Species and Varieties, Their Origin by Mutation Part 29

Thousands of seeds with perhaps wide circles of variability are ripened each year, but only those that belong to the existing old narrow circles survive. How different would Nature appear to us if she were free to evolve all her potentialities!

Darwin himself was struck with this lack of harmony between common observations and the probable real state of things. He discussed it in connection with the cranesbill of the Pyrenees (_Geranium pyrenaic.u.m_).

He described how this fine little plant, which has never been extensively cultivated, had escaped from a garden in Staffords.h.i.+re and had succeeded in multiplying itself so as to occupy a large area.

In doing so it had evidently found place for an uncommonly large number of plantlets from its seeds and correspondingly it had commenced to vary in almost all organs and qualities and nearly in all imaginable directions. It displayed under these exceptional circ.u.mstances a capacity which never had been exceeded and [600] which of course would have remained concealed if its multiplication had been checked in the ordinary way.

Many species have had occasion to invade new regions and cover them with hundreds of thousands of individuals. First are to be cited those species which have been introduced from America into Europe since the time of Columbus, or from Europe into this country. Some of them have become very common. In my own country the evening-primroses and Canada fleabane or are examples, and many others could be given. They should be expected to vary under these circ.u.mstances in a larger degree. Have they done so? Manifestly they have not struck out useful new characters that would enable their bearers to found new elementary species. At least none have been observed. But poor types might have been produced, and periods of mutability might have been gone through similar to that which is now under observation for Lamarck's evening primrose in Holland.

From this discussion we may infer that the chances of discovering new mutating species are great enough to justify the utmost efforts to secure them. It is only necessary to observe large numbers of plants, grown under circ.u.mstances which allow the best opportunities for [601]

all the seeds. And as nature affords such opportunities only at rare intervals, we should make use of artificial methods. Large quant.i.ties of seed should be gathered from wild plants and sowed under very favorable conditions, giving all the nourishment and s.p.a.ce required to the young seedlings. It is recommended that they be sown under gla.s.s, either in a gla.s.s-house or protected against cold and rain by gla.s.s-frames. The same lot of seed will be seen to yield twice or thrice as many seedlings if thus protected, compared with what it would have produced when sown in the field or in the garden. I have nearly wholly given up sowing seeds in my garden, as circ.u.mstances can be controlled and determined with greater exact.i.tude when the sowing is done in a gla.s.shouse.

The best proof perhaps, of the unfavorable influence of external conditions for slightly deteriorated deviations is afforded by variegated leaves. Many beautiful varieties are seen in our gardens and parks, and even corn has a variety with striped leaves. They are easily reproduced, both by buds and by seeds, and they are the most ordinary of all varietal deviations. They may be expected to occur wild also. But no real variegated species, nor even good varieties with this attribute occurs in nature. [602] On the other hand occasional specimens with a single variegated leaf, or with some few of them, are actually met with, and if attention is once drawn to this question, perhaps a dozen or so instances might be brought together in a summer. But they never seem to be capable of further evolution, or of reproducing themselves sufficiently and of repeating their peculiarity in their progeny. They make their appearance, are seen during a season, and then disappear.

Even this slight incompleteness of some spots on one or two leaves may be enough to be their doom.

It is a common belief that new varieties owe their origin to the direct action of external conditions and moreover it is often a.s.sumed that similar deviations must have similar causes, and that these causes may act repeatedly in the same species, or in allied, or even systematically distant genera. No doubt in the end all things must have their causes, and the same causes will lead under the same circ.u.mstances to the same results. But we are not justified in deducing a direct relation between the external conditions and the internal changes of plants. These relations may be of so remote a nature that they cannot as yet be guessed at. Therefore only direct experience may be our guide. Summing up the result of our facts and discussions [603] we may state that wild new elementary species and varieties are recorded to have appeared from time to time. Invariably this happened by sudden leaps and without intermediates. The mutants are constant when propagated by seed, and at once const.i.tute a new race. In rare instances this may be of sufficient superiority to win a place for itself in nature, but more often it has qualities which have led to its introduction into gardens as an ornamental plant or into botanical gardens by reason of the interest afforded by their novelty, or by their anomaly.

Many more mutations may be supposed to be taking place all around us, but artificial sowings on a large scale, combined with a close examination of the seedlings and a keen appreciation of the slightest indications of deviation seem required to bring them to light.

