Chapter D: Eversporting Varieties (7)

[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.