Species and Varieties, Their Origin by Mutation Part 10

Varieties have to be freed by selection from all such impurities, since isolation is a means which is quite impracticable under ordinary circ.u.mstances. Isolation is a scientific requirement that should never be neglected in experiments, indeed it may be said to be the first and most important requisite for all exact research in questions of variability and inheritance. But in cultivating large fields of allied varieties for commercial purposes, it is impossible to grow them at such distances from each other [197] as to prevent cross-pollination by the visits of bees.

This purification must be done in nearly every generation. The oldest varieties are to be subjected to it as well as the latest. There is no regular amelioration, no slow progression in the direction of becoming free from these admixtures. Continuous selection is indispensable to maintain the races in the degree of purity which is required in commerce, but it does not lead to any improvement. Nor does it go so far as to become unnecessary in the future. This shows that there must be a continuous source of impurities, which in itself is not neutralized by selection, but of which selection can only eliminate the deteriorating elements.

The same selection is usually applied to new varieties, when they occasionally arise. In this case it is called "fixing," as gardeners generally believe that through selection the varieties are brought to the required degree of purity. This belief seems to rest mainly on observations made in practice, where, as we have seen, isolation is of very rare application. Most varieties would no doubt be absolutely pure from the first moment of their existence, if it were only possible to have them purely fertilized. But in practice this is seldom to be obtained. Ordinarily the breeder is content with such slow [198]

improvement as may be obtained with a minimum of cost, and this mostly implies a culture in the same part of the nursery with older varieties of the same species. Three, four or five years are required to purify the novelty, and as this same length of time is also required to produce sufficient quant.i.ties of seed for commercial purposes, there is no strong desire to shorten the period of selection and fixation. I had occasion to see this process going on with sundry novelties at Erfurt in Germany. Among them a chamois-colored variety of the common stock, a bluish _Clarkia elegans_ and a curiously colored opium-poppy may be mentioned. In some cases the crossfertilization is so overwhelming, that in the next generation the novelty seems entirely to have disappeared.

The examples given may suffice to convey a general idea of the phenomenon, ordinarily called atavism by gardeners, and considered mostly to be the effect of some innate tendency to revert to the ancestral form. It is on this conception that the almost universal belief rests, that varieties are distinguished, as such, from species by their inconstancy. Now I do not deny the phenomenon itself. The impurity of seeds and cultures is so general and so manifest, and may so easily be tested by every one [199] that it cannot reasonably be subjected to any doubt. It must be conceded to be a fact, that varieties as a rule revert to their species under the ordinary circ.u.mstances of commercial culture. And I cannot see any reason why this fact should not be considered as stating a princ.i.p.al difference between varieties and species, since true species never sport into one another.

My objection only refers to the explanation of the observed facts.

According to my view nearly all these ordinary reversions are due to crosses, and it is for this reason that I proposed to call them by a separate name, that of "vicinists." Varieties then, by means of such spontaneous intercrossing sport into one another, while species either do not cross, or when crossing produce hybrids that are otherwise const.i.tuted and do not give the impression of atavistic reversion.

I must not be content with proposing this new conception, but must give the facts on which this a.s.sumption rests. These facts are the results of simple experiments, which nevertheless are by no means easy to carry out, as they require the utmost care to secure the absolute purity of the seeds that are employed. This can only be guaranteed by previous cultures of isolated plants or groups of plants, or by artificial pollination.

[200] Once sure of this preliminary condition, the experiment simply consists in growing a variety at a given distance from its species and allowing the insects to transfer the pollen. After harvesting the seed thus subjected to the presumed cause of the impurities, it must be sown in quant.i.ties, large enough to bring to light any slight anomaly, and to be examined during the period of blooming.

The wild seash.o.r.e aster, _Aster Tripolium_, will serve as an example. It has pale violet or bluish rays, but has given rise to a white variety, which on testing, I have found pure from seed. Four specimens of this white variety were cultivated at a distance of nearly 100 meters from a large lot of plants of the bluish species. I left fertilization to the bees, harvested the seeds of the four whites separately and had from them the following year more than a thousand flowering plants. All of them were of the purest white, with only one exception, which was a plant with the bluish rays of the species, wholly reverting to its general type. As the variety does not give such reversions when cultivated in isolation, this sport was obviously due to some cross in the former year. In the same way I tried the white Jacob's ladder, _Polemonium coeruleum_ alb.u.m in the neighborhood of the blue-flowered species, the distance [202] in this case being only 40 meters. Of two hundred seeds one became a blue atavist, or rather vicinist, while all others remained true to the white type. The same was observed in the white creeping thyme, or _Thymus Serpyllum alb.u.m_, and the white self-heal, _Brunella vulgaris alba_, gave even so much as 28% seedlings with purple corollas out of some 400 specimens, after being cultivated in close proximity to its parent-species. I have tried many other species, but always with the same result. Such atavists only arise by cultivation in the proximity of allied varieties, never in isolation.