[604]

LECTURE XXI

MUTATIONS IN HORTICULTURE

It is well known that Darwin based his theory of natural selection to a large extent upon the experience of breeders. Natural and artificial selection exhibit the same general features, yet it was impossible in Darwin's time to make a critical and comparative a.n.a.lysis of the two processes.

In accordance with our present conception there is selection of species and selection within the species. The struggle for life determines which of a group of elementary species shall survive and which shall disappear. In agricultural practice the corresponding process is usually designated by the name of variety-testing. Within the species, or within the variety, the sieve of natural selection is constantly eliminating poor specimens and preserving those that are best adapted to live under the given conditions. Some amelioration and some local races are the result, but this does not appear to be of much importance. On the contrary, the selection [605] within the race holds a prominent place in agriculture, where it is known by the imposing term, race-breeding.

Experience and methods in horticulture differ from those in agriculture in many points. Garden varieties have been tested and separated for a long time, but neither vegetables nor flowers are known to exhibit such motley groups of types as may be seen in large forage crops.

New varieties which appear from time to time may be ornamental or otherwise in flowers, and more or less profitable than their parents in vegetables and fruits. In either case the difference is usually striking, or if not, its culture would be unprofitable.

The recognition of useful new varieties being thus made easy, the whole attention of the breeder is reduced to isolating the seeds of the mutants that are to be saved and sown separately, and this process must be repeated during a few years, in order to produce the quant.i.ty of seed that is needed for a profitable introduction of the variety into commerce. In proportion to the abundance of the harvest of each year this period is shorter for some and longer for other species.

Isolation in practice is not so simple nor so easy an affair as it is in the experimental garden. Hence we have constant and nearly unavoidable [606] cross-fertilizations with the parent form or with neighboring varieties, and consequent impurity of the new strain. This impurity we have called vicinism, and in a previous lecture have shown its effects upon the horticultural races on one hand, and on the other, on the scientific value that can be ascribed to the experience of the breeder.

We have established the general rule that stability is seldom met with, but that the observed instability is always open to the objection of being the result of vicinism. Often this last agency is its sole cause; or it may be complicated with other factors without our being able to discern them.

Though our a.s.sertion that the practice of the horticulturist in producing new varieties is limited to isolation, whenever chance affords them, is theoretically valid, it is not always so. We may discern between the two chief groups of varieties. The retrograde varieties are constant, the individuals not differing more from one another than those of any ordinary species. The highly variable varieties play an important part in horticulture. Double flowers, striped flowers, variegated leaves and some others yield the most striking instances. Such forms have been included in previous lectures among the ever-sporting varieties, because their peculiar characters oscillate between two extremes, viz: [607] the new one of the variety and the corresponding character of the original species.

In such cases isolation is usually accompanied by selection: rarely has the first of a double, striped or variegated race well filled or richly striped flowers or highly spotted leaves. Usually minor degrees of the anomaly are seen first, and the breeder expects the novelty to develop its features more completely and more beautifully in subsequent generations. Some varieties need selection only in the beginning, in others the most perfect specimens must be chosen every year as seed-bearers. For striped flowers, it has been prescribed by Vilmorin, that seeds should be taken only from those with the smallest stripes, because there is always reversion. Mixed seed or seed from medium types would soon yield plants with too broad stripes, and therefore less diversified flowers.

In horticulture, new varieties, both retrograde and ever-sporting, are known to occur almost yearly. Nevertheless, not every novelty of the gardener is to be considered as a mutation in the scientific sense of the word. First of all, the novelties of perennial and woody species are to be excluded. Any extreme case of fluctuating variability may be preserved and multiplied in the vegetative way. Such types are designated [608] in horticulture as varieties, though obviously they are of quite another nature than the varieties reproduced by seed. Secondly, a large number, no doubt the greater number of novelties, are of hybrid origin. Here we may discern two cases. Hybrids may be produced by the crossing of old types, either of two old cultivated forms or newly introduced species, or ordinarily between an old and an introduced variety. Such novelties are excluded from our present discussion.

Secondly, hybrids may be produced between a true, new mutation and some of the already existing varieties of the same species. Examples of this obvious and usual practice will be given further on, but it must be pointed out now that by such crosses a single mutation may produce as many novelties as there are available varieties of the same species.

