original method was a very simple one. Polarization was still +undiscovered in his time. He determined the specific weight of his +beets, either by weighing them as a whole, or by using a piece cut from +the base of the roots and deprived of its bark, in order to test only +the sugar tissues. The pieces were floated in solutions of salt, which +were diluted until the pieces [808] began to sink. Their specific weight +at that moment was determined and considered to be a measure of the +corresponding value of the beet. This principle was afterwards improved +in two ways. The first was a selection after the salt solution method, +but performed on a large scale. After some few determinations, a +solution was made of such strength as to allow the greater number of the +beets to float, and only the best to sink down. In large vessels +thousands of beets could be tested in this way, to select a few of the +very heaviest. The other improvement was the determination of the +specific weight of the sap, pressed out from the tissue. It was more +tedious and more expensive, but more direct, as the influence of the air +cavities of the tissue was excluded. It prepared the way for +polarization. + +This was introduced about the year 1874 in Germany, and soon became +generally accepted. It allowed the amount of sugar to be measured +directly, and with but slight trouble. Thousands of beets could be +tested yearly by this method, and the best selected for the production +of seed. In some factories a standard percentage is determined by +previous inquiries, and the mass of the beets is tested only by it. In +others the methods of taking samples and clearing the sap have been +improved so far as to allow the [809] exact determination of three +hundred thousand polarization values of beets within a few weeks. Such +figures give the richest material for statistical studies, and at once +indicate the best roots, while they enable the breeder to change his +standard in accordance with the results at any time. Furthermore they +allow the mass of the beets to be divided into groups of different +quality, and to produce, besides the seeds for the continuation of the +race, a first class and second-class product and so on. In the factory +of Messrs. Kuhn & Co., at Naarden, Holland, the grinding machine has +been markedly improved, so as to tear all cell walls asunder, open all +cells, and secure the whole of the sap within less than a minute, and +without heating. + +It would take too long to go into further details, or to describe the +simultaneous changes that have been applied to the culture of the elite +strains. The detailed features suffice to show that the chief care of +the breeder in this case is a continuous amelioration of the method of +selecting. It is manifest that the progression of the race is in the +main due to great technical improvements, and not solely to the +repetition of the selection. + +Similar facts may be seen on all the great lines of industrial +selection. An increasing appreciation [810] of all the qualities of the +selected plants is the common feature. Morphological characters, and the +capacity of yielding the desired products, are the first points that +strike the breeder. The relation to climate and the dependence on manure +soon follow; but the physiological and chemical sides of the problem are +usually slow of recognition in the methods of selection. When visiting +Mr. de Vilmorin at Paris some years ago, I inspected his laboratory for +the selection of potatoes. In the method in use, the tubers were rubbed +to pulp and the starch was extracted and measured. A starch percentage +figure was determined for each plant, and the selection of the tubers +for planting was founded upon this result. In the same way wheat has +been selected by Dippe at Quedlinburg, first by a determination of its +nitrogenous contents in general, and secondly by the amount of the +substances which determine its value for baking purposes. + +The celebrated rye of Schlanstedt was produced by the late Mr. Rimpau in +a similar manner and was put on the market between 1880 and 1890 and was +received with great favor throughout central Europe, especially in +Germany and in France. It is a tall variety, with vigorous stems and +very long heads, the kernels of which are nearly double the size of +those of the [811] ordinary rye, and are seen protruding, when ripe, +from between the scales of the spikelets. It is unfit for poor soils, +but is one of the very best varieties for soils of medium fertility in a +temperate climate. It is equal in the production of grain to the best +French sorts, but far surpassing them in its amount of straw. It was +perfected at the farm of Schlanstedt very slowly, according to the +current conceptions of the period. The experiment was started in the +year 1866, at which time Rimpau collected the most beautiful heads from +among his fields, and sowed their kernels in his experiment garden. From +this first culture the whole race was derived. Every year the best ears +of the strain were chosen for repeated culture, under experimental care, +while the remainder was multiplied in a field to furnish the seeds for +large and continually increasing areas of his farms. + +Two or three years were required to produce the quantity of seed of each +kind required for all the fields of Schlanstedt. The experiment garden, +which through the kindness of Mr. Rimpau I had the good fortune of +visiting more than once between 1875 and 1878, was situated in the +middle of his farm, at some distance from the dwellings. Of course it +was treated with more care, and especially kept [812] in better +conditions of fertility than was possible for the fields at large. A +continued study of the qualities and exigencies of the elite plants +accompanied this selection, and gave the means of gradually increasing +the standard. Resistance against disease was observed and other +qualities were ameliorated in the same manner. Mr. Rimpau repeatedly +told me that he was most anxious not to overlook any single character, +because he feared that if any of them might become selected in the wrong +way, perchance unconsciously, the whole strain might suffer to such a +degree as to make all the other ameliorations quite useless. With this +purpose the number of plants per acre was kept nearly the same as those +in the fields, and the size of the culture was large enough every year +to include the best kernels of quite a number of heads. These were never +separated, and exact individual pedigrees were not included in the plan. +This mixture seemed to have the advantage of keeping up an average value +of the larger number of the characters, which either from their nature +or from their apparent unimportance had necessarily to be neglected. + +After ten years of continuous labor, the rye of Rimpau caught the +attention of his neighbors, being manifestly better than that of +ordinary [813] sowings. Originally he had made his cultures for the +improvement of his own fields only. Gradually however, he began to sell +his product as seed to others, though he found the difference still very +slight. After ten years more, about 1886, he was able to sell all his +rye as seed, thereby making of course large profits. It is now +acknowledged as one of the best sorts, though in his last letter Mr. +Rimpau announced to me that the profits began to decline as other +selected varieties of rye became known. The limit of productiveness was +reached, and to surmount this, selection had to be begun again from some +new and better starting point. + +This new starting point invokes quite another principle of selection, a +principle which threatens to make the contrast