Lecture Vii

FALSE ATAVISM OR VICINISM

About the middle of the last century Louis de Vilmorin showed that it was possible to subject plants to the methods of amelioration of races then in use for domestic animals, and since that time atavism has played a large part in all breeding-processes. It was considered to be the greatest enemy of the breeder, and was generally spoken of as a definite force, working against and protracting the endeavors of the horticulturist.

No clear conception as to its true nature had been formulated, and even the propriety of designating the observed phenomena by the term atavism seemed doubtful. Duchesne used this word some decades ago to designate those cases in which species or varieties revert spontaneously, or from unknown internal causes, to some long-lost characters of their ancestors. Duchesne's definition was evidently a sharp and useful one, since it developed for the first time the idea of latent or dormant qualities, formerly active, and awaiting probably through centuries an occasion to awaken, and to display the lost characters.

Cases of apparent reversion were often seen in nurseries, especially in flower culture, which under ordinary circumstances are rarely wholly pure, but always sport more or less into the colors and forms of allied varieties. Such sporting individuals have to be extirpated regularly, otherwise the whole variety would soon lose its type and its uniformity and run over to some other form in cultivation in the vicinity. For this reason atavism in nurseries causes much care and labor, and consequently is to be dealt with as a very important factor.

From time to time the idea has suggested itself to some of the best authorities on the amelioration of plants, that this atavism was not due to an innate tendency, but, in many cases at least, was produced by crosses between neighboring varieties. It is especially owing to Verlot that this side of the question was brought forward. But breeders as a rule have not attached much importance to this supposition, chiefly because of the great practical difficulties attending any attempt to guard the species of the larger cultures against intermixture with other varieties. Bees and humble-bees fly from bud to bud, and carry the pollen from one [187 ] sort to another, and separation by great distances would be required to avoid this source of impurity. Unfortunately the arrangements and necessities of large cultures make it impossible to isolate the allied varieties from each other.

From a theoretical point of view the origin of these impurities is a highly important question. If the breeders' atavism is due to crosses, and only to this cause, it has no bearing at all on the question of the constancy of varieties. And the general belief, that varieties are distinguished from true species by their repeated reversion and that even such reversibility is the real distinction of a variety, would not hold.

For this reason I have taken much trouble in ascertaining the circumstances which attend this form of atavism. I have visited a number of the leading nurseries of Europe, tested their products in various ways, and made some experiments on the unavoidable conditions of hybridizing and on their effect on the ensuing generations. These investigations have led me to the conclusion, that atavism, as it is generally described, always or nearly always is due to hybridization, and therefore it is to be considered as untrue or false atavism.

True atavism, or reversion caused by an innate latent tendency, seems to be very rare, and limited to such cases as we have spoken of under our last heading. And since the definition, given to this term by its author, Duchesne, is generally accepted in scientific works, it seems better not to use it in another sense, but rather to replace it in such cases by another term. For this purpose I propose the word vicinism, derived from the Latin vicinus or neighbor, as indicating the sporting of a variety under the influence of others in its vicinity. Used in this way, this term has the same bearing as the word atavism of the breeders, but it has the advantage of indicating the true cause thereof.

It is well known that the term variability is commonly employed in the broadest possible sense. No single phenomenon can be designated by this name, unless some primary restriction be given. Atavism and vicinism are both cases of variability, but in wholly different sense. For this reason it may be as well, to insert here a short survey of the general meanings to be conveyed by the term variation. It implies in the first place the occurrence of a wide range of forms and types, irrespective of their origin, and in the second place the process of the change in such forms. In the first signification it is nearly identical with polymorphy, or richness of types, especially so when these types are themselves quite stable, or when it is not at all intended to raise the question of their stability. In scientific works it is commonly used to designate the occurrence of subspecies or varieties, and the same is the case in the ordinary use of the term when dealing with cultivated plants. A species may consist of larger or smaller groups of such units, and they may be absolutely constant, never sporting if hybridization is precluded, and nevertheless it may be called highly variable. The opium-poppy affords a good instance. It "varies" in height, in color of foliage and flowers; the last are often double or laciniated; it may have white or bluish seeds, the capsules may open themselves or remain closed and so on. But every single variety is absolutely constant, and never runs into another, when the flowers are artificially pollinated and the visits of insects excluded. So it is with many other species. They are at the same time wholly stable and very variable.