They are not real atavists, but only vicinists.

In order to show this yet more clearly, I made another experiment with the white selfheal. I had a lot of the pinnate-leaved variety with purple flowers and somewhat stouter stems, and cultivated single plants of the whiteflowering sort at distances that varied from 2-16 meters.

The seeds of each plant were collected and sown separately, those of the nearest gave up to 5 or 6 hybrids from the seeds of one parent, while those of the farthest gave only one purple-flowered plant for each parent. Evidently the chance of the pollen being carried by bees is much greater on short than on longer distances.

True hybrids between species may arise in quite the same way, and since it is obviously impossible to attribute them to an innate tendency to reversion, they afford an absolutely irrefutable proof of the a.s.sertion that pollen is often brought by insects from one lot of plants to another. In this way I obtained a hybrid between the common Jacob's ladder and the allied species _Polemonium dissectum_. With a distance of 100 meters between them I had two hybrid seeds among a hundred of pure ones. At a similar distance pollen was carried over from the wild radish, _Rapha.n.u.s Raphanistrum_, to the allied _Rapha.n.u.s caudatus_, and I observed the following year some very nice hybrids among my seedlings.

A hybrid-bean between _Phaseolus na.n.u.s_ and _P. multiflorus_, and some hybrids between the yellow daisy, _Chrysanthemum segetum_ and the allied _Chrysanthemum coronarium_ or ox-eye daisy which also arose spontaneously in my garden between parents cultivated at recorded distances, might further be noted. Further details of these experiments need not be given. Suffice to say, that occasional crosses between species do occur, and not even rarely, that they are easily recognized as such and cannot be confused with cases of atavism, and that therefore they give proof to the a.s.sumption that in the same way crosses ordinarily occur also between varieties [203] of the same species, if cultivated at small distances apart, say 40-50 meters or even more.

Vicinism therefore, may play a part in all such cultures, enough to account for all the impurities observed in the nurseries or in commercial seed-samples.

Of course this whole discussion is limited to such species as are not only as a rule visited by insects, but are dependent on these visits for their fertilization. Most of our garden-flowers are included in this category. If not then we may expect to find the cultures and seeds pure, irrespective of the distances between allied varieties, as for instance with peas, which are known to be self-fertilizing. Another instance is given by the barley. One of the most curious anomalous varieties of this cereal, is the "Nepaul-barley," with its small advent.i.tious flowers on the palets or inner scales. It is a very old, widely cultivated sort, which always comes true from seed, and which has been tested in repeated experiments in my garden. The spikelets of this curious plant are oneflowered and provided with two linear glumes or outer scales. Of the inner scales or palets, the outer one is three-lobed at the summit, hence the varietal name of _Hordeum vulgare trifurcatum_. The central lobe is oblong and hollow, covering a small supernumerary floret inserted [204] at its base. The two lateral lobes are narrower, sometimes linear, and are often prolonged into an awn, which is generally turned away from the center of the spike. The central lobe sometimes bears two florets at its base, although but one is usually present and it may be incomplete.

I might give one more instance from my own experience. A variety of the evening-primrose with small linear petals was once found by one of my sons growing wild near Amsterdam. It was represented by only one individual, flowering among a great many of the ordinary type with broad petals. But the evening-primroses open their anthers in the morning, fertilize themselves during the day, and only display their beautiful flowers in the evening, after the pollination has been accomplished.

They then allure evening moths, such as _Agrotis_ and _Plusia_, by their bright color, their sweet honeysmell and their nectar. Since the fertilization is accomplished many hours before opening, crosses are effected only in rare instances, and the seeds commonly remain true to the parent type. The seeds of this one plant, when sown separately in my garden, produced exclusively flowers with the small linear petals of their parent. Although I had a hundred individuals bearing many thousands of flowers, there was not an instance of reversion. And such would [205] immediately have been observed, had it occurred, because the hybrids between the cruciate and the normal flowers are not intermediate, but bear the broad petals of the _O. biennis_.