between artificial and +natural selection still greater. In fact it is nothing new, being in use +formerly in the selection of domestic animals, and having been applied +by Vilmorin to his sugar beets more than half a century ago. Why it +should ever have been overlooked and neglected in the selection of sugar +beets now is not clear. + + The principle in itself is very simple. It agrees that the visible + characters of an animal or a plant are only an imperfect measure for + its hereditary qualities, instead of being the real criterion to be + relied upon, as is the current belief. [814] It further reasons that a + direct appreciation of the capacity of inheritance can only be derived + from the observation of the inheritance itself. Hence it concludes that + the average value of the offspring is the only real standard by which + to judge the representatives of a race and to found selection upon. + +These statements are so directly opposed to views prevalent among plant +breeders, that it seems necessary to deal with them from the theoretical +and experimental, as well as from the practical side. + +The theoretical arguments rest on the division of the fluctuating +variability into the two large classes of individual or embryonic, and +of partial deviations. We have dealt with this division at some length +in the previous lecture. It will be apparent at once, if we choose a +definite example. Let us ask what is the real significance of the +percentage figure of a single plant in sugar beets. This value depends +in the first place, on the strain or family from which the beet has been +derived, but this primary point may be neglected here, because it is the +same for all the beets of any lot, and determines the average, around +which all are fluctuating. + +The deviation of the percentage figure of a single beet depends on two +main groups of external [815] causes. First come those that have +influenced the young germs of the plant during its most sensitive +period, when still an embryo within the ripening seed. They give a new +limitation to the average condition, which once and forever becomes +fixed for this special individual. In the second place the young +seedling is affected during the development of its crown of leaves, and +of its roots, by numerous factors, which cannot change this average, but +may induce deviations from it, increasing or decreasing the amount of +sugar, which will eventually be laid down in the root. The best young +beet may be injured in many ways during periods of its lifetime, and +produce less sugar than could reasonably be expected from it. It may be +surpassed by beets of inferior constitution, but growing under more +favorable circumstances. + +Considered from this point of view the result of the polarization test +is not a single value, but consists of at least two different factors. +It may be equal to the algebraic sum of these, or to their difference, +according to whether the external conditions on the field were locally +and individually favorable or unfavorable. A large amount of sugar may +be due to high individual value, with slight subsequent deviation from +it, [816] or to a less prominent character combined with an extreme +subordinate deviation. + +Hence it is manifest that even the results of such a highly improved +technical method do not deserve the confidence usually put in them. They +are open to doubt, and the highest figures do not really indicate the +best representatives of the race. In order to convey this conception to +you in a still stronger manner, let us consider the partial variability +as it usually shows itself. The various leaves of a plant may noticeably +vary in size, the flowers in color, the fruits in flavor. They fluctuate +around an average, which is assumed to represent the approximate value +of the whole plant. But if we were allowed to measure only one leaf, or +to estimate only one flower or fruit, and be compelled to conclude from +it the worth of the whole plant, what mistakes we could make! We might +indeed hit upon an average case, but we might as easily get an extreme, +either in the way of increase or of decrease. In both cases our judgment +would be badly founded. Now who can assure us that the single root of a +given beet is an average representative of the partial variability? The +fact that there is only one main root does not prove anything. An annual +plant has only one stem, but a perennial species has many. The average +height of the last is a [817] reliable character, but the casual height +of the former is very uncertain. + +So it is with the beets. A beet may be divided by its buds and give +quite a number of roots, belonging to the same individual. These +secondary roots have been tested for the amount of sugar, and found to +exhibit a manifest degree of variability. If the first root corresponded +to their average, it might be considered as reliable, but if not anyone +will grant that an average is more reliable than a single determination. +Deviations have as a fact been observed, proving the validity of our +assertion. These considerations at once explain the disappointment so +often experienced by breeders. Some facts may be quoted from the Belgian +professor of agriculture at Gembloux, the late Mr. Laurent. He selected +two beets, from a strain, with the exceptional amount of 23% sugar, but +kept their offspring separate and analyzed some 60 of each. In both +groups the average was only 11-12%, the extremes not surpassing 14-15%. +Evidently the choice was a bad one, notwithstanding the high +polarization value of the parent. Analogous cases are often observed, +and my countrymen, Messrs. Kuhn & Co., go so far as to doubt all +excessive variants, and to prefer beets with high, but less +extraordinary percentages. Such are to be had in larger numbers [818] +and their average has a good chance of exemption from a considerable +portion of the doubts adhering to single excessive cases. + +It is curious to note here what Louis de Vilmorin taught concerning this +point in the year 1850. I quote his own words: "I have observed that in +experiments on heredity it is necessary to individualize as much as +possible. So I have taken to the habit of saving and sowing separately +the seeds of every individual beet, and I have always found that among +the chosen parent plants some had an offspring with a better average +yield than others. At the end I have come to consider this character +only, as a standard for amelioration." + +The words are clear and their author is the originator of the whole +method of plant breeding selection. Yet the principle has been +abandoned, and nearly forgotten under the impression that polarization +alone was the supreme guide to be relied upon. However, if I understand +the signs rightly, the time is soon coming when Vilmorin's experience +will become once more the foundation for progress in breeding. + +Leaving the theoretical and historical aspects of the problem, we will +now recall the experimental evidence, given in a former lecture, dealing +with the inheritance of monstrosities. I have shown that in many +instances monstrosities [819] constitute double races, consisting of +monstrous and of normal individuals. At first sight one might be induced +to surmise that the monstrous ones are the true representatives of the +race, and that their seeds should be exclusively sown, in order to keep +the strain up to its normal standard. One might even suppose that the +normal individuals, or the so-called atavists, had really reverted to +the original type of the species and that their progeny would remain +true to this. + +My experiments, however, have shown that quite the contrary is the case. +No