Summarizing these introductory remarks we must lay stress on the fact that only a small part of the horticultural novelties are real mutations, although they do occur from time to time. If useful, they are as a rule isolated and multiplied, and if necessary, improved by selection. They are in many instances, as constant from seed as the unavoidable influence of vicinism allows them to be. Exact observations on the origin, or on the degree of constancy, are usually lacking, [609]

the notes being ordinarily made for commercial purposes, and often only at the date of introduction into trade, when the preceding stages of the novelty may have been partly forgotten.

With this necessary prelude I will now give a condensed survey of the historical facts relating to the origin of new horticultural varieties.

An ample description has been given recently by Kors.h.i.+nsky, a Russian writer, who has brought together considerable historical material as evidence of the sudden appearance of novelties throughout the whole realm of garden plants.

The oldest known, and at the same time one of the most accurately described mutations is the origin of the cut-leaved variety of the greater celandine or _Chelidonium majus_. This variety has been described either as such, or as a distinct species, called _Chelidonium laciniatum_ Miller.

It is distinguished from the ordinary species, by the leaves being cut into narrow lobes, with almost linear tips, a character which is, as we have seen on a previous occasion, repeated in the petals. It is at present nearly as commonly cultivated in botanical gardens as the _C.

majus_, and has escaped in many localities and is observed to thrive as readily as the native wild [610] plants. It was not known until a few years before the close of the 16th century. Its history has been described by the French botanist, Rose. It was seen for the first time in the garden of Sprenger, an apothecary of Heidelberg, where the _C.

majus_ had been cultivated for many years. Sprenger discovered it in the year 1590, and was struck by its peculiar and sharply deviating characters. He was anxious to know whether it was a new plant and sent specimens to Clusius and to Plater, the last of whom transmitted them to Caspar Bauhin. These botanists recognized the type as quite new and Bauhin described it some years afterwards in his Phytopinax under the name of _Chelidonium majus foliis quernis_, or oak-leaved celandine. The new variety soon provoked general interest and was introduced into most of the botanical gardens of Europe. It was recognized as quite new, and repeated search has been made for it in a wild state, but in vain. No other origin has been discovered than that of Sprenger's garden.

Afterwards it became naturalized in England and elsewhere, but there is not the least doubt as to its derivation in all the observed cases.

Hence its origin at Heidelberg is to be considered as historically proven, and it is of course only legitimate to a.s.sume that it originated in [611] the year 1590 from the seeds of the _C. majus_. Nevertheless, this was not ascertained by Sprenger, and some doubt as to a possible introduction from elsewhere might arise. If not, then the mutation must have been sudden, occurring without visible preparation and without the appearance of intermediates.

From the very first, the cut-leaved celandine has been constant from seed. Or at least it has been propagated by seed largely and without difficulty. Nothing, however, is known about it in the first few years of its existence. Later careful tests were made by Miller, Rose and others and later by myself, which have shown its stability to be absolute and without reversion, and it has probably been so from the beginning. The fact of its constancy has led to its specific distinction by Miller, as varieties were in his time universally, and up to the present time not rarely, though erroneously, believed to be less stable than true species.

Before leaving the laciniate celandine it is to be noted that in crosses with _C. majus_ it follows the law of Mendel, and for this reason should be considered as a retrograde variety, the more so, as it is also treated as such from a morphological point of view by Stahl and others.

We now come to an enumeration of those cases in which the date of the first appearance [612] of a new horticultural variety has been recorded, and I must apologize for the necessity of again quoting many variations, which have previously been dealt with from another point of view. In such cases I shall limit myself as closely as possible to historical facts. They have been recorded chiefly by Verlot and Carriere, who wrote in Paris shortly after the middle of the past century, and afterwards by Darwin, Kors.h.i.+nsky, and others. It is from their writings and from horticultural literature at large that the following evidence is brought together.

