Chapter 8

[120] Indeed it may be surmised that this has been its chief and prominent function. Taking up again our metaphor of the sieve we can assert that in such cases climate and soil exercise sifting action and in this way the application of the metaphor becomes more definite. Of course, next to the climate and soil in importance, come ecological conditions, the vegetable and animal enemies of the plants and other influences of the same nature.

In conclusion it is to be pointed out that this side of the problem of natural selection and the struggle for life appears to offer the best prospects for experimental, or for continued statistical inquiry. Direct observations are possible and any comparison of numerical proportions of species in succeeding years affords clear proof of the part it plays. And above all, such observations can be made quite independently of doubtful theoretical considerations about presumed changes of character.

The fact of natural selection is plain and it should be studied in its most simple conditions.

[121] C. RETROGRADE VARIETIES

LECTURE V

CHARACTERS OF RETROGRADE VARIETIES

Every one admires the luxuriance of garden-flowers, and their diversity of color and form. All parts of the world have contributed to their number and every taste can find its preference among them. New forms produced by the skill of the breeder are introduced every year. This has been done mostly by crossing and intermingling the characters of introduced species of the same genus. In some of the cases the history of our flowers is so old that their hybrid origin is forgotten, as in the case of the pansies. Hybridizations are still going on in other groups on a large scale, and new forms are openly claimed to be of hybrid origin.

Breeders and amateurs generally have more interest in the results than in the way in which they have been brought about. Excellent flowers and fruit recommend themselves and there seems to be no reason for inquiring [122] about their origin. In some cases the name of the originator may be so widely known that it adds weight to the value of the new form, and therefore may advantageously be coupled with it. The origin and history of the greater part of our garden-flowers, fruits and vegetables are obscure; we see them as they are, and do not know from whence they came. The original habitat for a whole genus or for a species at large, may be known, but questions as to the origin of the single forms, of which it is built up, ordinarily remain unanswered.

For these reasons we are restricted in most cases to the comparison of the forms before us. This comparison has led to the general use of the term "variety" in opposition to "species." The larger groups of forms, which are known to have been introduced as such are called species. All forms which by their characters belong to such a species are designated as varieties, irrespective of their systematic relation to the form, considered as the ancestor of the group.

Hence, we distinguish between "hybrid varieties" and "pure varieties" according to their origin from different parents or from a single line of ancestors. Moreover, in both groups the forms may be propagated by seeds, or in the vegetative way by buds, by grafting or [123] by cutting, and this leads to the distinction of "seed-varieties" and "vegetative varieties." In the first case the inheritance of the special characters through the seeds decides the status of the variety, in the latter case this point is left wholly out of consideration.

Leaving aside all these different types, we are concerned here only with the "seed-varieties" of pure origin, or at least with those, that are supposed to be so. Hybridization and vegetative multiplication of the hybrids no doubt occur in nature, but they are very rare, when compared with the ordinary method of propagation by seed. "Seed-varieties" may further be divided into constant and inconstant ones. The difference is very essential, but the test is not always easy to apply. Constant varieties are as sharply defined and as narrowly limited as are the best wild species, while inconstant types are cultivated chiefly on account of their wide range of form and color. This diversity is repeated yearly, even from the purest seed. We will now discuss the constant seed-varieties, leaving the inconstant and eversporting types to a subsequent lecture.

In this way we may make an exact inquiry into the departures from the species which are ordinarily considered to constitute the essential character of such a constant and pure seed-variety [124] and need only compare these differences with those that distinguish the elementary species of one and the same group from each other.

Two points are very striking. By far the greatest part of the ordinary garden-varieties differ from their species by a single sharp character only. In derivative cases two, three or even more such characters may be combined in one variety, for instance, a dwarfed variety of the larkspur may at the same time bear white flowers, or even double white flowers, but the individuality of the single characters is not in the least obscured by such combinations.

The second point is the almost general occurrence of the same variety in extended series of species. White and double flowers, variegated leaves, dwarfs and many other instances may be cited. It is precisely this universal repetition of the same character that strikes us as the essential feature of a variety.