doubt, the seeds of the monstrous specimens are trustworthy, but the +seeds of the atavists are not less so. Fasciated hawkweeds and twisted +teasels gave the same average constitution of the offspring from highly +monstrous, and from apparently wholly normal individuals. In other words +the fullest development of the visible characteristic was not in the +slightest degree an indication of better hereditary tendencies. In +unfavorable years a whole generation of a fasciated race may exhibit +exclusively normal plants, without transmitting a trace of this +deficiency to the following generation. As soon as the suitable +conditions return, the monstrosity reassumes its full development. The +accordance of these facts with the experience [820] of breeders of +domestic animals, and of Louis de Vilmorin, and with the result of the +theoretical considerations concerning the factors of fluctuation has led +me to suggest the method of selecting, which I have made use of in my +experiments with tricotyls and syncotyls. + +Seedling variations afford a means of counting many hundreds of +individuals in a single germinating pan. If seed from one parent plant +is sown only in each pan, a percentage figure for the amount of +deviating seedlings may be obtained. These figures we have called the +hereditary percentages. I have been able to select the parent plants +after their death on the sole ground of these values. And the result has +been that from varieties which, on an average, exhibited 50-55% +deviating seedlings, after one or two years of selection this proportion +in the offspring was brought up to about 90% in most of the cases. +_Phacelia_ and mercury with tricotylous seedlings, and the Russian +sunflower with connate seed leaves, may be cited as instances. + +Besides these tests, others were performed, based only on the visible +characters of the seedlings. The result was that this characteristic was +almost useless as a criterion. The atavists gave, in the main, nearly +the same hereditary percentages as the tricotyls and syncotyls, and +[821] their extremes were in each case far better constituted than the +average of the chosen type. Hence, for selection purposes, the atavists +must be considered to be in no way inferior to the typical specimens. + +If it had been possible to apply this principle to twisted and fasciated +plants, and perhaps even to other monstrosities, I think that it will +readily be granted that the chance of bringing even these races up to a +percentage of 90% would have been large enough. But the large size of +the cultures required for the counting of numerous groups of offspring +in the adult state has deterred me from making such trials. Recently +however, I have discovered a species, _Viscaria oculata_ which allows of +counting twisted specimens in the pans, and I may soon be able to obtain +proofs of this assertion. The validity of the hereditary percentage as a +standard of selection has, within the last few years, been recognized +and defended by two eminent breeders, W.A. Hays in this country and Von +Lochow in Germany. Both of them have started from the experience of +breeders of domestic animals. Von Lochow applied the principle to rye. +He first showed how fallacious the visible characters often are. For +instance the size of the kernels is often dependent on their number in +the head, and if this number is [822] reduced by the injurious varietal +mark of lacunae (Luckigkeit), the whole harvest will rapidly deteriorate +by the selection of the largest kernels from varieties which are not +quite free from this hereditary deficiency. + +In order to estimate the value of his rye plants, he gathers the seed of +each one separately and sows them in rows. Each row corresponds to a +parent plant and receives 200 or 150 seeds, according to the available +quantity. In this way from 700 to 800 parent plants are tested yearly. +Each row is harvested separately. The number of plants gives the average +measure of resistance to frost, this being the only important cause of +loss. Then the yield in grain and straw is determined and calculated, +and other qualities are taken into consideration. Finally one or more +groups stand prominent above all others and are chosen for the +continuation of the race. All other groups are wholly excluded from the +"elite," but among them the best groups and the very best individuals +from lesser groups are considered adequate for further cultivation, in +order to produce the commercial product of the race. + +As a matter of fact the rye of Von Lochow is now one of the best +varieties, and even surpasses the celebrated variety of Schlanstedt. It +was only after obtaining proof of the validity [823] of his method that +Von Lochow decided to give it to the public. + +W.M. Hays has made experiments with wheat at the Minnesota Agricultural +Experiment Station. He chose a hundred grains as a proper number for the +appreciation of each parent plant, and hence has adopted the name of +"centgener power" for the hereditary percentage. + +The average of the hundred offspring is the standard to judge the parent +by. Experience shows at once that this average is not at all +proportional to the visible qualities of the parent. Hence the +conclusion that the yield of the parent plant is a very uncertain +indication of its value as a parent for the succeeding generation. Only +the parents with the largest power in the centgener of offspring are +chosen, while all others are wholly discarded. Afterwards the seeds of +the chosen groups are propagated in the field until the required +quantities of seed are obtained. + +This centgener power, or breeding ability, is tested and compared for +the various parent plants as to yield, grade, and percentage of +nitrogenous content in the grain, and as to the ability of the plant to +stand erect, resist rust, and other important qualities. It is evident +that by this test of a hundred specimens a far better [824] and much +more reliable determination can be made than on the ground of the +minutest examination of one single plant. From this point of view the +method of Hays commands attention. But the chief advantage lies in the +fact that it is a direct proof of that which it is desired to prove, +while the visible marks give only very indirect information. + +Thus the results of the men of practice are in full accordance with +those of theory and scientific experiment, and there can be little doubt +that they open the way for a rapid and important improvement. Once +attained, progress however, will be dependent on the selection +principle, and the hereditary percentage, or centgener power or breeding +ability, must be determined in each generation anew. Without this the +race would soon regress to its former condition. + +To return to our starting point, the comparison of artificial and +natural selection. Here we are at once struck by the fact that it is +hardly imaginable, how nature can make use of this principle. In some +measure the members of the best centgener will manifestly be at an +advantage, because they contain more fit specimens than the other +groups. But the struggle for existence goes on between individuals, and +not between groups of brethren against groups of [825] cousins. In every +group the best adapted individuals will survive, and soon the breeding +differences between the parents must vanish altogether. Manifestly they +can, as a rule, have no lasting result on the issue of the struggle far +existence. + +If now we remember that in Darwin's time this principle, breeding +ability, enjoyed a far more general appreciation than at present, and +that Darwin must have given it full consideration, it becomes at once +clear that