A very well-known instance is that of the dwarf variety of _Tagetes signata_, which arose in the nursery of Vilmorin in the year 1860. It was observed for the first time in a single individual among a lot of the ordinary _Tagetes signata_. It was found impossible to isolate it, but the seeds were saved separately. The majority of the offspring returned to the parental type, but two plants were true dwarfs. From these the requisite degree of purity for commercial purposes was reached, the vicinists not being more numerous than 10% of the entire number. The same mutation had been observed a year earlier in the same nursery in a lot of _Saponaria calabrica_. The seeds of this dwarf repeated the variety in the next generation, but in the third none were observed. Then the variety was [613] thought to be lost, and the culture was given up, as the Mendelian law of the splitting of varietal hybrids was not known. According to our present knowledge we might expect the atavistic descendants of the first dwarf to be hybrids, and to be liable to split in their progeny into one-fourth dwarfs and three-fourths normal specimens. From this it is obvious that the dwarfs would have appeared a second time if the strain had been continued by means of the seeds of the vicinistic progeny.

In order to avoid a return to this phase of the question, another use of the vicinists should at once be pointed out. It is the possibility of increasing the yield of the new variety. If s.p.a.ce admits of sowing the seeds of the vicinists, a quarter of the progeny may be expected to come true to the new type, and if they were partly pollinated by the dwarfs, even a larger number would do so. Hence it should be made a rule to sow these seeds also, at least when those of the true representatives of the novelty do not give seed enough for a rapid multiplication.

Other dwarfs are recorded to have sprung from species in the same sudden and unexpected manner, as for instance _Ageratum coeruleum_ of the same nursery, further _Clematis Viticella nana_ and _Acer campestre nanum_.

_Prunus Mahaleb nana_ was discovered in 1828 in one [614] specimen near Orleans by Mme. LeBrun in a large culture of Mahaleb. _Lonicera tatarica nana_ appeared in 1825 at Fontenay-aux Roses. A tall variety of the strawberry is called "Giant of Zuidwijk" and originated at Boskoop in Holland in the nursery of Mr. van de Water, in a lot of seedlings of the ordinary strawberry. It was very large, but produced few runners, and was propagated with much difficulty, for after six years only 15 plants were available. It proved to be a late variety with abundant large fruit, and was sold at a high price. For a long time it was prominent in cultures in Holland only.

Varieties without p.r.i.c.kles are known to have originated all of a sudden in sundry cases. _Gleditschia sinensis_, introduced in 1774 from China, gave two seedlings without spines in the year 1823, in the nursery of Caumzet. It is curious in being one of the rare instances where a simultaneous mutation in two specimens is acknowledged, because as a rule, such records comply with the prevailing, though inexact, belief that horticultural mutations always appear in single individuals.

From Kors.h.i.+nsky's survey of varieties with cut leaves or laciniate forms the following cases may be quoted. In the year 1830 a nurseryman named Jacques had sown a large lot of elms, [615] _Ulmus pedunculata_. One of the seedlings had cut leaves. He multiplied it by grafting and gave it to the trade under the name of _U. pedunculata urticaefolia_. It has since been lost.

Laciniate alders seem to have been produced by mutation at sundry times.

Mirbel says that the _Alnus glutinosa laciniata_ is found wild in Normandy and in the forests of Montmorency near Paris. A similar variety has been met with in a nursery near Orleans in the year 1855. In connection with this discovery some discussion has arisen concerning the question whether it was probable that the Orleans strain was a new mutation, or derived in some way from the trees cited by Mirbel. Of course, as always in such cases, any doubt, once p.r.o.nounced, affects the importance of the observation for all time, since it is impossible to gather sufficient historical evidence to fully decide the point. The same variety had appeared under similar circ.u.mstances in a nursery at Lyons previously (1812).

Laciniated maples are said to be of relatively frequent occurrence in nurseries, among seedlings of the typical species. Loudon says that once 100 laciniated seedlings were seen to originate from seed of some normal trees. But in this case it is rather probable that the presumed [616]

normal parents were in reality hybrids between the type and the laciniated form, and simply split according to Mendel's law. This hypothesis is partly founded on general considerations and partly on experiments made by myself with the cut-leaved celandine, previously alluded to, which I crossed with the type. The hybrids repeated the features of the species and showed no signs of their internal hybrid const.i.tution. But the following year one-fourth of their progeny returned to the cut-leaved form. If the same thing has taken place in the case of Loudon's maples, but without their hybrid origin being known, the result would have been precisely what he observed.

