Chapter 15

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.