And again these two characteristics may now be considered separately. Let us begin with the sharpness of the varietal characters. In this respect varieties differ most obviously from elementary species. These are distinguished from their nearest allies in almost all organs. There is no prominent distinctive feature between the single forms of _Draba_ [125] _Verna_, _Helianthemum_ or of _Taraxacum_; all characters are almost equally concerned. The elementary species of _Draba_ are characterized, as we have seen, by the forms and the hairiness of the leaves, the number and height of the flower-stalks, the breadth and incision of the petals, the forms of the fruits, and so on. Every one of the two hundred forms included in this collective species has its own type, which it is impossible to express by a single term. Their names are chosen arbitrarily. Quite the contrary is the case with most of the varieties, for which one word ordinarily suffices to express the whole difference.

White varieties of species with red or blue flowers are the most common instances. If the species has a compound color and if only one of the constituents is lost, partially colored types arise as in _Agrostemma Coronaria bicolor_. Or the spots may disappear and the color become uniform as in _Gentiana punctata concolor_ and the spotless Arum or _Arum maculatum immaculatum_. Absence of hairs produces forms as _Biscutella laevigata glabra_; lack of prickles gives the varieties known as _inermis, as for instance, _Ranunculus arvensis inermis_. _Cytisus prostratus_ has a variety _ciliata_, and _Solanum Dulcamara_, or the bitter-sweet, has a variety called _tomentosum_. The curious monophyllous [126] variety of the strawberry and many other forms will be discussed later.

To enlarge this list it would only be necessary to extract from a flora, or from a catalogue of horticultural plants, the names of the varieties enumerated therein. In nearly every instance, where true varieties and not elementary species are concerned, a single term expresses the whole character.

Such a list would also serve to illustrate the second point since the same names would recur frequently. Long lists of varieties are called alba, or inermis, or canescens or lutea, and many genera contain the same appellations. In some instances the systematists use a diversity of names to convey exactly the same idea, as if to conceal the monotony of the character, as for instance in the case of the lack of hairs, which is expressed by the varietal names of _Papaver dubium glabrum_, _Arabis ciliata glabrata_, _Arabis hirsuta glaberrima_, _Veronica spicata nitens_, _Amygdalus persica laevis_, _Paeonia corallina Leiocarpa_, &c.

On the contrary we find elementary species in different genera based on the greatest possible diversity of features. The forms of _Taraxacum_ or _Helianthemum_ do not repeat those of _Draba_ or _Viola_. In roses and brambles the distinguishing features are characteristic of the type, as [127] they are evidently derived from it and limited to it. And this is so true that nobody claims the grade of elementary species for white roses or white brambles, but everyone recognizes that forms diverging from the nearest species by a single character only, are to be regarded as varieties.

This general conviction is the basis on which we may build up a more sharply defined distinction between elementary species and varieties. It is an old rule in systematic botany, that no form is to be constituted a species upon the basis of a single character. All authors agree on this point; specific differences are derived from the totality of the attributes, not from one organ or one quality. This rule is intimately connected with the idea that varieties are derived from species. The species is the typical, really existing form from which the variety has originated by a definite change. In enumerating the different forms the species is distinguished by the term of genuine or typical, often only indicated as _a_ or the first; then follow the varieties sometimes in order of their degree of difference, sometimes simply in alphabetical order. In the case of elementary species there is no real type; no one of them predominates because all are considered to be equal in rank, and the systematic species to which they [128] are referred is not a really existing form, but is the abstraction of the common type of all, just as it is in the case of a genus or of a family.

Summarizing the main points of this discussion, we find that elementary species are of equal rank and together build up the collective or systematic ideal species. Varieties on the other hand are derived from a real and commonly, still existing type.

I hope that I have succeeded in showing that the difference between elementary species, or, as they are often called, smaller or subspecies, on the one hand and varieties on the other, is quite a marked one. However, in order to recognize this principle it is necessary to limit the term variety, to those propagating themselves by seed and are of pure and not of hybrid origin.

But the principle as stated here, does not involve an absolute contrast between two groups of characters. It is more a difference in our knowledge and appreciation of them than a difference in the things themselves. The characters of elementary species are, as a rule, new to us, while those of varieties are old and familiar. It seems to me that this is the essential point.

And what is it that makes us familiar with them? Obviously the continuous recurrence of the same changes, because by a constant repetition they must of course lose their novelty.

[129] Presently we shall look into these characters more in detail and then we shall find that they are not so simple as might be supposed at first sight; but precisely because we are so familiar with them, we readily see that their different features really belong to a single character; while in elementary species everything is so new that it is impossible for us to discern the unities of the new attributes.