The terms variation and variety are used frequently when speaking of hybrids. By crossing forms, which are already variable in the sense just mentioned, it is easy to multiply the number of the types, and even in crossing pure forms the different characters may be combined in different ways, the resulting combinations yielding new, and very often, valuable varieties. But it is manifest that this form of variation is of quite another nature from the variations of pure races. Many hybrid varieties are quite constant, and remain true to their type if no further crosses are made; many others are artificially propagated only in a vegetative way, and for this reason are always found true. Hybrid varieties as a rule were formerly confused with pure varieties, and in many instances our knowledge as to their origin is quite insufficient for sharp distinctions. To every student of nature it is obvious, that crossing and pure variability are wholly distinct groups of phenomena, which should never be treated under the same head, or under the same name.

Leaving aside polymorphy, we may now discuss those cases of variability, in which the changes themselves, and not only their final results play a part. Of such changes two types exist. First, the ever-recurring variability, never absent in any large group of individuals, and determining the differences which are always to be seen between parents and their children, or between the children themselves. This type is commonly called "individual variability" and since this term also has still other meanings, it has of late become customary to use instead the term "fluctuating variability." And to avoid the repetition of the latter word it is called "fluctuation." In contrast to these fluctuations are the so-called sports or single varieties, not rarely denominated spontaneous variations, and for which I propose to use the term "mutations." They are of very rare occurrence and are to be considered as sudden and definite steps.

Lastly, we have to consider those varieties, which vary in a much wider range than the ordinary ones, and seem to fluctuate between two opposite extremes, as for instance variegated leaves, cultivated varieties with variegated or striped flowers, double flowers and some other anomalies. They are eversporting and ever-returning from one type to the other. If however, we take the group of these extremes and their intermediates as a whole, this group remains constant during the succeeding generations. Here we find once more an instance of the seemingly contradictory combination of high variability and absolute constancy. It means that the range of variability has quite definite limits, which in the common course of things, are never transgressed.

We may infer therefore that the word variability has such a wide range of meanings that it ought never be used without explanation. Nothing indeed, is more variable than the signification of the term variable itself.

For this reason, we will furthermore designate all variations under the influence of neighbors with the new and special term "vicinism." It always indicates the result of crossing.

Leaving this somewhat lengthy terminological discussion, we now come to the description of the phenomenon itself. In visiting the plantations of the seedsmen in summer and examining the large fields of garden-flowers from which seed is to be gathered, it is very rare to find a plot quite pure. On the contrary, occasional impurities are the rule. Every plot shows anomalous individuals, red or white flowers among a field of blue, normal among laciniated, single among double and so on. The most curious instance is afforded by dwarf varieties, where in the midst of hundreds and thousands of small individuals of the same height, some specimens show twice their size. So for instance, among the dwarfs of the larkspur, Delphinium Ajacis.

Everywhere gardeners are occupied in destroying these "atavists," as they call them. When in full bloom the plants are pulled up and thrown aside. Sometimes the degree of impurity is so high, that great piles of discarded plants of the same species lie about the paths, as I have seen at Erfurt in the ease of numerous varieties of the Indian cress or Tropaeolum.

Each variety is purified at the time when it shows its characters most clearly. With vegetables, this is done long before flowering, but with flowers only when in full bloom, and with fruits, usually after fertilization has been accomplished. It needs no demonstration to show that this difference in method must result in very diverging degrees of purity.

We will confine ourselves to a consideration of the flowers, and ask what degree of purity may be expected as the result of the elimination of the anomalous plants during the period of blooming.

