Species and Varieties, Their Origin by Mutation Part 8

And in fact we have two varieties which exhibit the two causes of this attribute separately. One of them is called "Delila," and has the red color limited to the lips, whilst the tube is pure white. The other is called "Fleshy," and is of a pale pink throughout the whole corolla.

Adding these two units to one another, we get the original dark red of the wild type, and it may be briefly stated here, that the way of effecting such an addition is given us in the crossing of the "Fleshy"

and the "Delila" variety, the hybrid showing the two colors and returning thereby to the old prototype.

Other cases of compound flower colors or of color patterns might be given as in the _Mimulus_ and the poppy, and in most of these cases some varieties are to be seen in our gardens which show only the single const.i.tuents of the group.

[152] Many dark flowers have an intermediate bright hued form besides the white variety, as in the case of roses, asters, _Nicandra_ and so on.

Intermediate forms with respect to stature may also be seen. The opium-poppy, the snapdragon, peas, the _Nicandra_, and many other garden-plants have not only dwarf varieties, but also some of intermediate height. These, though they are intermediate between the tall and dwarf types, cannot be considered as transitions, as between them and the extremes, intermediates are, as a rule wholly lacking.

Instances of the same occurrence of three types may be seen in the seeds of maize ("Cuzco," "Horse-dent" and "Gracillima") of beans and some other plants. The _Xanthium Wootoni_, above referred to, with only part of the p.r.i.c.kles of Xanthium commune is also a very curious instance of the demonstration of the compound nature of a character.

Summarizing the conclusions that may be drawn from the evidence given in this lecture, we have seen that varieties differ from elementary species in that they do not possess anything really new. They originate for the greater part in a negative way, by the apparent loss of some quality, and rarely in a positive manner by acquiring a character, already seen in allied species. These characters are not of the nature of [153]

morphologic ent.i.ties, but are to be considered as physiologic units, present in all parts of the organisms, and manifesting themselves where ever occasion is afforded. They are units in the sense that they may appear and disappear singly. But very often they are combined to yield compound characters, which are capable of a.n.a.lysis. Opportunities for such an a.n.a.lysis are afforded by these groups of cultivated varieties, of which some members show a single distinguis.h.i.+ng quality, or a number of them.

[154]

LECTURE VI

STABILITY AND REAL ATAVISM

It is generally believed that varieties are princ.i.p.ally distinguished from species by their inconstancy. This conception is derived from some special cases and transferred to others, and in its common form this belief must have originated from the confusion which exists as to the meaning of the term variety. It is true that vegetative varieties as a rule run back, when propagated by seeds; they are an obvious instance of inconstancy. In the second place we have considered the group of inconstant or sporting varieties, which of course we must exclude when studying the stability of other types. However, even these sporting varieties are unstable only to a certain degree, and in a broader sense will prove to be as true to their character as the most constant types.

Having separated these two groups, which include also the wide range of hybrid forms, we may next consider only those varieties of pure origin, and ordinarily propagated by seeds, [155] which have been discussed in former chapters. Their general character lies in their fidelity to type, and in the fact that this is single, and not double, as in the sporting varieties.

But the current belief is, that they are only true to their peculiarities to a certain degree, and that from time to time, and not rarely, they revert to the type from which they have arisen. Such reversion is supposed to prove that they are mere varieties, and at the same time to indicate empirically the species from which they have sprung.

In the next lecture we shall examine critically the evidence on which this a.s.sumption rests. Before doing so however, it will be necessary to collate the cases in which there is no reversion at all, or in which the reversion is absent at least in experimental and pure sowings.

In the present state of our knowledge it is very difficult to decide, whether or not true reversion occurs in constant varieties. If it does occur, it surely does so very rarely and only under unusual circ.u.mstances, or in particular individuals. However when such individuals are multiplied by buds and especially when they are the only representatives of their type, the reversion, though theoretically rare, will be shown by nearly every specimen of the variety. Examples of this will be given below.