We may now take up another phase of the question, that of the running out of new varieties, shortly after their introduction into a new country, or later.

The most widely known instance of this is that of the American corn in Baden, recorded by Metzger and quoted by Darwin as a remarkable instance of the direct and prompt action of climate on a plant. It has since been considered as a reversion to the old type. Such reversions invariably occur, according to Wallace, in cases of new varieties, which have been produced quickly. But as we now know, such reversions are due to spontaneous crosses with the old form, and to the rule, that the hybrids of such origin are not intermediate, but a.s.sume the features of the older of the two parents. In the light of this experience, Metzger's observation becomes a typical instance of vicinism. It relates to the "Tuscarora" corn of St. Louis, a variety with broad and flat white seeds.

About the year 1840, this corn was introduced into Baden in Germany, and cultivated by Metzger. In the first year it came true to type, and [206]

attained a height of 12 feet, but the season did not allow its seeds to ripen normally. Only a few kernels were developed before the winter.

From this seed plants of a wholly different type came the next year, of smaller stature, and with more brownish and rounded kernels. They also flowered earlier and ripened a large number of seeds. The depression on the outer side of the seed had almost disappeared, and the original white had become darker. Some of the seeds had even become yellow and in their rounded form they approached the common European maize. Obviously they were hybrids, a.s.suming the character of their pollen-parent, which evidently was the ordinary corn, cultivated all around. The observation of the next year showed this clearly, for in the third generation nearly all resemblance to the original and very distinct American species was lost. If we a.s.sume that only those seeds ripened which reverted to the early-ripening European type, and that those that remained true to the very late American variety could not reach maturity, the case seems to be wholly comprehensible, without supposing any other factors to have been at work than those of vicinism, which though unknown at the period of Metzger's and Darwin's writings, seems now to be fully understood. No innate tendency to run out and no changing influence of the climate are required for an adequate explanation of the facts.

In the observation quoted, what astonishes us most, is the great rapidity of the change, and the short time necessary for the offspring of the accidental crosses to completely supplant the introduced type. In the lecture on the selection of elementary species, closely a.n.a.logous cases were described. One of them was the wild oat or _Avena fatua_ which rapidly supplants the cultivated oats in bad years in parts of the fields. Other instances were the experiments of Risler with the "Galland" wheat and the observation of Rimpau on "Rivett's bearded"

wheat.

Before leaving the question of vicinism and its bearing on the general belief of the instability of varieties, which when tested with due care, prove to be quite stable, it may be as well to consider the phenomena from another point of view. Our present knowledge of the effects of crosses between varieties enables us to formulate some general rules, which may be used to calculate, and in some way to predict, the nature of the impurities which necessarily attend the cultivation of allied species in close vicinity. And this mode of cultivation being in almost universal use in the larger nurseries, [208] we may, by this discussion, arrive at a more scientific estimation of the phenomena of vicinism, hitherto described.

The simplest case that may be given, is when an ordinary retrograde variety is cultivated with the species to which it belongs. For instance, if dwarfs are cultivated next to the taller type, or a white variety next to the red or blue-flowering species, or thornless forms in neighboring beds with the armed species. Bees and b.u.mble-bees, b.u.t.terflies and moths are seen flying from flower to flower, collecting the honey and carrying pollen. I frequently saw them cross the limits of the neighboring beds. Loaded with the pollen of the variety they visit the flowers of the different species and impregnate the stigma with it.

And returning to the variety they bring about similar crosses in the flowers of the latter. Hybrid seeds will develop in both cases and become mixed with the crop. We now have to ask the question, what sort of plants will arise from these hybrid seeds. As a general rule we may state, first, that the hybrids of either form of cross are practically the same, secondly that they are not intermediate, but that the character of one parent prevails to the almost absolute exclusion of the other and in the third place that the older character dominates the younger.

[209] The hybrid offspring will therefore, in the main, have the character of the species and be indistinguishable from it, or show only such differences as escape ordinary observation. When occurring in the seeds of the variety they betray themselves as soon as the differential characters are displayed. Between the thousands of flowering plants of a white variety the hybrids will instantly catch the eye by their red or blue corollas. Quite the contrary effect results from the admixture of hybrids with the seeds of the species itself. Here no difference will show itself, even in the fullest bloom. The effect of the spontaneous crosses will pa.s.s un.o.bserved. The strain, if pure in the first year, will seem to be still in the same condition. Or in other terms, the unavoidable spontaneous crosses will disturb the purity of the variety in the second year, while they do not seem to interfere at all with the uniformity of the species. The direct effect of the visits of the insects is evident in the first case, but pa.s.ses un.o.bserved in the latter.