If we bear in mind all these difficulties we cannot wonder at the confusion on this question that seems to prevail everywhere. Some authors following Linnaeus simply call all the subdivisions of species, varieties; others follow Jordan and avoid the difficulty by designating all smaller forms directly as species. The ablest systematists prefer to consider the ordinary species as collective groups, calling their constituents "The elements of the species," as was done by A.P. De Candolle, Alph. De Candolle and Lindley.

By this method they clearly point out the difference between the subdivisions of wild species as they ordinarily occur, and the varieties in our gardens, which would be very rare, were they not singled out and preserved.

Our familiarity with a character and our grounds for calling it an old acquaintance may result from two causes, which in judging a new [130] variety are essentially different. The character in question may be present in the given species or it may be lacking, but present in the other group. In the first case a variety can only be formed by the loss of the character, in the second case it arises by the addition of a new one.

The first mode may be called a negative process, while the second is then to be designated as positive. And as it is more easy to lose what one has than to obtain something new, negative varieties are much more common than are positive ones.

Let us now take an instance of a character that is apt to vary in both ways, for this is obviously the best way of making clear what is meant by a negative and a positive change.

In the family of the composites we find a group of genera with two forms of florets on each flower-head. The hermaphrodite ones are tubular with 5, or rarely 4, equal teeth, and occupy the center of the head. These are often called the flosculous florets or disk-florets. Those of the circumference are ligulate and ordinarily unisexual, without stamens. In many cases they are sterile, having only an imperfect ovary. They are large and brightly colored and are generally designated as ray-florets. As instances we may cite the camomile (_Anthemis nobilis_), the wild camomile (_Matricaria Chamomilla_), [131] the yarrow (_Achillea Millefolium_), the daisies, the Dahlia and many others. Species occur in this group of plants from time to time that lack the ray-florets, as in the tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus of the marigolds or _Bidens_ is noted for containing both of these types. The smaller and the three-toothed marigold (_B. cernua_ and _B. tripartita_) are very common plants of wet soil and swamps, ordinarily lacking the ray-florets, and in some countries they are very abundant and wholly constant in this respect, never forming radiate flower-heads. On the other hand the white-flowered and the purple marigold (_B. leucantha_ and _B. atropurpurea_) are cultivated species of our gardens, prized for their showy flower-heads with large white or deeply colored, nearly black-purple florets.

Here we have opportunity to observe positive and negative varieties of the same character. The smaller, and the three-toothed marigold occur from time to time, provided with ray florets, showing a positive variation. And the white marigold has produced in our gardens a variety without rays. Such varieties are quite constant, never returning to the old species. Positive and negative varieties of this kind are by no means rare among the compositae.

[132] In systematic works the positive ones are as a rule called "radiate," and the negative ones "discoid." Discoid forms of the ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and of some centauries have been described. Radiate forms have been observed in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio vulgaris_). Taken broadly the negative varieties seem to be somewhat more numerous than the positive ones, but it is very difficult to come to a definite conclusion on this point.

Quite the contrary is the case with regard to the color-varieties of red and blue flowers. Here the loss of color is so common that every one could give long lists of examples of it. Linnaeus himself supposed that no blue or red-colored wild species would be without a white variety. It is well known that he founded his often criticized prescript never to trust to color in recognizing or describing a species, on this belief.

On the other hand there are some red varieties of white-flowered species. But they are very rare, and little is known about their characters or constancy. Blue varieties of white species are not found. The yarrow (_Achillea Millefolium_) has a red-flowered form, which occurs [133] from time to time in sunny and sandy localities. I have isolated it and cultivated it during a series of years and during many generations. It is quite true to its character, but the degree of its coloring fluctuates between pink and white and is extremely variable. Perhaps it can be considered as an inconstant variety. A redflowered form of the common _Begonia semperflorens_ is cultivated under the name of "Vernon," the white hawthorn (_Crataegus Oxyacantha_) is often seen with red flowers, and a pink-flowered variety of the "Silverchain" or "Bastard acacia" (_Robinia Pseud-Acacia_) is not rarely cultivated. The "Crown" variety of the yellow wall-flower and the black varieties, are also to be considered as positive color variations, the black being due in the latter cases to a very great amount of the red pigment.