[156] They are generally called atavists or reversionists, but even these terms are sometimes used in a different sense.

Lastly it is to be said that the empirical and experimental evidence as to the question of constancy is not as extensive as it should be. The experimental conditions are seldom described, and it is only recently that an interest in the matter has been awakened. Much remains to be done. Among other things the innumerable varieties of trees, shrubs and perennial herbs should be tested as to their constancy when grown from purely fertilized seeds. Many of them may be included among the number that sport constantly.

Leaving aside the doubtful or insufficiently studied cases, we may now turn our attention to the facts that prove the absolute stability of a large number of varieties, at least as far as such completeness can be attained by experiment or observation.

The best proof is afforded by the varieties which grow wild in localities where they are quite isolated from the species, and where for this reason, no possibility of crossing disturbs the significance of the proof. As one instance the rayless form of the wild camomile, or the _Matricaria Chamomilla discoidea_ may be mentioned. Many systematists have been so strongly [157] impressed with its absolute constancy and its behavior as an ordinary species, that they have elevated it, as it is called, to the rank of a species. As such it is described under the name of _Matricaria discoidea_ DC. It is remarkable for its rapid and widespread distribution, as of late years it has become naturalized in different parts of America and of Europe, where it is to be seen especially in France and in Norway. Experimentally I raised in succeeding years between 1000 and 2000 seedlings, but observed no trace of reversion, either in the strongest or in the numerous very small and weak individuals which appeared in the cultures.

The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a second instance. It is a perennial herb with short rootstocks and stout stems bearing numerous short-peduncled heads in large compact corymb; it multiplies itself abundantly by seeds and is very common on the sand dunes of Holland. It has two forms, differing only in the occurrence or the lack of the ray florets. But these two varieties occupy different localities and are even limited to different provinces. As far as I have been able to ascertain on numerous excursions during a series of years, they never sport, and are only intermingled on the outskirts of their habitats. The rayless form is generally considered as the [158] variety but it is quite as stable as the radiate species.

The radiate varieties of marigold, quoted in a former lecture, seem to be equally constant, when growing far away from their prototypes. I sowed the seeds of a single plant of the radiate form of _Bidens cernua_, and found all of the seedlings came true, and in the next year I had from their seed between 2,000 and 3,000 flowering individuals, all equally radiate. Many species of composites have been tried, and they are all constant. On the other hand rare sports of this kind have been observed by Murr and other authors.

Many kinds of vegetables and of fruits give instances of stability.

White strawberries, green grapes, white currants, crisped lettuce, crisped parsley and some other crisped forms may be cited. The spinage without p.r.i.c.kles is a widely known instance. White-flowered flax never reverts to the blue prototype, if kept pure. Sugar-peas and sugar-corn afford further instances. Strawberries without runners have come true from seed ever since their first appearance, over a hundred years ago.

Many garden-varieties, the stability of which under ordinary circ.u.mstances is doubtful, because of their being sown too close to other varieties of the same species, have been tested in [159] respect to their stability by different writers and at different times. In doing this it is plain that it is very essential to be sure of the purity of the seed. Specimens must be grown in positions isolated from their allies, and if possible be pollinated artificially with the exclusion of the visits of insects. This may be done in different ways. If it is a rare species, not cultivated in the neighborhood, it is often sufficient to make sure of this fact. Pollen may be conveyed by bees from distances of some ten or twenty meters, or in rare cases from some hundred meters and more, but a greater distance is ordinarily sufficient for isolation.

If the flowers fertilize themselves, as is more often the case than is generally supposed, or if it is easy to pollinate them artificially, with their own pollen or in small groups of similar individuals, the best way is to isolate them by means of close coverings. When flowering, the plants are as a rule too large to be put under bell-gla.s.ses, and moreover such coverings would keep the air moist, and cause the flower-buds to be thrown off. The best coverings are of netting, or of canvas of sufficiently wide mesh, although after a long experience I greatly prefer cages of fine iron-wire, which are put around and over the whole plant or group of plants, and fastened securely and tightly to the ground.