this old, but recently revived principle, is not adequate to +support the current comparison between artificial and natural selection. + +In conclusion, summing up all our arguments, we may state that there is +a broad analogy between breeding selection in the widest sense of the +word, including variety testing, race improvement and the trial of the +breeding ability on one side, and natural selection on the other. This +analogy however, points to the importance of the selection between +elementary species, and the very subordinate role of intraspecific +selection in nature. It strongly supports our view of the origin of +species by mutation instead of continuous selection. Or, to put it in +the terms chosen lately by Mr. Arthur Harris in a friendly criticism of +my views: "Natural selection may explain the survival [826] of the +fittest, but it cannot explain the arrival of the fittest." + + + +A + +_Abies concolor fastigiata_, 618 +_Acacia_, 176, 196, 217, 458, 697 + bastard, 343, 617, 618, 664, 665, 666 +_Acer compestre nanum_, 612 +_Achillea millefolium_, 131, 132, 441 +Adaptation, 702 + double, 430, 451, 452, 454, 455, 457, 458, 642 +_Aegilops ovata_, 265 + _speltaeformis_, 265 +_Agave vivipara_, 684 +_Ageratum coeruleum_, 612 +_Agrostemma Coronaries bicolor_, 125 + _Githago_, 282 + _nicaeensis_, 162 +_Agrotis_, 204 +Alder, cut-leaved, 147, 596 +Alfalfa, 264 +Algae, 699 +Allen, Grant, 237 +_Alliaria_, 638 +_Alnus glutinosa laciniata_, 615 +Alpine plants, 437, 695, 794 +_Althaea_, 490 +Amaranth, 282, 452 +_Amaranthus caudatus_, 282 +_Amaryllis_, 272, 275, 762 + brasiliensis_, 275 + leopoldi_, 275 + pardina_, 275 + psittacina_, 275 + vittata_, 275 +Amen-Hotep, 697 +_Ampelopsis_, 239 +_Amygdalus persica laevis_, 126 +_Anagallis arvensis_, 162 +_Androsace_, 634 +_Anemone_, 266, 331 + _coronaria_, 241, 491 + var. "Bride," 510 + _magellanica_, 266 + _sylvestris_, 266 +_Anemone_, garden, 241 +Annee, 760 +Anomalies, taxonomic, 658, 685 +_Anthemis_, 236 + _nobilis_, 130 +_Anthurium scherzerianum_, 639 +_Antirrhinum majus_, 315 + _luteum rubro-striatum_, 315 +Apetalous flowers, 622 +Apples, 134, 240, 328, 454, 806 + elementary species, 75 + method of cultivating, 76 + origin of cultivated varieties, 73 + use by the Romans, 74 + "Wealthy," 78, 79 + wild, 73, 74, 75, 76 +_Aquilegia chrysantha_, 161 +_Arabis ciliata glabrata_ + _hirsuta glaberrima_, 126 +_Aralia crassifolia_, 662 +Arbres fruitiers ou Pomonomie belge, 76 +_Aralia papyrifera_, 662 +Arctic flora, 695 +_Arnica_, 494 + _montana_, 236 +Aroids, 222, 631, 639 +Artemisias, 131 +Artificial selection, 18, 71, 77, 93, 95, 743, 744, 798, 826 + first employed, 72, 92 + nature of, 19 +_Arum maculatum immaculatum_, 125 +Ascidia, 310, 366, 367, 427, 428, 669, 670, 671, 672, 673, + 674, 675 +Ash, 135, 341 + one-bladed, 666, 667 + weeping, 196, 596 +Ashe, 343 +Aster, 132, 152, 242 + seashore, 200, 282 +_Aster Tripolium_, 132, 200, 236, 282, 410 +_Astragalus alpinus_, 696 +Atavism, 154, 170, 172, 175, 176, 178, 182, 185, 187, 188, + 198, 220, 222, 226, 235, 344, 354, 399, 405, 411, + 660, 661 + bud, 183, 226 + definition of, 170, 631 + false, 185, 187 + negative, 344 + positive, 344 + seed, 176 + systematic, 174, 222, 630-657 +Atavists, 156, 201 + heredity of, 412 +_Atropa Belladonna lutea_, 592 +_Aubretia_, 241 +_Avena fatua_, 100, 207 +_Azalea_, 178, 322 +_Azolla caroliniana_, 239 + +B + +Babington, _Manual of British Botany_, 36, +Bailey, 78, 306, 684 +Balsams, 334 +Bananas, 90, 134 +Banyan, 244 +Barberry, 133, 180 + European, 270 + purple, 596 +_Barbarea vulgaris_, 427 +Barley, 98, 105, 133, 203, 678, 679 + "Nepaul," 203, 676, 677, 679, 681, 682 +Bastard-acacia, 133, 136, 140 +Bateson, 250 +Bauhin, Caspar, 72, 610 +Baumann, 618 +Beans, 90, 152, 327, 727, 735 +Bedstraw, 648 +Beech, 133, 135, 242 + cut-leaved, 179, 196, 616 + laciniated, 196 + oak-leaved, 595 + purple, 196, 593, 595 +Beeches, 427 + fern-leaved, 147 +Beets, 68, 72, 92, 93, 792, 796, 801, 815, 817, 818 + Californian, 796 + European, 796 + forage, 71, 72, 791 + salad, 71 +Beet-sugar, 67, 68, 69, 70, 71, 109, 165, 717, 791, 807, + 813, 814 +_Begonia_, 218, 366, 509, 765 + ever-flowering, 148 + tuberous, 272 + _clarkii_, 272 + _davisii_, 272 + _rosiflora_, 272 + _sedeni_, 273 + _semperflorens_, 133, 148, 620 +_Begonia_ + bulbous, 372 + _veitchi_, 272 +Behrens, 804 +Belladonna, 145 +_Bellis perennis_, 236 + _perennis plena_, 195 +Bentham, 237 +Bentham & Hooker, _Handbook of British Flora_, 36 +_Berberis_, 133, 180, 455 + _ilicifolia_, 270 + _vulgaris_, 270 +Bertin, 596 +_Berula angustifolia_, 457 +Bessey, 660 +_Beta maritima_, 69 + _patula_, 69, 70 + _vulgaris_, 69, 70 +_Betula_, 132 +Between-race, 358 +Bewirkung, Theorie der directen (Nageli), 448 +_Biastrepsis_, 402 +_Bidens_, 131 + _atropurpurea_, 131 + _cernua_, 131, 158 + _leucantha_, 131 + _tripartite_, 131 +Bilberries, 577 +Bindweed, 41924 +Binomium, of Newton, 767 +Birch, 133, 243 + cut-leaved, 596, 616 + fastigiate, 618 + fern-leaved, 179 +_Bisoutella_, 282 + _laevigata glabra_, 125 +Bitter-sweet, 125 +Blackberry, 268, 768 + "Paradox," 769 +Blue-bells, variation in, 54, 491, 577 +Blueberries, 769 +Blue-bottle, 499, 507, 509, 510 +Blueflag, atavism of, 172 +_Boehmeria_, 675 + _bilboa_, 685 +Bonnier, 439, 441, 442, 444, 451, 795 +Boreau, 663 +Brambles, 126, 127, 147, 239, 244, 245, 268, 740, + 769, 663 +_Brassica_, 244 +Braun, 738 +Braun and Schimper, 494 +Bread-fruits, 90 +Briot, 618 +Britton and Brown's Flora, 162 +Brooks, 711 +Broom, 140 + prickly, 217 +Broom-rape, 220 +_Broussonetia papyifera dissecta_, 616 +_Brunella_, 146, 268 + _vulgaris_, 577 + _vulgaris alba_, 201 +_Bryophyllum calycinum_, 218 +Buckwheat, 452 +Bud-variation, 750 +Buds, adventitious, 218 +Burbank, Luther, 57, 79, 116, 134, 268, 758, 768, + 769, 784 +Buttercup, 331, 357, 410, 725, 740 + Asiatic, 241 + +C + +Cabbages, 428, 684 + atavism in, 638 + origin of varieties, 621 +Cactuses, 444 +Cactus-dahlia, 625 +_Calamintha Acinos_, 437, 452 +Calamus root, 222 +_Calendula officinalis_, 502 +_Calliopsis tinctoria_, 195 +_Calluna_, 146 + _vulgaris_, 437, 577 +_Caltha_, 490 + _palustris_, 331 +_Camelina_, 684 +_Camellia_, 178, 323 + _japonica_, 368 +Camellias, 331 +Camomile, 130, 132, 156, 366, 494, 503, 509, 512 +_Campanula persicifolia_, 151, 234 + _rotundifolia_, 437 +Campion, 283, 302, 304 + evening, 281 + red, 238 +_Canna_, 751, 759, 761 + _indica_, 760 + "Madame Crozy," 760, 761 + _nepalensis_, 760 + _warczewiczii_, 760 +_Capsella Bursa-pastoris apetala_, 585 + _heegeri_, 22, 582, 583, 684 +_Carex_, 53 +Carnation, 178, 241, 491 + wheat-ear, 227 +_Carpinus Betulus heterophylla_, 180 +Carriere, 491, 596, 612, 806 +Carrots, 806 +Catch-fly, 419 +Carboniferous period, 699 +_Casuarina quadrivalvis_, 649 +Cauliflowers, origin of, 621 +Caumzet, 614 +Causation, theory of direct, (Nageli), 448 +Cedar, pyramidal, 618 +Celandine, 147, 245, 280, 365 + oak-leaved, 603, 610, 611 +_Celosia_, 621 +_Celosia cristata_, 327, 411 +_Centaurea_, 242 +Centgener power, 20, 822 +_Centranthus macrosiphon_, 424 +_Cephalotaxus_, 170, 226 + _pedunculata fastigiata_, 169 +Cereals, 105, 106, 107, 119, 801, 804 + origin of cultivation, 104 +Character-units, 632 +Charlock, 424 +_Cheiranthus_, 490 +_Cheiri_, 370 +_Cheiri gynantherus_, 371 +_Chelidonium laciniatum_, 22, 609 + _majus_, 147, 365, 600, 610, 611 + _majus foliis quernis_, 610 +Cherries, 79 +Cherry, bird's, 617 +Chestnuts, 427 +Chromosomes, 306 +_Chrysanthemum_, 178, 274 + corn, 739 +_Chrysanthemum carinatum_, 494 + _coronarium_, 161, 202, 510 + _grandiflorum_, 739 + _imbricatum_, 494 + _indicum_, 490 + _inodorum_, 503 + _inodorum plenissimum_, 336 + new double, 501 + _segetum_, 202, 493, 504, 729 + _segetum_, var. _grandiflorum_, 43, 495, 498, 504, + 504 +_Chrysopogon montanus_, 450 +Cieslar, 804 +_Cineraria cruenta_, 514 +Cinquefoil, 52 +_Clarkia_, 420 + _elegans_, 198 + _pulchella_, 282 + _pulchella carnea_, 162 +_Clematis Vitalba_, 662 + _Viticella nana_, 612 +Clover, 80, 102, 674 + crimson (Italian), 353, 358, 359, 360 + five-leaved, 340, 362, 374, 431, 509, 789 + four-leaved, 340, 346, 352 + red, 235, 281 + white, 133, 366 +Clusius, 610 +_Cochlearia anglica_, 52 + _danica_, 52 + _officinalis_, 52 +Coconut, 67, 82, 83, 87, 88, 89 + dispersal of, 85, 89 + geographic origin of, 88,89 +Coconut-palm, 84, 88 +Cockerell, T.D.A., 139, 140, 591 +Cocklebur, 139 +Cockscomb, 165, 327, 356, 411, 621 +_Cocos nucifera stupposa_, 83, 84 + _cupuliformis_, 82 + _rutila_, 82 +_Codiaeum appendicularum_, 673 +_Colchicum_, 490 +_Coleus_, 132 +Columbine, 725 + yellow, 161 +Columbus, 89, 118 +Columella, 106 +Composites, 130, 131, 336, 723, 778 +Conifers, 168, 226, 239, 455 + weeping, 617 +Connation, of petals, 660, 661 +"Conquests," 242 +Contra-selection, 425 +Cook, 84, 86, 88, 89 +Corn, 81, 90, 118, 119, 135, 283, 287, 288, 775, + 786, 788, 804 + American, 205 +Corn-cockle, 162 +Corn-chrysanthemum, 739 +Corn-flowers, 491, 92 +Corn, "Forty-day," 118 + "Harlequin," 327 + sterile variety of, 622 + sugar, 135, 158 + "Tuscarora," 205 +Corn-marigold, 493, 494 +Cornel berry, yellow, 196 +Cornaceae, 675 +_Cornu_, 338 +_Cornus Mas_, 196 +Correlation, 142 +_Corylus_, 133 + _Avellana_, 181 + _tubulosa_, 181 +Cotton, 725 +Cotyledon, 674 + variation in, 416 +_Crambe maritima_, 621 +Cranesbill, 599 + European, 628 + meadow, 322 +_Crataegus_, 196 + _oxyacantha_, 132 +Crowfoot, 331 + corn, 283 +_Crepis biennis_, 410, 411 +Cress, Indian, 192 +Crosses + bisexual, 255, 276, 294, 298 + reciprocal, 279 + unisexual, 255, 261 + varietal (see Hybrids) +_Croton_, 673, 674 +Crozy, 760, 762 +Crucifers, 222, 635 +_Cryptomeria_, 169, 226 + _japonica_, 239 +Cucumbers, 118 +_Cucumis_, 52 +_Cucurbita_, 52 +Cultivated plants, 65, 66 + elementary species of, 62 + improvement of, 92 + mixed nature of, 96, 118 + origin of, 91 +Currants, 79 + Californian, 270 + flowering, 166 + "Gordon's," 270 + Missouri, 270 + white, 158 + white-flowered, 167 +Cuttings, 721 +_Cyclamen_, 323, 355, 627, 684 + Butterfly, 627 + _vernum_, 619 +_Cypripedium caudatum_, 487 +_Cytisus adami_, 271 + _candicans Attleyanus_, 367 + _Laburnum_, 271 + _prostratus_, 139 + _prostratus ciliata_, 125 + _purpureus_, 271 + _spinescens_, 139 + +D + +_Dahlia_, 131, 241, 272, 625 + cactus, 625 + "Jules Chretien," 628 + purple-leaved, 626 + "surprise," 230 + tubular, 627 + [sic] 274, 490, 764 + first double ones, 490 + green, 227, 229, 230 +Daisies, 131, 132, 494 + double, 195 + hen-and-chicken, 514 + ox-eye, 202 +Shasta, 769 + yellow, 202 +Dandelion, 411 + parthenogenesis, 61 + variations in, 60 +Daphne Mezereum, 146 +Darwin, 1, 2, 3, 4, 5, 6, 7, 18, 76, 85, 93, 109, + 110, 180, 196, 205, 206, 242, 306, 324, 338, + 448, 571, 604, 612, 689, 702, 710, 715, 743, + 798, 825 +Darwin, George, 711 +Darwinian theory, 461 + basis of, 5 +Date, 134 +_Datura Stramonium_, 139, 142 + _Stramonium inermis_, 300 + _Tatula_, 139, 142, 300 +Dead-nettle, 237 +De Bary, 38, 47, 49 +De Candolle, 76, 84, 85, 89, 228, 370, 403, 621 + Alphonse, 74, 129, 226 + A.P., 129 + Casimir, 659, 676 +De Graaff, 275 +_Delphinium Ajacis_, 192 +Deniau, 617 +Descent, theory of, 690, 694, 702, 707, 716, 798 +De Serres, Olivier, 72 +_Desmodium gyrans_, 655, 656, 663, 664, 65 +Dewberry, California, 269 +_Dianthus barbatus_, 322, 648 + twisted variety, 408 +Diatoms, 699 +Dictoyledons + ancestors of monocotyledons, 15 +_Digitalis parviflora_, 161, 640 + _purpurea_, 483 + pelorism of, 482 +Dimorphism, 445, 447, 454, 457, 458 +Dippe, 810 +_Dipsacus fullonum_, 402 + sylvestris_, 402, 402 +Dominant character, 280 +Double flowers + poppies 490 + production of, 489 + types of, 330 +Double races (see also ever-sporting varieties), + 419, 427, 428 +Dubois, Eugene, 712 +Duchesne, 185, 188, 596 +Duckweed, 222 +_Draba_, 692, 693 + verna, 47, 50, 51, 53, 125, 126, 518, 533, + 546, 547, 561 +_Dracocephalum moldavicum_, 419 +Dragon-head, 419 +_Drosera anglica_, 268 + _filiformis_, 268 + _intermedia_, 268 + _obovata_, 267 + _rotundifolia_, 268 + +E + +Earth, age of, 710 +Edelweiss, 438 +Eichler, 660 +Election, 801 +Electric light, growth in, 442 +Elementary species, 11, 13, 32, 67, 74, 76, 77, + 78, 79, 91, 95, 116, 119, 124, 126, 128, 129, + 207, 238, 252, 256, 307, 430, 435, 695, 696, + 698, 702, 715, 787, 798, 800, 825 + apples, 75 + coconut, 82 + corn, 81 + cultivated plants, 62 + definition of, 12, 35, 127 + flax, 80 + how produced, 16, 248 + hybrids of, 253, 255 + mutation of, 141 + origin of, 459, 603 + origin of, how studied, 463 + selection of, 92 + varieties vs., 14, 15, 141, 152, 224, 243, + 247, 251, 495 +Elm, 136, 219, 239, 427 +_Epilobium_, 268 + _hirsutum_, 683 + _hirsutum cruciatum_, 588 + _montanum_, 269 + _tetragonum_, 269 +_Equisetum Telmateja_, 642, 649 +_Erica Tetralix_, 577, 661 +Ericaceae, 146, 660 +_Erigeron _Asteroides_, 450 + _canadensis_, 132, 236, 453, 600, 695 +_Erodium_, 146 + _cicutarium album_, 161 +_Erucastrum_, 630, 638, 639 + _pollichii_, 222, 637 +_Eryngium campestre_, 674 + _maritimum_, 674 +_Erysimum cheiranthoides_, 638 +_Erythraea pulchella_, 452 +_Erythrina_, 621 + _Crista-galli_, 620 +Eschcholtzias, 59 +Esimpler, 337 +_Eucalyptus citriodora_, 669 + _Globulus_, 217 +_Euphorbia Ipecacuanha_, 55 +Evening-primrose, 62, 204, 256, 424, 686, 687, + 688, 690, 691, 694, 695, 699, 702, 703, + 705, 707, 708, 713, 747, 793 +Evolution, 93, 685, 686, 689, 704, 707, 709, + 710, 713, 718 + degressive, 222, 223, 249 + progression in, 630 + progressive, 221, 222, 223, 248 + regression in, 630 + regressive, 221, 222; 223, 24 + retrograde, 221, 631 +Extremes, asexual multiplication of, 742, 769 + +F + +Fabre, 265 +_Fagus_, 133 +_Fagus sylvatica pectinata_, 179 +Fan, genealogical, 700 +Fasciated stems, 409, 412 +Ferns, 63 + cristate, 427 + plumose, 427 +_Ficaria_, 53 +_Ficus radicans_, 436 + _religiosus_, 244 + _repens_, 436 + _stipulata_, 436 + _ulmifolia_, 436 +Figs, 436 +_Filago_, 52 +Fir, 134, 804 +Fittest, survival of, 826 +Flax, 80, 805 + springing, 80 + threshing, 80 + white-flowered, 158, 160 +Fleabane, Canada, 132, 236 +Flowers, gamopetalous, 660 +Fluctuability + embryonic, see Fluctuation, individual +Fluctuation, 708, 715, 716, 718, 719, 724, 737, 741 + curves of, 729, 794 + defined, 191 + individual, 718, 723, 732, 741, 745, 749, 788 + mutation vs. 7, 16, 719 + partial, 718, 723, 732, 741, 745, 748, 749, + 771 + inadequate for evolution, in elementary species, + 19 + nature of, 18 + specific and varietal characters vs. 17 +Forget-me-not, 368 +Fothergill, John, 521 +Foxglove, 163 + peloric, 164, 356, 367 + yellow, 161, 640 +_Fraxinus excelsior monophylla_, 667 + _exheterophylla_, 667 + _simplici folio_, 667 +French flora (Grenier and Godron), 433 +Fries + on _Hieracium_, 60 +Frostweed, 440 + species of, 52 +_Fuchsia_, 272, 355 +Fuchsias, 491 + +G + +Gaertner, 279 +_Galeopsis Ladanum canescens_, 139 +_Galium_, 648 + _Aparine_, 409, 648 + _elatum_, 52 + _erectum_, 52 + _Mollugo_, 62 + _verum_, 648 +Gallesio, 138 +Galton, 736, 776 +Gamopetaly, 662 +Garden-pansy, origin of, 38 +Garlic, 638 +Gauchery, 452 +Geikie, 711 +Genera + artificial character of, 36 + polymorphous, 692 +_Gentiana punctata concolor_, 125 +Gentians, 577 +Georgics (Vergil), 106 +_Geranium pratense_, 323, 628 + _album_, 628 + _pyreniacum_, 599 +German flora (Koth), 432 +Geum, 282 +Gherkins, 118 +Gideon, Peter M., 78 +Glacial period, 696 +_Gladiolus_, 241, 272, 274, 368, 765 + _cardinalis_, 275 + _gandavensis_, 275 + _psittacinus_, 275 + _purpureo-auratus_, 275 +_Glaucium_, 241 +_Gleditschia sinensis_, 614 + _triacanthos