From this it would seem, that spontaneous crosses are hurtful to varieties, but are innocuous to true species. Certainly this would be so, were there no selection. But it is easily seen, that through this operation the effect becomes quite the opposite. For when the fields [210] are inspected at the time of the fullest display of the varietal characters, the obvious hybrids will be eliminated, but the hidden ones will of necessity be spared, as they are concealed among the species by the similarity of their type. Hence, the harvest of the variety may be rendered pure or nearly so, while the harvest of the species will retain the seeds of the hybrids. Moreover it will contain seeds originated by the spontaneous but numerous crosses of the true plants with the spa.r.s.ely intermingled hybrids.

This brings us to the question, as to what will be the visible consequences of the occurrence of such invisible hybrids in the following generation. In opposition to the direct effects just described, we may call them indirect. To judge of their influence, we must know how hybrid seeds of the first generation behave.

In one of our lectures we will deal with the laws that show the numerical relations known as the laws of Mendel. But for our present purpose, these numerical relations are only of subordinate importance.

What interests us here is the fact that hybrids of varieties do not remain constant in the second generation but usually split as it is said, remaining hybrid only in part of their offspring, the other portion returning to the parental types. This however, will show itself only in those individuals [211] which rea.s.sume the character of the varietal parent, all the others apparently remaining true to the type of the species. Now it is easy to foresee what must happen in the second generation if the first generation after the cross is supposed to be kept free from new vicinistic influences, or from crosses with neighboring varieties.

We may limit ourselves in the first place to the seeds of the un.o.bserved hybrids. For the greater part they will repeat the character of their parents and still remain concealed. But a small number will display the varietal marks, as for example showing white flowers in a field of blue ones. Hence, the indirect consequence of the spontaneous crosses will be the same in the species, as was the direct effect in the variety, only that it appears a year later. It will then be eliminated in the process of selection.

Obviously, this elimination conduces only to a partial purification. The conspicuous plants will be destroyed, but a greater number of hybrids will remain, still concealed by their resemblance to the general type and will be spared to repeat the same process next year. So while the variety may be freed every year from the impurities brought into it in the preceeding summer, the admixtures of the species [212] will continue during a number of years, and it may not be possible to get rid of them at all.

It is an often recurring a.s.sertion that white varieties of colored species are the most stable of all horticultural races. They are often said to be at least as constant as the species itself, and even to surpa.s.s it in this quality. With our present state of knowledge, the explanation of this general experience is easily given. For selection removes the effect of spontaneous crosses from the variety in each year, and renders it practically pure, while it is wholly inadequate to produce the same effects on the species, because of the concealed hybrids.

The explanation given in this simple instance may be applied to the case of different varieties of the same species, when growing together and crossed naturally by insects.

It would take too long to go into all the details that present themselves here to the student of nature and of gardens. I will only state, that since varieties differ princ.i.p.ally from their species by the lack of some sharp character, one variety may be characterized by the lack of color of the flowers, another by the lack of p.u.b.escence, a third by being dwarfed, and so on. Every character must be studied separately in its effects on the offspring [213] of the crosses. And it is therefore easily seen, that the hybrids of two varieties may resemble neither of them, but revert to the species itself. This is necessarily and commonly the case, since it is always the older or positive characters that prevail in the hybrids and the younger or negative that lie hidden. So for instance, a blue dwarf larkspur, crossed with a tall white variety, must give a tall blue hybrid, rea.s.suming in both characters the essentials of the species.

Keeping this rule in view, it will be easy to calculate what may be expected from spontaneous crosses for a wide range of occurrences, and thus to find an explanation of innumerable cases of apparent variability and reversion in the principle of vicinism. Students have only to recollect that specific characters prevail over varietal ones, and that every character competes only with its own antagonist. Or to give a sharper distinction: whiteness of flowers cannot be expected to be interchanged with p.u.b.escence of leaves.

In concluding I will point out another danger which in the principle of vicinism may be avoided. If you see a plant in a garden with all the characteristics of its species, how can you be sure that it is truly a representative of the species, and not a hybrid? The prevailing [214]

characters are in either case the same. Perhaps on close inspection you may find in some cases a slight difference, some character being not as fully developed in the hybrid as in the species. But when such is not the case, or where the opportunity for such a closer examination is wanting, a hybrid may easily be taken for a specimen of the pure race.