[160] Paper bags also may be made use of. They are slipped over the flowering branches, and bound together around the twigs, thus enclosing the flowers. It is necessary to use prepared papers, in order that they may resist rain and wind. The best sort, and the one that I use almost exclusively in my fertilization-experiments, is made of parchment-paper.

This is a wood-pulp preparation, freed artificially from the so-called wood-substance or lignin. Having covered the flowers with care, and having gathered the seeds free from intermixtures and if possible separately for each single individual, it only remains to sow them in quant.i.ties that will yield the greatest possible number of individuals.

Reversions are supposed to be rare and small groups of seedlings of course would not suffice to bring them to light. Only sowings of many hundreds or thousands of individuals are decisive. Such sowings can be made in one year, or can be extended over a series of years and of generations. Hildebrand and Hoffman have preferred the last method, and so did Hofmeister and many others. Hildebrand sowed the white hyacinth, and the white varieties of the larkspur, the stock and the sweet pea.

Hoffman cultivated the white flax and many other varieties and Hofmeister extended his sowings [161] over thirty years with the white variety of the yellow foxglove (_Digitalis parviflora_). White-flowered varieties of perennial garden plants were used in my own experiments. I bought the plants, flowered them under isolation in the way described above, gathered the seeds from each individual separately and sowed them in isolated groups, keeping many hundreds and in some cases above a thousand plants up to the time of flowering. Among them I found only one inconstant variety, the white form of the yellow columbine, _Aquilegia chrysantha_. It evidently belonged to the group of sporting varieties already referred to. All others came absolutely true to type without any exception. The species experimented with, were _Campanula persicifolia_, _Hyssopus officinalis_, _Lobelia syphilitica_, _Lychnis chalcedonica_, _Polemonium dissectum_, _Salvia sylvestris_ and some others. Tested in the same way I found the white varieties of the following annual plants also quite true: _Chrysanthemum coronarium_, _G.o.detia amoena_, _Linum usitatissimum_, _Phlox drummondi_, and _Silene Armeria_. To these may be added the white hemlock stork's-bill (_Erodium cicutarium alb.u.m_) which grows very abundantly in some parts of my fatherland, and is easily recognizable by its pure green leaves and stems, even when not flowering. I cultivated it, in large numbers [162] during five succeeding generations, but was never able to find even the slightest indication of a reversion to the red prototype. The scarlet pimpernel or _Anagallis arvensis_ has a blue variety which is absolutely constant.

Even in Britton and Brown's "Flora," which rarely enumerates varieties, it is mentioned as being probably a distinct species. Eight hundred blooming seedlings were obtained from isolated parents, all of the same blue color. The New Zealand spinage (_Tetragonia expansa_) has a greenish and a brownish variety, the red color extending over the whole foliage, including the stems and the branches. I have tried both of them during several years, and they never sported into each other. I raised more than 5,000 seedlings, from the different seeds of one lot of the green variety in succeeding years, but neither those germinating in the first year, nor the others coming into activity after two, three or four years of repose gave any sign of the red color of the original species.

It is an old custom to designate intermediate forms as hybrids, especially when both the types are widely known and the intermediates rare. Many persons believe that in doing so, they are giving an explanation of the rarer forms. But since the laws of hybridism are coming to be known we shall have to break with [163] all such usages. So for instance there are numerous flowers which are of a dark red or a dark blue color, and which, besides a white variety, have a pink or a pale blue form. Such pale varieties are of exactly the same value as others, and on testing they are found to be equally stable. So for instance the pink variety of the Sweet William (_Silene Armeria rosea_), the _Clarkia pulch.e.l.la carnea_ and the pale variety of the corn-c.o.c.kle, called usually _Agrostemma Githago nicaeensis_ or even simply _A.

nicaeensis_. The latter variety I found pure during ten succeeding generations. Another notable stable intermediate form is the poppy bearing the Danish flag (_Papaver somniferum Danebrog_). It is an old variety, and absolutely pure when cultivated separately. A long list of other instances might easily be given.