pendula_, 617 +_Gloxinia_, 282, 485 + erect, 626 +_Gloxinia erecta_, 485 + peloric variety, 485 +_Gnaphalium Leontopodium_, 438 +_Godetia amoena_, 161 +Godetias, 59, 232 +Godron, 265, 432 +Goeppert, 370 +Gooseberry, 79, 140, 626 + red, 133, 165, 241 +Grapes, 90, 158, 328 +Grape-hyacinth, _plumosa_, 134 +Grasses, 102, 631, 681 +Grenier, 433 +Groundsel, 132 +Growth, nutrition and, 714, 720, 722 +Guelder-rose, 134, 239 +Gum-tree, Australian, 217 +_Gypsophila paniculata_ + twisted variety, 409 + +H + +Haeckel, 707 +Half-races, 358, 372, 409, 419, 424, 427, 428 +Hall, 444 +Hallet, F.F., 109 +Harebell, 232 + peach-leaved, 234 +Harris, Arthur, 825 +Harshberger, John W., 591 + on _Euphorbia_ in New Jersey, 55 +Hawksbeard, 410, 411, 412 +Hawkweed, 411, 439, 443, 819 +Hawkweeds + seeding without fertilization, 61 +Hawthorn, white, 132 +Hays, W.M. + on individual selection, 20, 94, 95, 117, + 821, 823, 824 +Hazelnut, 133, 181, 242 +Hazels, cut-leaved, 596,-616 +Heath family, 146, 222, 660 +Heaths, origin of, 662 +Heather, 577 +_Hedera Helix arborea_, 437 +Hedgehog burweed, 140 +_Hedys_Arum_, 664 +Heeger, 582 +Heer, Oswald, 74, 105 +Heinricher, 172, 173, 174 +_Helianthemum_, 53, 125, 126, 561 + _apenninum_, 52 + _pilosum_, 52 + _polifolium_, 52 + _pulverulentum_, 52 + _vulgare_, 440 +_Helichrysum_, 420 +_Helwingia_, 678, 678, 682 + _rusciflora_, 675 +Hemp, 419 +Henbane, 282 +_Hepatica_, 322, 490 +Heredity, 731, 734, 818 + bearers of, 632 + in teasels, 642 +_Hesperis_, 241, 322 + _matronalis_, 323, 411 +_Heylandia latebrosa_, 450 +_Hibiscus Moscheutos_, 591 +_Hieracium_, 59, 439 + _alpinum_, 696 +Hildebrand, 160, 240, 241 +Hoffman, 160, 662 +Hofmeister, 160, 370, 480 +Holbein, 164, 596 +Holly, 140, 196 +Holtermann, 449, 451 +Hollyhock, 427 +Honeysuckle, 674 + ground, 443 +_Hordeum distichum_, 677 + _hexastichum_, 677, 678 + _tetrastichum_, 677 + _trifurcatum_, 676, 678 + _vulgare trifurcatum_, 203 +Hornbeam, European, 180 +Horse-chestnut, 219 + thornless, 234 +Horsetail, Canadian, 695 + European, 649 +Horsetail, family, 641 +Horse-weed, 132 + Canadian, 452 +_Hortensia_, 134, 181 +Horticulture, mutations in, 604 +Houseleek, 370, 371 +Hunneman, John, 521 +Hyacinths, 178, 322 + white, 160 +Hybrids, 58, 201, 202, 206, 250, 575 + between elementary species, 253 + constant, 263, 264, 265, 266, 267, 268, 269 + law of varietal, 716 + Mendelian, 324 + nature of, 20 + species, 256, 260 + splitting of, 210 + varietal, 208, 209, 247, 277, 278, 279, 281, + 285, 293, 294 +Hybridization, 706, 751, 752, 758, 759, 764 +_Hydrocotyle_, 668 +_Hyoscyamus niger_, 282 + _pallidus_, 283 +_Hypericum perforatum_, 725 +_Hyssopus officinalis_, 161 + +I + +_Iberis umbellata rosea_, 195 +Improved races, inconstancy of 770-797 +Indian cress, 668 + pelorism of, 485 +Indian pipe, 661 +Ipecac spurge, 55 +_Iris_, 456 + _falcifolis_, 172 + _kaempferi_, 174 + _lortetii_, 521 + _pallida_, 172 + _pallida abavia_, 681 +Isolation, 108 +Ivy, 436 + +J + +Jacob's ladder, 200, 202 +Jacques, 614 +Jacquin, 52, 632 +Jaggi, 594, 595 +Jaeger, 228, 662 +Jalappa, 165 +Janczewski, 266 +Japanese plum, 58 +_Jasminum Sambac_, 662 +Joly, 712 +Jordan, Alexis, 45, 47, 49, 50, 129 + experiments with species, 37, 40 +_Juncus effusus spiralis_, 684 +Juniper, 684 + +K + +Kapteyn, 716 +Kelvin, Lord, 720, 711 +Kerner von Marilaun, 266, 267 +Keteleer, 618 +Knight, 390, 719, 720 +Koch, 433, 667 +Koelreuter, 279 +Korshinsky, 609, 612, 614, 617, 667 +Krelage, 510, 619 +Kuhn & Co., Messrs., 801, 809, 817 + +L + +_Labiates_, 237 + pelories of, 577 +_Labiatiflorae_, pelorism of, 468 +Labrador tea, 661 +_Laburnum_, 270, 284, 342 + oak-leaved 147, 179 + pelorism of, 485 +_Lactuca_, 52 + _Scariola_, 456 +Lagasca, Mariano, 96, 97, 114 +Lamarck, 1, 447, 461, 522, 522 +Lamarckism + objections to, 449 +_Lamium album_, 237 + _maculatum_, 237 + pelorism of, 486 + _purpureum_, 237 +Larch, 804 +Larkspur, 124, 192, 311, 452 + hybrid, 213 + white, 160 +Latency, 657 + individual, 219 + specific, 246 + systematic, 219, 220, 235 + varietal, 246 +Latent characters, 216 +_Lathyrus odoratus_, 776 +_Laurea pinnatifida_, 450 +Laurel, lady's, 146 +Laurent, 802 +Leaves, cleft, 685 + variegated, 426, 431 +LeBrun, Mme., 614 +Le Couteur, 96, 97, 107, 108, 114, 115, 116, 742 +_Ledum_, 222, 661 +_Lemna_, 222 +Lemoine, 762, 762 +Lettuce, 684 + crisped, 158 + prickly, 456 +Life, struggle for, 103, 119, 120 +Lilacs, 59, 769 + double, 762 +_Lilium candidum flore pleno_, 331 + _pardalium_, 116 +Lime-tree, 355, 366, 428, 669 + fern-leaved, 147 +_Linaria_, 467, 471, 480 + _dalmatica_, 482 + _genistifolia_, 267 + _italica_, 267 + _vulgaris_, 267, 471 + _vulgaris peloria_, 464 +Lindley, 63, 129, 506 +Linnaeus, 32, 33, 129, 132, 256, 663 + on the idea of species, 11, 13 + on origin of species, 2, 34 + on primroses, 52 +_Linum angustifolium_, 80 + _crepitans_, 81 + _usitatissimum_, 80, 161 +Link, 466 +Liver-leaf, 322 +_Lobelia syphilitica_, 161 +_Lonicera etrusca_, 640 + _tartarica nana_, 614 +Lorenz, Chr., 482 +Lothelier, 454 +_Lotus corniculatus_, 442 + _corniculatus hirsutus_, 139 +London, 615, 616, 667 +Lucerne, 264 +Ludwig, 738 +Lupines, 90 +_Lychnis_, 282 + _chalcedonica_, 161 + _diurna_, 238, 578 + _preslii_, 578 + _vespertina_, 238, 281, 585 +_Lycium_, 455 +_Lycopersicum_, 655 + _grandifolium_, 654 + _latifolium_ (see _L. grandifolium_). + _solanopsis_, 854, 656 + _validum_ (see _L. solanopsis_). +Lyell, 1, 710 +_Lysimachia vulgaris_, 684 + +M + +MacDougal, D.T., 62, 575, 590 +Macfarlane, 56, 255, 268 +_Madia elegans_, 779 +_Magnolia_, 355, 366, 428, 674, 675 + _obovata_, 355, 669 +_Magnus_, 228 +_Mahonia aquifolia_, 270 +Maize, 134, 775 + "Cuzco," 152 + European, 206 + "Gracillima," 152 + "Horse-dent," 152 + "Quarantino," 118 +Mallow, 663, 684 +_Malva crispa_, 684 +Maples, laciniate, 615 +Marchant, 592 +Marigold, 131, 158 + corn, 729 + field, 503, 505, 508 + garden, 503 + Japanese, 490, 494, 495 +Marsh-marigold, 331 +Martinet, 80 +Measart, 434 +Masters, 228, 370, 372 +_Matricaria Chamomilla_, 130 + _Chamomilla discoidea_, 156 +Matricaria discoidea, D.C., 157 +May-thorn, red, 196 +_Medicago media_, 264 + _falcata_, 264 +_Melanium_, 39 +Melons, 118 +Mendel, 6, 210, 294, 296, 306, 308 +Mendel's law, 276, 293, 294, 298, 299, 300, 301, + 307, 612, 613, 616, 716 +Mendelism, 307 +Mentha, 52 +_Mercurialis annua_, 420 + _annua laciniata_, 592 +Mercury, 420, 422, 425, 820 +Methods of investigation, 21 +Metzger, 205, 206 +Milde, 38 +Milfoil, 441 +Millardet, 266 +Miller, 611 +Millet, 105 +_Mimulus_, 151 + _quinquevulnerus_, 725 +_Mimusops_, 697 +Miocene period, 698 +Miquel, 83 +_Mirabilis_, 241 + _Jalappa_, 322 +Mirbel, 615 +_Monardella macrantha_, 444 +Monstrosities, 400, 401, 445, 446, 447 +Monkey-flower, 725 +Monocotyledons + ancestry of, 1, 5 + regression in, 630 +_Monotropa_, 222, 661 +Morphologic units, 145, 152 +Monstrosities, 818 +Morgan + on mutation-theory, 9 +Morren, 244, 762 +Mountain-ash, 342 +Muller, Fritz, 775, 776, 780 +Multiplication, vegetative (see Asexual propagation) +Munting, Abraham, 164, 165, 490, 762 +Munting's drawings, 512 +Murr, 158, 236 +_Muscari comosam_, 134 +Museum d'Histoire Naturelle, Paris, 522 +Mutability vs. fluctuating variability, 568 +Mutation, 659, 674, 677, 685, 686, 694, 713, 716, + 825 + absence of intermediate steps in, 474, 480 + conditions for observing, 601 + decided within the seed, 28 + definition of, 7 + easily observed, 30 + experimental, 688 + few observations of, 8 + fluctuation vs., 7, 16, 719 + influence of on variability, 335 + iterative nature of, 476, , 703 + laws of, 556, 558, 560, 562, 564, 566, 568, + 570 + limited in time, 29 + observation of, 16 + in _Oenothera_, 521, 525, 690 + oldest known, 609 + oldest recorded, 22 + periodic, 690, 692, 694 + perodicity of, 519 + progressive, 