Chapter 2

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.