Now it is evident that the colors and forms of the flowers can only be clearly distinguished, when they are fully displayed. Furthermore it is impossible to destroy every single aberrant specimen as soon as it is seen. On the contrary, the gardener must wait until all or nearly all the individuals of the same variety have displayed their characters, as only in this way can all diverging specimens be eliminated by a single inspection. Unfortunately the insects do not wait for this selection. They fertilize the flowers from the beginning, and the damage will have been done long before the day of inspection comes around. Crosses are unavoidable and hybrid seeds will unavoidably come into the harvest. Their number may be limited by an early eradication of the vicinists, or by the elimination of the first ripe seeds before the beginning of the regular harvest, or by other devices. But some degree of impurity will remain under ordinary circumstances.

It seems quite superfluous to give more details. In any case in which the selection is not done before the blooming period, some impurities must result. Even if it is done before that time, errors may occur, and among hundreds and thousands of individuals a single anomalous one may escape observation.

The conclusion is, that flower seeds as they are offered in commerce, are seldom found absolutely pure. Every gardener knows that he will have to weed out aberrant plants in order to be sure of the purity of his beds. I tested a large number of samples of seeds for purity, bought directly from the best seed growers. Most of them were found to contain admixtures and wholly pure samples were very rare.

I will now give some illustrative examples. From seeds of a yellow snapdragon, I got one red-flowered specimen among half a hundred yellow ones, and from the variety "Delila" of the same species two red ones, a single white and two belonging to another variety called "Firefly." Calliopsis tinctoria has three varieties, the ordinary type, a brown-flowered one and one with tubular rays. Seeds of each of these three sorts ordinarily contain a few belonging to the others. Iberis umbellata rosea often gives some white and violet examples. The "Swan" variety of the opium-poppy, a dwarfish double-flowered form of a pure white, contained some single-flowered and some red-flowered plants, when sown from commercial seed are said to be pure. But these were only occasional admixtures, since after artificial fertilization of the typical specimens the strain at once became absolutely pure, and remained so for a series of generations, as long as the experiment was continued. Seeds of trees often contain large quantities of impurities, and the laciniated varieties of birch, elder and walnut have often been observed to come true only in a small number of seedlings.

In the case of new or young varieties, seed merchants often warn their customers as to the probable degree of purity of the seeds offered, in order to avoid complaints. For example the snow-white variety of the double daisy, Bellis perennis plena, was offered at the start as containing as much as 20% of red-flowered specimens.

Many fine varieties are recorded to come true from seed, as in the case of the holly with yellow fruits, tested by Darwin. Others have been found untrue to a relatively high degree, as is notorious in the case of the purple beech. Seeds of the laciniated beech gave only 10% of laciniated plants in experiments made by Strasburger; seeds of the monophyllous acacia, Robinia Pseud-Acacia monophylla, were found to be true in only 30% of the seedlings. Weeping ashes often revert to the upright type, red May-thorns (Crataegus) sometimes revert nearly entirely to the white species and the yellow cornel berry is recorded to have reverted in the same way to the red berries of the Cornus Mas.

Varieties have to be freed by selection from all such impurities, since isolation is a means which is quite impracticable under ordinary circumstances. 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 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 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 quantities 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 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 circumstances of commercial culture. And I cannot see any reason why this fact should not be considered as stating a principal 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 constituted 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 assumption 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.

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 quantities, large enough to bring to light any slight anomaly, and to be examined during the period of blooming.

The wild seashore 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 album in the neighborhood of the blue-flowered species, the distance 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 album, 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 assertion 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, Raphanus Raphanistrum, to the allied Raphanus caudatus, and I observed the following year some very nice hybrids among my seedlings. A hybrid-bean between Phaseolus nanus 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 assumption that in the same way crosses ordinarily occur also between varieties 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 adventitious 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 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 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 assume 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 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, assuming 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 assume 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 analogous 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, 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 Bumble-bees, butterflies 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.

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 pass unobserved. 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 passes unobserved 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 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 sparsely 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 which reassume 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 unobserved 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 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 assertion 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 surpass 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 principally 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 pubescence, a third by being dwarfed, and so on. Every character must be studied separately in its effects on the offspring 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, reassuming 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 pubescence 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 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 assisting 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 sparse literature on sports, numerous cases will be found, which cannot stand this test. In many instances crossing must be looked to as an explanation, and in other cases the evidence relied upon does not suffice to exclude this assumption. 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 unobserved spontaneous crosses.