307 + repetition of, 476 + in _Saponaria calabrica_, 612 + simultaneous, 614 + in tomato, 655 +Mutations, 141, 275, 280, 445, 449, 573, 608, 620, + 626, 678, 685, 686, 701, 704, 712, 713, 716, + 800 + artificial, 402 + chance for useful, 598 + defined, 191 + frequency of, 597 + in garden-flowers, 488 + in horticulture, 604, 706 + latent, 703 + mode of appearance, 517 + numerical proportion of, 475 + original production of, 702 + peloric, 707 + periodic, 686, 705 + progressive, 704 + retrograde, 704 + stray, 704, 705, 706 + synonyms of, 191 +Mutation-period, 714 +_Myosotis azorica_, 368 +_Myrtus communis_, 684 + +N + +Nageli, 60, 439, 443, 448, 795 +Nagelian principle, 448, 450, 451 +Natural selection, 18, 119, 120, 445, 456, 682, + 694, 703, 743, 744, 798-826 + basis, 604 + nature of, 6, 19 +Naudin, 118 +Nectarines, 137, 138, 226, 627 +Nemec, 578 +Neo-Lamarckians + principle of, 8 +Neo-Lamarckism 447 +_Nepenthes_, 671, 672, 673, 674 +Newton, 1, 732, 767 +_Nicandra_, 152 +_Nigella_, 134 +Nightshade, 298 + black, 282 +Nourishment + meaning of, 732 + variability and 771 +_Nuphar_, 268 +Nutrition and growth, 720, 722 +_Nymphaea_, 698 + +O + +Oats, 98, 100, 101, 105, 112, 113, 115, 119, 133, + 452 + "Early Angus," 115 + "Early Fellow," 115 + "Fine Fellow," 115 + "Hopetown," 112 + "Longfellow," 115 + "Make-him-rich," 112 + wild, 207, 803 +Oak, 136, 239 +_Oenothera_, 260, 262, 279, 700, 706, 708, 709 + European species, source of, 575 + mutation in, 521, 525, 585, 690, 708 + new species of, 516-546 + _albida_, 537, 553, 555, 563, 565, 573 + _biennia_, 82, 205, 256, 257, 258; 259, 262, + 263, 264, 521, 524, 527, 574, 575, 586, + 587, 683, 690, 708 + _biennis cruciata_, 22, 587 + _brevistylis_, 263, 280, 526, 529, 530, 547, + 563, 564, 565, 573, 574, 702, 706 + _cruciata_, 575, 585, 586, 589, 590, 683 + _elliptica_, 540, 545, 555, 562 + _gigas_, 533, 534, 535, 536; 537, 553, 554, + 563, 565, 566, 567, 573, 574, 702 + _glauca_, 424 + _hirtella_, 262 + _laevifolia_, 526, 528, 529, 547, 563, 564, + 573, 574, 701, 706 + _lamarckiana_, 17, 262, 262, 522, 523, 527, + 528, 529,, 533, 574, 575, 586, 690, 699 + pollination of, 524 + _lata_, 540, 541, 542, 549, 550, 551, 552, + 555, 559, 563, 566, 573, 574, 702 + _leptocarpa_, 540 + _muricata_, 256, 257, 258, 259, 262, 263, + 264, 513, 575, 690 + pollination of, 524 + _nanella_, 526, 531, 549, 50, 551, 552, 555, + 563, 564, 565, 566, 703 + _oblonga_, 537, 538, 552, 555, 563, 565, 566, + 572 + _rubrinervis_, 533, 534, 536, 537, 550, 551, + 552, 555, 563, 565, 568, 573, 574 + _scintillans_, 540, 543, 553, 555, 563, 566, + 573, 574 + mutability of, 544 + _semilata_, 540 + _suaveolens_, 521 +_Oleander_, 684 +_Onagra_, 262, 708, 709 +Onions, wild, 684 +_Ononis repens_, 577 +Orange, 90, 133, 134 +Orchids, 631 +Origin of species (Darwin), 109 +_Orobanche_, 220 +_Othonna crassifolia_, 442 +Otin, 618 +Oviedo, 89 + +P + +_Paeonia corallina leiocarpa_, 126 +Paillat, 618 +Pangenes, 306 +Pangenesis, 306, 689 +_Panicum_, 105 +Pansies, 640 +Pansy, 118, 121 +_Papaver alpinum_, 139 + _bracteatum_, 661 + _bracteatum monopetalum_, 661 + _commutatum_, 357 + _dubium glabrum_, 126 + hybridism, 662 + _somniferum Danebrog, 162 + _somniferum monstruosum_, 371 + _somniferum polycephalum_, Parris, 754 +Parsley + crisped, 158, 181 +Parsnip, water, 457 +Pea-family, 344 +Peach, 138, 226, 240 +Peach-almond, 769 +Pears, 79, 90, 134, 147, 152, 203, 283 +Pearson, Karl, 716 +Peas, sugar, 135, 158 +_Pedicularis_, 410 + _palustris_, 410 +Pedigree-culture, 109 + experimental, 547 +_Pelargonium_, 272, 355 +Peloria, definition of, 164 +Peloric toad-flax + first record of, 466 + origin of, 459, 464, 472 + sterility of, 467 +Pelorism + _Antirrhinum majus_ (see snapdragon) + _Digitalis purpurea_, 482 + _Gloxinia_, 484, 485 + labiates, 486 + _Laburnum_, 485 + _Lamium_, 486. + _Linaria_, see Toad-flax + _Linaria dalmatica_, 482 + _Linaria vulgaris_, 464 + orchids, 479, 486, 487 + _Salvia_, 486 + _Scrophularia nodosa_, 486 + snapdragon, 481 + toad-flax, 459-487 + _Tropaeolum majus_, 485 + _Uropedium Lindenii_, 487 + wild sage, 486 +_Peltaria alliacea_, 663 +Pennywort, marsh, 668 +Penzig, 638 +Periodicity, law of, 365, 368, 721, 722 +Periods, mutative, 706, 708 +Periwinkles, 322 +Persicaria, water, 433, 434, 435, 643 +Petalomany, 330 +Petunia, 491, 626 +_Phacelia_, 420, 422, 820 +_Phaseolus lunatus_, 592 + _multiflorus_, 202 + _nanus_, 202 +_Phleum alpinum_, 696 +_Phlox_, 232 + _drummondi_, 161 +_Phyllonoma ruscifolia_, 676 +Physiologic units, 144, 153, 249 +_Picris hieraoioides_, 411 +Pimpernel, scarlet, 162 +Pinacothec, Munich, 164 +Pine, 368, 804 +Pine-apples, 90, 134 +Pinks, 178 +_Pinus sylvestris_, 368 +Pistillody in poppies, 369, 370, 372 +Pitcher-plants, 671 +Plankton, 711 +_Plantago_, 53 + lanceolata_, 520, 671, 684 +Plantain, 684 +Plater, 610 +Plum, 79, 134, 789 + beach, 58 + Japanese, 58 + purple-leaved, 619 +_Plusia_, 204 +_Poa alpina vivipara_, 684 +_Podocarpus koraiana_, 169 +_Polemonium coeruleum_, 282 + _coeruleum album_, 200 + _dissectum_, 161, 202 +_Polygala_, 242 +_Polygonum amphibium_, 432 + var. _natans_ Moench, 433, 434 + var. _terrestris_ Wench, 433, 434 + _Convolvulus_, 419, 424 + _viviparum_, 684 +Polymorphy, 188 +Pomegranate, 90 +Pond-lily, yellow, 268 +Poplar, fastigiate, 623, 624 + Italian, 623 +_Populus italica_, 622 + _nigra_, 624 +Poppy, 146, 151, 152, 163, 165, 241, 356, 640, + 723 + "Danebrog," 283, 291 + garden, 661 + "Mephisto," 283, 291 + opium, 89, 189, 195, 198, 282, 291, 369, + 371, 373, 379, 383, 391, 405, 406, 420, 452, + 720, 789 + pistillody in, 369 + pistilloid, 508 + polycephalous, 405 +Potatoes, 765, 810 +_Potentilla Tormentilla_, 52 +Pre-Linnean attitude, 2 +Primrose, 268, 372, 410 + evening (see evening-primrose). +_Primula acaulis_, 52, 632 + _elatior_, 52, 633, 635 + _grandiflora_, 268 + _imperialis_, 697 + _japonica_, 410 + _officinalis_, 52, 268, 633, 635 + _variabilis_, 268 + _veris_, 52, 633, 634 +Prodromus (De Candolle) 370 +Progression, 430, 705, 774, 775, 777, 779, 805 + in evolution, 630 +Propagation + asexual, 745, 751, 766, 767, 770, 774, 777 + sexual, 745, 777 + vegetative (see asexual) +Proskowetz, Em. von, 70 +Prototype + definition of, 170 +_Prunus_, 52 + _cerasifera_, 619 + _Mahaleb_, 617 + _nana_, 612 + _maritima_, 59 + _Padus_, 617 + _Pissardi_, 619 + variation in, 56 +_Pyrethrum roseum_, 511 +_Pyrola_, 222, 661 + +Q + +Quartile, 736, 737, 767 +_Quercus pedunculata fastigata_, 596 +Quetelet's law, 463, 716, 717, 725, 730, 734, + 738, 748, 753, 759, 767, 775, 779, 780, 806 + +R + +Races, inconstancy of improved, 770-797 +Raciborsky, 682 +Radishes, 325, 806 +Ragwort, tansy, 157 +Raisins, 134 +Rameses, 697 +_Ranunculus_, 331 + _acris_, 331 + _arvensis_, 282 + _arvensis inermis_, 125 + _asiaticus_, ,241 + _bulbosus_, 357, 410, 740 +Ra-n-Woser, King, 104 +_Raphanus Raphanistrum_, 202, 424,520 + _caudatus_, 202 +Rasor, John, 588, 589 +Raspberry, 268, 768 + "Phenomenal," 268 + "Primus," 269 + Siberian, 269 +Ratzeburg, 467 +Raunkiaer + on variation in _Taraxacum_, 60 +Recessive character, 280 + Sports, 191, 715, 689 + bud, 427 + +S + +Sprenger, 610, 611 +Stability, 155 +Stahl, 611 +_Stellaria Holostea apetala_, 585 +Stocks, 146, 322, 328, 329, 332, 334, 336, 338, + 432 +Stock + "Brompton," 329 + chamois-colored, 198 + "Queen," 324 + white, 160 +Stork's-bill, white hemlock, 161 +Strasburger, 196, 448 +Strawberry, 158, 266, 342 + "Gaillon," 135 + "Giant of Zuidwijk," 614 + one-leaved, 164, 596, 666 + white, 158, 165 +Striped flowers, 309, 374, 431, 606, 607 + races, types of, 328 +Struggle for life, 674, 571, 682, 702, 799, 803, + 824, 825 +St. Johnswort, 725 +St. Sebastian, 164 +Sub-species (see also Elementary species), 224, 225 +Sugar-beets (see Beets, sugar) +Sugar-cane, 