Many garden-varieties, that are still universally prized and cultivated are very old. It is curious to note how often such forms have been introduced as novelties. The common foxglove is one of the best examples. It has a monstrous variety, which is very showy because it bears on the summit of its raceme and branches, large erect cup-shaped flowers, which have quite a different aspect from the normal thimbleshaped side-blossoms. These flowers are ordinarily described as belonging to the anomaly [164] known as "peloria," or regular form of a normally symmetric type; they are large and irregular on the stems and the vigorous branches but slender and quinate on the weaker twigs. Their beauty and highly interesting anomalous character has been the cause of their being described many times, and nearly always as a novelty; they have been recently re-introduced into horticulture as such, though they were already cultivated before the middle of the last century. About that time very good descriptions with plates were published in the journal "Flora" by Vrolik, but afterwards they seem to have been forgotten. The peloric variety of the foxglove always comes true from seed, though in the strict sense of the word which we have chosen for our discussion, it does not seem to be a constant and pure variety.

It is very interesting to compare old botanical books, or even old drawings and engravings containing figures of anomalous plants. The celebrated Pinacothec of Munich contains an old picture by Holbein (1495-1543) representing St. Sebastian in a flower-garden. Of the plants many are clearly recognizable, and among others there is one of the "one-leaved" variety of the strawberry, which may still be met with in botanical gardens. In the year 1671 a Dutch botanist, Abraham Munting published [165] a large volume on garden-plants, containing a great number of very good engravings. Most of them of course show normal plants, but intermixed with these are varieties, that are still in cultivation and therefore must be at least two centuries old. Others, though not figured, are easily recognized by their names and descriptions. The c.o.c.ks...o...b..is the most widely known, but many white or double flowered varieties were already cultivated at that time. The striped Jalappa, the crested Sedum, the fasciated crown-imperial, white strawberries, red gooseberries and many others were known to Munting.

Some varieties are as old as culture itself, and it is generally known that the Romans cultivated the white form of the opium-poppy and used the foliage of the red variety of the sugarbeet as a vegetable.

In our time flowers and fruits are changing nearly as rapidly as the fancies and tastes of men. Every year new forms are introduced and usurp the place of older ones. Many are soon forgotten. But if we look at old country gardens, a goodly number of fine and valued old sorts are still to be found. It would be worth while to make special collections of living plants of old varieties, which surely would be a good and interesting work and bring about a conviction [166] of the stability of pure strains. Coming now to the other side of the question, we may consider those cases of reversion which have been recorded from time to time, and which always have been considered as direct proofs of the varietal character of the reverting form. Reversion means the falling back or returning to another type, and the word itself expresses the idea that this latter type is the form from which the variety has arisen.

Some instances of atavism of this kind are well known, as they are often repeated by individuals that are multiplied by buds or by grafting.

Before looking attentively into the different features of the many cases of rare reversions it will be advisable to quote a few examples.

The flowering-currant of the Pacific Coast or North American scarlet ribes (_Ribes sanguineum_), a very popular ornamental shrub, will serve as a good example. It is prized because of its brilliant red racemes of flowers which blossom early in the spring, before the appearance of the leaves. From this species a white form has arisen, which is an old and widely cultivated one, but not so highly prized because of its pale flowers. These are not of a pure white, but have retained a faint reddish hue. The young twigs and the stalks of the [167] leaves afford an instance of correlated variability since in the species the red color shows itself clearly mixed with the green, while in the variety this tinge is wholly wanting.