Now take the seeds of your plant and sow them. If you had not supposed it to be hybrid you will be astonished at finding among its progeny some of a wholly different type. You will be led to conclude that you are observing a sudden change in structure such as is usually called a sport.

Or in other words you may think that you are a.s.sisting at the origination of a new variety. If you are familiar with the principle of vicinism, you will refrain from such an inference and consider the supposition of a hybrid origin. But in former times, when this principle was still unknown and not even guessed at, it is evident that many mistakes must have been made, and that many an instance, which until now has been considered reliable proof of a so-called single variation, is in fact only a case of vicinism. In reading the spa.r.s.e literature on sports, numerous cases will be found, which cannot stand this test. In many instances crossing must be looked to as an explanation, [215] and in other cases the evidence relied upon does not suffice to exclude this a.s.sumption. Many an old argument has of late lost its force by this test.

Returning to our starting point we may now state that regular reversions to a specific type characterize a form as a variety of that species.

These reversions, however, are not due to an innate tendency, but to un.o.bserved spontaneous crosses.

[217]

LECTURE VIII

LATENT CHARACTERS

No organism exhibits all of its qualities at any one time. Many of them are generally dormant and await a period of activity. For some of them this period comes regularly, while in others the awakening depends upon external influences, and consequently occurs very irregularly. Those of the first group correspond to the differences in age; the second const.i.tute the responses of the plant to stimuli including wound-injuries.

Some ill.u.s.trative examples may be quoted in order to give a precise idea of this general conception of dormant or latent characters. Seed leaves are only developed in the seed and the seedling; afterwards, during the entire lifetime of the plant, the faculty of producing them is not made use of. Every new generation of seeds however, bears the same kind of seed leaves, and hence it is manifest that it is the same quality, which shows itself from time to time.

The primary leaves, following the seed-leaves, are different in many species, from the later ones, and the difference is extremely p.r.o.nounced in some cases of reduction. Often, when leaves are lacking in the adult plant, being replaced by flattened stalks as in the case of the acacias, or by thorns, or green stems and twigs as in the p.r.i.c.kly broom or _Ulex europaeus_, the first leaves of the young plant may be more highly differentiated, being pinnate in the first case and bearing three leaflets in the second instance. This curious behavior which is very common, brings the plants, when young, nearer to their allies than in the adult state, and manifestly implies that the more perfect state of the leaves is latent throughout the life of the plant, with the exception of the early juvenile period.

_Eucalyptus Globulus_, the Australian gum tree, has opposite and broadly sessile leaves during the first years of its life. Later these disappear and are replaced by long sickle-shaped foliage organs, which seem to be scattered irregularly along the branches. The juvenile characters manifestly lie dormant during the adult period, and that this is so, may be shown artificially by cutting off the whole crown of the tree, when the stem responds by producing numerous new branches, which a.s.sume the [218] shape proper to the young trees, bearing sessile and opposite leaves.

It seems quite unnecessary to give further instances. They are familiar to every student. It is almost safe to say that every character has its periods of activity and of inactivity, and numbers of flowers and fruits can be mentioned as ill.u.s.trations. One fact may be added to show that nearly every part of the plant must have the power of producing all or nearly all the characters of the individual to which it belongs. This proof is given by the formation of advent.i.tious buds. These, when once formed, may grow out into twigs, with leaves and flowers and roots. They may even be separated from the plants and used as cuttings to reproduce the whole. Hence we may conclude that all tissues, which possess the power of producing advent.i.tious buds, must conceal in a latent state, all the numerous characters required for the full development of the whole individual.

Advent.i.tious buds may proceed from specialized cells, as on the margin of the leaves of _Bryophyllum calycinum_; or from the cells of special tissues, as in the epidermis of the begonias; or they may be provoked by wounds in nearly every part of the plant, provided it be able to heal the wound by swelling tissues or [219] callus. The best instance is afforded by elms and by the horse-chestnut. If the whole tree is hewn down the trunk tries to repair the injury by producing small granulations of tissue between the wood and the bark, which gradually coalesce while becoming larger. From this new ring of living matter innumerable buds arise, that expand into leafy branches, showing clearly that the old trunk possesses, in a latent state, all the qualities of the whole crown. Indeed, such injured stumps may be used for the production of copses and hedges.

All the hitherto recorded cases of latency have this in common, that they may become active during the life-time of any given individual once, or oftener. This may be called the ordinary type of latency.