731, 752 + "Black Manilla," 753 + "Cheribon," 753, 755, 756 + "Chunnic," 753 + "Hawaii," 755, 756 + seeds of, 754 + "White Manilla," 752 +Sundew, 268 +Sunflower, 410, 425, 820 +Sweet-flag, 222 +Sweet-pea, 160, 776 +Sweet William, 163, 282, 322, 648 + twisted variety, 408, 648 +Syncotyls, 417, 424 +_Syringa vulgaris axurea plena_, 763 +Systematic species, 12, 64, 101, 128 + nature of, 54, 62 +Systematic units, 61, 91 + +T + +_Tagetes africana_, 510 + _signata_, 612 +"Talavera de Bellevue," 97 +_Tanacetum vulgare_, 131, 132, 236 +Tansy, 131, 132, 236 +_Taraxacum_, 125, 126 + officinale, 59, 411 +_Tares_, 105 +_Taxus_, 136 + _baccata_, 169 + _baccata fastigiata_, 170, 618 + _minor_, 169 +Teasels, 402, 642, 645, 674, 675 + twisted, 405, 412, 446, 447, 643, 646, 647, + 648, 819 +_Tetragonia expansa_, 162 +Theatre d'Agriculture, 72 +Thibault, 618 +Thomson, Sir William (see Kelvin, Lord) +Thorn-apples, 139, 142, 143, 145, 238, 283, 300, + 452 + thornless, 234 +Thorn-broom, 457 +_Thrincia hirta_, 411 +Thuret, 38, 47, 49 +Thyme, white creeping, 201 +_Thymus Serphyllum album_, 201 + _vulgaris_, 577 +_Tilia parvifolia_, 355, 669 +Toad-flax, 267, 282, 707 + cross pollination of, 471 + experiment with, described, 468 + invisible dimorphous state of, 470, 471, 478 + latent tendency to mutation in, 479 + peloric, see Peloric toad flax + sterility of mutants, 477 + unusual pelorism, 486 +Tomato, 653 + "Acme," 656, 657 + "Mikado," 654 + mutation of, 655 + upright, 654 + "Washington," 657 +Tournefort + author of genera, 32 +Tracy, W.W. 592 +Trees, genealogic, 707, 708 +Tricotyls, 416, 419, 420 +_Trifolium incarnatum_, 352 +_Triticum dicoccum_, 105 +_Tropaeolum_, 193, 668 + _majus_, pelorism of, 485 +"True Exercises with Plants" (hunting), 490 +Tulips, 149, 178, 274, 322 + black, 620 +Turnip, 244, 621 +Twisted stems, 402, 403, 405, 413 +Twisted varieties + atavists of, 406 + +U + +_Ulex europaeus_, 140, 217 +_Ulmus pedunculata_, 615 + _pedunculata urticaefolia_, 615 +Umbellifers, 457 +_Umbilicus_, 669 +Unger, 105 +Unit-characters, 249, 261, 306, 307, 313, 658, + 689, 715, 716 +Urban, 265 +_Uropedium lindenii_, 487 +Utility, 685, 724 +Utricularia, 672 + +V + +_Vaccinium Myrtillus_, 577 +Valerian, 402, 409, 648 + twisted, 403 +_Valeriana officinalis_, 402 +_Vallisneria_, 684. +Van den Berg, 625 +Van de Water, 614 +Van Mons, 76, 77, 78, 806 +Variability (see also Fluctuation ), 188, 190, 191 + analogous, 244 + apple, 75 + asexual, 320 + correlative, 142, 143, 148, 167 + cultivated plants, 66 + embryonic, 770, 771, 814 + ever-recurring, 190 + fluctuating (see also individual), 62, 142, + 190, 233, 375, 416, 454, 698, 759, 762, 765, + 766, 767, 770, 771, 789, 805, 814 + fluctuating vs. mutability 569 + homologous, 244 + individual (see also fluctuating), 190, 716, + 718, 746, 749, 770, 814 + influence of mutation on, 335 + kinds of, 715 + nutrition and, 390, 391, 719, 771 + parallel, 243 + partial, 440, 444, 718, 746, 748, 753, 814, + 816 + repeated, 242 + restricted, 598 + sectional, 317 + sexual, 320 + sources of, 758 +Variation + bud, 176, 178, 180, 284, 317, 318, 321, 338, + 427, 750 + definition of, 188 + partial, 788, 789 + seed, 750 + spontaneous, 191 + use of term, 189 +Variegation, 426, 427 +Varietal marks, origin of, 275 +Varieties, 84, 95, 126, 127, 128, 129, 132, 142 + broom-like, 618, 624 + constancy of, 532 + constant, 135 + crosses of species with, 247, 277, 278, 281 + elementary species vs. 459 + ever-sporting, 178, 309, 310, 311, 312, 313, + 321, 324, 328, 329, 332, 333, 334, 350, 358, + 365, 368, 372, 399, 413, 420, 430, 431, 432, + 434, 445, 606, 607, 628, 740, 789, 790, 795 + fasciated (see Fasciated stems). + groups of, 606 + horticultural, 607, 609 + hybrid, 122, 190, 608 + hybrids of, 210, 254, 255 + inconstant, 135, 154; 155, 161 + mutation of, 141 + negative (retrogressive), 131, 132, 134, 224, + 226, 238, 245, 277 + positive, 131, 132, 134, 224, 238, 245 + pure, 122, 190 + retrograde, 14, 15, 16, 95, 121, 208, 430, + 435, 606, 607 + retrogressive (see negative). + seed, 122 + single, 191 + spontaneous crosses, 209 + sporting (see inconstant) + stability of, 207 + sterile, 622 + types of, 142 + variable, 606 + vegetative, 122 + weeping, 617 +Variety, 130 + definition of, 11, 12 + elementary species vs. 141, 152, 154, 224, + 243, 247, 251 + origin of, 141, 152, 224 + use of term, 189, 435 +Variety-testing, 95, 97, 116, 119, 743, 799, 825 +Varro, 106 +Veitch & Sons, 272 +Venus' looking-glass, 367 +Verlot, 186, 612 +Vernon, 132 +_Vernonia cinerea_, 450 +_Veronica longifolia_, 282, 284 + _scutellata_, 139 + _spicata nitens_, 126 +_Viburnum Opulus_, 134, 239 +Vicinism, 185, 188, 203, 205, 206, 213, 214, 776 + definition of, 188, 192, 606 +Vicinist, 199, 201 +_Vicoa aurioulata_, 450 +_Victoria regia_, 668 +Villars + on _Draba verna_, 49 +Vilmorin, 570, 607, 612, 622, 661, 662, 773, 775, + 776; 792, 795, 796, 797, 806, 807, 810, 813, + 818, 820 +Vilmorin, Louis de, 72, 92, 93, 97, 108, 109, 110, + 114, 185, 818 +Vilmorin, Messrs., 322 +_Vinca_, 242, 490 + _minor_, 322 +Vine, parsley-leaved, 179 +_Viola_, 126, 546, 547, 692 + _agrestis_, 45 + _alpestris_, 40 + _altaica_, 39 + _anopetala_, 44 + _arvensis_, 39, 40, 41, 44 + _curtisepala_, 45 + _striolata_, 45 + _aurobadia_, 44 + _caloarata_, 39 + _cornuta_, 39, 281 + _lutea_, 38 + _lutescens_, 44 + _nemausensis_, 45 + _ornatissima_, 44 + _palescens_, 45 + _patens_, 45 + _roseola_, 44 + _segetatis_, 45 + _stenochila_, 41 + _tricolor_, 38, 40, 41, 44, 46 + _ammotropha_, 41 + _coniophila_, 41 + _genuina_, 42 + _versicolor_, 42 +Violets, 63, 232, 233, 490 +Violet, dame's, 322, 323, 411 + long-spurred, 281 +Virgil, 105, 106, 108 +_Viscaria oculata_, 4, 648, 821 + twisted variety, 408 +_Vitis_, 52 +Volckamer, 228 +Von Lochow, 821, 822, 822 +Von Rumker, 94 +Von Wettstein, 448, 805 +Vrolik, 164, 483 + +W + +"Waare Oeffeninge der Planten" (Munting), 490 +Wallace, 5, 7, 8, 30, 205 +Wall-flower, 370, 371 +Walnut, 243, 766 + cut-leaved, 616 + one-bladed, 666 +Water-lilies, 668 +Weber, 228 +Weeping-willow, 180 + crisped, 181 +Weigelias, 740 +_Wellingtonia_, 618 +Wheat, 96, 98, 105, 113, 119, 283, 810, 823 + bearded, 98 + "Blue-stem," 117 + "Galland," 100, 207 + "Hopetown," 112, 112 + "Hunter's," 111, 112 + "Minnesota No. 169," 117 + "Mungoswell's," 110, 111 + "Pedigree," 109 + "Pringle's," 114 + "Rivett's bearded," 207 + "Sheriff's bearded red," 114 + "Sheriff's bearded white," 114 + "White Hunter's," 112 +Wheat-ear carnation, 227 +White, C.A., 656, 657 +White varieties, 577 +Whitlow-grasses, 63, 118, 119 +Whorls, ternate, 684 +Wild sage (see Salvia) +Willdenow, 468, 666, 667 +Williamson, 491 +Willows, 135, 267 +Willow + weeping (see Weeping-Willow) +Willow-herb, 268, 269, 682 +Wintercress, 427 +Wintergreen, 661 +Wittmack, 682 +Wittrock, 38, 40, 41, 42, 43, 44, 45, 46 +Wooton, E.O., 140 +Wormseed, 638 + +X + +_Xanthium canadense_, 140 + _commune_, 140, 152, 591 + _commune Wootoni_, 22 +Wootoni, 140, 152, 591 + +Y + +Yarrow, 131, 132 +Yew, 136, 169 + pyramidal, 618 + +Z + +_Zea Mays cryptosperma_, 641 + _tunicata_, 641 +_Zinnia_, 490 +Zioberg, 466 +Zocher & Co., 230 + + + + + + +End of the Project Gutenberg EBook of Species and Varieties, Their Origin by +Mutation, by Hugo DeVries + +*** END OF THE PROJECT GUTENBERG EBOOK SPECIES AND VARIETIES *** + +This file should be named spvrt10.txt or spvrt10.zip +Corrected EDITIONS of our eBooks get a new NUMBER, spvrt11.txt +VERSIONS based on separate sources get new LETTER, spvrt10a.txt + +Produced by Dave Gowan <dgowan@bio.fsu.edu> + +Project Gutenberg eBooks are often created from several printed +editions, all of which are confirmed as Public Domain in the US +unless a copyright notice 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