_Broussonetia papyriffera dissecta_ originated about 1830 at Lyons, and a second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts, birches, beeches and others have mostly been found in the wild state, as I have already pointed out in a previous lecture. A similar variety of the elder, _Sambucus nigra laciniata_, and its near ally, _Sambucus racemosa laciniata_, are often to be seen in our gardens. They have been on record since 1886 and come true from seed, but their exact origin seems to have been forgotten. Cut-leaved walnuts have been known since 1812; they come true from seed, but are extremely liable to vicinism, a nuisance which is [617] ascribed by some authors to the fact that often on the same tree the male catkins flower and fall off several weeks before the ripening of the pistils of the other form of flowers.

Weeping varieties afford similar instances. _Sophora j.a.ponica pendula_ originated about 1850, and _Gleditschia triacanthos pendula_ some time later in a nursery at Chateau-Thierry (Aisne, France). In the year 1821 the bird's cherry, or _Prunus Padus_, produced a weeping variety, and in 1847 the same mutation was observed for the allied _Prunus Mahaleb_.

Numerous other instances of the sudden origin of weeping trees, both of conifers and of others, have been brought together in Kors.h.i.+nsky's paper. This striking type of variation includes perhaps the best examples of the whole historical evidence. As a rule they appear in large sowings, only one, or only a few at a time. Many of them have not been observed during their youth, but only after having been planted out in parks and forests, since the weeping characters show only after several years.

The monophyllous b.a.s.t.a.r.d-acacia originated in the same way. Its peculiarities will be dealt with on another occasion, but the circ.u.mstances of its birth may as well be given here. In 1855 in the nursery of Deniau, at Brain-sur-l'Authion (Maine et Loire), it appeared in a lot of [618] seedlings of the typical species in a single individual. This was transplanted into the Jardin des Plantes at Paris, where it flowered and bore seeds in 1865. It must have been partly pollinated by the surrounding normal representatives of the species, since the seeds yielded only one-fourth of true offspring. This proportion, however, has varied in succeeding years. Briot remarks that the monophyllous b.a.s.t.a.r.d acacia is liable to petaloid alterations of its stamens, which deficiency may encroach upon its fertility and accordingly upon the purity of its offspring.

Broom-like varieties often occur among trees, and some are known for their very striking reversions by buds, as we have seen on a previous occasion. They are ordinarily called pyramidal or fastigiate forms, and as far as their history goes, they arise suddenly in large sowings of the normal species. The fastigiate birch was produced in this way by Baumann, the _Abies concolor fastigiata_ by Thibault and Keteleer at Paris, the pyramidal cedar by Paillat, the a.n.a.logous form of _Wellingtonia_ by Otin. Other instances could easily be added, though of course some of the most highly prized broom-like trees are so old that nothing is known about their origin. This, for instance, is the case with the pyramidal yew-tree, _Taxus baccata fastigiata_. [619] Others have been found wild, as already mentioned in a former lecture.

An a.n.a.logous case is afforded by the purpleleaved plums, of which the most known form is Prunus p.i.s.sardi. It is said to be a purple variety of _Prunus cerasifera_, and was introduced at the close of the seventies from Persia, where it is said to have been found in Tabris. A similar variety arose independently and unexpectedly in the nursery of Spath, near Berlin, about 1880, but it seems to differ in some minor points from the Persian prototype.

A white variety of _Cyclamen vernum_ made its appearance in the year 1836 in Holland. A single individual was observed for the first time among a large lot of seedlings, in a nursery near Haarlem. It yielded a satisfactory amount of seed, and the progeny was true to the new type.

Such plants propagate slowly, and it was only twenty-seven years later (1863) that the bulbs were offered for sale by the Haarlem firm of Krelage & Son. The price of each bulb was $5.00 in that year, but soon afterwards was reduced to $1.00 each, which was about thrice the ordinary price of the red variety.

The firm of Messrs. Krelage & Son has brought into commerce a wide range of new bulb-varieties, all due to occasional mutations, some by seed and others by buds, or to the accidental [620] transference of new qualities into the already existing varieties by cross-pollination through the agency of insects. Instead of giving long lists of these novelties, I may cite the black tulips, which cost during the first few years of their introduction about $25.00 apiece.

Horticultural mutations are as a rule very rare, especially in genera or species which have not yet been brought to a high degree of variability.