Occasionally this white-flowered currant reverts back to the original red type and the reversion takes place in the bud. One or two buds on a shrub bearing perhaps a thousand bunches of white flowers produce twigs and leaves in which the red pigment is noticeable and the flowers of which become brightly colored. If such a twig is left on the shrub, it may grow further, ramify and evolve into a larger group of branches. All of them keep true to the old type. Once reverted, the branches remain forever atavistic. It is a very curious sight, these small groups of red branches among the many white ones. And for this reason attention is often called to it, and more than once I myself have had the opportunity of noting its peculiarities. It seems quite certain that by planting such shrubs in a garden, we may rely upon seeing sooner or later some new buds reverting to the prototype.

Very little attention seems. .h.i.therto to have been given to this curious phenomenon, though in many respects it deserves a closer investigation.

The variety is said to have originated from seed in Scotland, many years ago, and [168] seems to be propagated only by cuttings or by grafting.

If this is true, all specimens must be considered as const.i.tuting together only one individual, notwithstanding their wide distribution in the gardens and parks of so many countries. This induces me to suppose, that the tendency to reversion is not a character of the variety as such, but rather a peculiarity of this one individual. In other words it seems probable that when the whitish variety arises a second time from the red species, it is not at all necessary that it should exhibit this same tendency to revert. Or to put it still in another way, I think that we may suppose that a variety, which might be produced repeatedly from the same original stock, would only in rare individuals have a tendency to revert, and in most cases would be as absolutely constant as the species itself.

Such a conception would give us a distinct insight into the cause of the rarity of these reversions. Many varieties of shrubs and trees have originated but once or twice. Most of them must therefore, if our supposition is correct, be expected to be stable and only a few may be expected to be liable to reversions.

Among the conifers many very good cases of reversions by buds are to be found in gardens and gla.s.shouses. They behave exactly like the whitish currant. But as the varietal characters [169] are chiefly found in the foliage and in the branches, these aberrations are to be seen on the plants during the whole year. Moreover they are in some cases much more numerous than in the first instance. The _Cryptomeria_ of j.a.pan has a variety with twigs resembling ropes. This is not caused by a twisting, but only by a curvature of the needles in such a way that they seem to grow in spiral lines around the twigs. This variety often reverts to the type with widely spread, straight needles. And on many a specimen four, five, or more reverted branches may be seen on different parts of the same shrub. Still more widely cultivated is the shrub called _Cephalotaxus pedunculata fastigiata_, and more commonly known under its old name of _Podocarpus koraiana_. It is the broomlike variety of a species, nearly allied to the common American and European species of yew, (_Taxus minor_ and _T. baccata_). It is a low shrub, with broadly linear leaves of a clear green. In the species the leaves are arranged in two rows, one to the left and one to the right of the horizontally growing and widely spreading branches. In the variety the branches are erect and the leaves inserted on all sides. When sporting, it returns to the bilateral prototype and flat wings of fan-shaped twigs are produced laterally on its dense broom-like tufts.

[170] Wherever this variety is cultivated the same reversion may be seen; it is produced abundantly, and even under seemingly normal circ.u.mstances. But as in the case of the _Ribes_ all the specimens are derived by buds from a single original plant. The variety was introduced from j.a.pan about the year 1860, but is probably much older. Nothing is known as to its real origin. It never bears flowers or fruits. It is curious to note that the a.n.a.logous variety of the European yew, _Taxus baccata fastigiata_, though much more commonly cultivated than the _Cephalotaxus_, never reverts, at least as far as I have been able to ascertain. This clearly corroborates the explanation given above.

After considering these rare instances of more widely known reversions, we may now examine the question of atavism from a broader point of view.

But in doing so it should once more be remembered, that all cases of hybridism and also all varieties sporting annually or frequently, are to be wholly excluded. Only the very rare occurrence of instances of atavism in varieties that are for the rest known to be absolutely constant, is to be considered.