Among fruits there are also some positive red varieties of greenish or yellowish species, as for instance the red gooseberry (_Ribes Grossularia_) and the red oranges. The red hue is far more common in leaves, as seen among herbs, in cultivated varieties of _Coleus_ and in the brown leaved form of the ordinary white clover, among trees and shrubs in the hazelnut (_Corylus_), the beech (_Fagus_), the birch (_Betula_), the barberry (_Berberis_) and many others. But though most of these forms are very ornamental and abundant [134] in parks and gardens, little is as yet known concerning the origin of their varietal attributes and their constancy, when propagated by seeds. Besides the ray-florets and the colors, there are of course a great many other characters in which varieties may differ from their species. In most of the cases it is easy to discern whether the new character is a positive or a negative one. And it is not at all necessary to scrutinize very narrowly the list of forms to become convinced that the negative form is the one which prevails nearly everywhere, and that positive aberrations are in a general sense so rare that they might even be taken for exceptions to the rule.

Many organs and many qualities may be lost in the origination of a variety. In some instances the petals may disappear, as in _Nigella_, or the stamens, as in the Guelder-rose (_Viburnum Opulus_) and the _Hortensia_ and in some bulbs even the whole flowers may be wanting, as in the beautiful "Plumosa" form of the cultivated grape-hyacinth or _Muscari comosum_. Fruits of the pineapples and bananas without seeds are on record as well as some varieties of apples and pears, of raisins and oranges. And some years ago Mr. Riviere of Algeria described a date growing in his garden that forms fruit without pits. The stoneless plum of Mr. [135] Burbank of Santa Rosa, California, is also a very curious variety, the kernel of which is fully developed but naked, no hard substance intervening between it and the pulp.

More curious still are the unbranched varieties consisting of a single stem, as may be seen sometimes in the corn or maize and in the fir. Fir-trees of some three or four meters in height without a single branch, wholly naked and bearing leaves only on the shoots of the last year's growth at the apex of the tree, may be seen. Of course they cannot bear seed, and so it is with the sterile maize, which never produces any seed-spikes or staminate flowers. Other seedless varieties can be propagated by buds; their origin is in most cases unknown, and we are not sure as to whether they should be classified with the constant or with the inconstant varieties.

A very curious loss is that of starch in the grains of the sugar-corn and the sugar-peas. It is replaced by sugar or some allied substance (dextrine). Equally remarkable is the loss of the runners in the so-called "Gaillon" strawberries.

Among trees the pendulous or weeping, and the broomlike or fastigiate forms are very marked varieties, which occur in species belonging to quite different orders. The ash, the beach, some willows, many other trees and some [136] finer species of garden-plants, as _Sophora japonica_, have given rise to weeping varieties, and the yew-tree or _Taxus_ has a fastigiate form which is much valued because of its ascending branches and pyramidal habit. So it is with the pyramidal varieties of oaks, elms, the bastard-acacia and some others.

It is generally acknowledged that these forms are to be considered as varieties on the ground of their occurrence in so wide a range of species, and because they always bear the same attributes. The pendulous forms owe their peculiarity to a lengthening of the branches and a loss of their habit of growing upwards; they are too weak to retain a vertical position and the response to gravity, which is ordinarily the cause of the upright growth, is lacking in them. As far as we know, the cause of this weeping habit is the same in all instances. The fastigiate trees and shrubs are a counterpart of the weeping forms. Here the tendency to grow in a horizontal direction is lacking, and with it the bilateral and symmetric structure of the branches has disappeared. In the ordinary yew-tree the upright stem bears its needles equally distributed around its circumference, but on the branches the needles are inserted in two rows, one to the left and one to the right. All the needles turn their upper surfaces upwards, [137] and their lower surfaces downwards, and all of them are by this means placed in a single horizontal plane, and branching takes place in the same plane. Evidently this general arrangement is another response to gravity, and it is the failure of this reaction which induces the branches to grow upwards and to behave like stems.

Both weeping and fastigiate characters are therefore to be regarded as steps in a negative direction, and it is highly important that even such marked departures occur without transitions or intermediate forms. If these should occur, though ever so rarely, they would probably have been brought to notice, on account of the great prospect the numerous instances would offer. The fact that they are lacking, proves that the steps, though apparently great, are in reality to be considered as covering single units, that cannot be divided into smaller parts. Unfortunately we are still in the dark as to the question of the inheritance of these forms, since in most cases it is difficult to obtain pure seed.