Atavism or reversion is the falling back to a prototype. But what is a prototype? We may take the word in a physiologic or in a systematic sense. Physiologically the signification is a [171] very narrowly restricted one; and includes only those ancestors from which a form is known to have been derived. But such evidence is of course historic. If a variety has been observed to spring from a definite species, and if the circ.u.mstances have been sufficiently ascertained not to leave the slightest doubt as to its pure origin, and if moreover all the evidence has been duly recorded, we may say that the origin of the variety is historically known. In most cases we must be content with the testimony, given somewhat later, and recorded after the new variety had the opportunity of showing its greater merits.

If it now happens that such a variety of recorded origin should occasionally revert to its parent-species, we have all we can wish for, in the way of a thoroughly proved case of atavism. But such instances are very rare, as the birth of most varieties has only been very imperfectly controlled.

Next to this comes the systematic relation of a variety to its species.

The historic origin of the variety may be obscure, or may simply be forgotten. But the distinguis.h.i.+ng marks are of the order described in our last lecture, either in the positive or in the negative direction, and on this ground the rarer form is considered to be a variety of the more wide-spread one. If [172] now the presumed variety sports and runs over to the presumed type, the probability of the supposed relation is evidently enhanced. But it is manifest that the explanation rests upon the results of comparative studies, and not upon direct observations of the phenomena themselves.

The nearer the relations between the two types in question, the less exposed to doubt and criticism are the conclusions. But the domain of atavism is not restricted to the cases described. Quite on the contrary the facts that strike us most forcibly as being reversions are those that are apt to give us an insight into the systematic affinity of a higher degree. We are disposed to make use of them in our attempts to perfect the natural system and to remould it in such a way as to become a pedigree of the related groups. Such cases of atavism no doubt occur, but the anomalies referred to them must be interpreted merely on the ground of our a.s.sumptions as to the relative places in the system to be a.s.signed to the different forms.

Though such instances cannot be considered as belonging strictly to the subject we are dealing with, I think it may be as well to give an example, especially as it affords an occasion for referring to the highly important researches of Heinricher on the variability and atavistic [173] tendencies of the pale blue flag or _Iris pallida_. The flowers of the blue flags have a perianth of six segments united below into a tube. The three outer parts are dilated and spreading, or reflexed, while the three inner usually stand erect, but in most species are broad and colored like the outer ones. Corresponding to the outer, perianth-segments are the three stamens and the three, petal-like divisions of the style, each bearing a transverse stigma immediately above the anther. They are pollinated by b.u.mble-bees, and in some instances by flies of the genus _Rhingia_, which search for the honey, brush the pollen out of the anthers and afterwards deposit it on the stigma. According to systematic views of the monocotyledons the original prototype of the genus _Iris_ must have had a whorl of six equal, or nearly equal perianth-segments and six stamens, such as are now seen in the more primitive types of the family of the lilies, as for instance in the lilies themselves, the tulips, hyacinths and others. As to the perianth this view is supported by the existence of one species, the _Iris falcifolia_, the perianth of which consists of six equal parts.

But species with six stamens are wholly lacking. Heinricher however, in cultivating some anomalous forms of _Iris pallida_, succeeded in filling out this gap and in producing [174] flowers with a uniform perianth and six stamens, recalling thereby the supposed ancestral type. The way in which he got these was as follows: he started from some slight deviations observed in the flowers of the pale species, sowed the seeds in large numbers and selected from the seedlings only those which clearly showed anomalies in the expected atavistic direction. By repeating this during several generations he at last reached his goal and was able to give reality to the prototype, which formerly was only a hypothetical one. The _Iris kaempferi_, a large-flowered j.a.panese species much cultivated in gardens, is very variable in the number of the different parts of its flowers, and may in some instances be seen even with six stamens. If studied in the same way as Heinricher's iris, it no doubt will yield highly interesting and confirmatory results.

Many other instances of such systematic atavism could be given, and every botanist can easily add some from memory. Many anomalies, occurring spontaneously, are evidently due to the same principle, but it would take too long to describe them.