Species and Varieties, Their Origin by Mutation Part 9

Reversion may occur either by buds or by seeds. It is highly probable that it occurs more readily by s.e.xual than by as.e.xual propagation. But if we restrict the discussion to the limits [175] hitherto observed, seed-reversions must be said to be extremely rare. Or rather cases which are sufficiently certain to be relied upon, are very rare, and perhaps wholly lacking. Most of the instances, recorded by various writers, are open to question. Doubts exist as to the purity of the seeds and the possibility of some un.o.bserved cross disturbing the results.

In the next lecture we shall deal in general with the ordinary causes and results of such crosses. We shall then see that they are so common and occur so regularly under ordinary circ.u.mstances that we can never rely on the absolute purity of any seeds, if the impossibility of an occasional cross has not been wholly excluded, either by the circ.u.mstances themselves, or by experimental precautions taken during the flowering period.

For these reasons cases of atavism given without recording the circ.u.mstances, or the precautions that guarantee the purity of the fertilization, should always be disregarded. And moreover another proof should always be demanded. The parent which yielded the seeds might be itself a hybrid and liable to reversions by the ordinary laws of the splitting up of hybrids. Such cases should likewise be discarded, since they bring in confusing elements. If we review the long list of recorded cases by these [176] strict methods of criticism very few instances will be found that satisfy legitimate demands. On this ground it is by far safer in the present state of our knowledge, to accept bud-variations only as direct proofs of true atavism. And even these may not always be relied on, as some hybrids are liable to split up in a vegetative way, and in doing so to give rise to bud-variations that are in many respects apparently similar to cases of atavism. But fortunately such instances are as yet very rare.

After this discussion it would be bold indeed to give instances of seed-atavism, and I believe that it will be better to refrain wholly from doing so.

Many instances of so-called atavism are of purely morphologic nature.

The most interesting cases are those furnished by the forms which some plants bear only while young, and which evidently connect them with allied species, in which the same features may be seen in the adult state. Some species of the genus _Acacia_ bear bipinnate leaves, while others have no leaves at all, but bear broadened and flattened petioles instead. The second type is presumed to be descended from the first by the loss of the leaflets and the modification of the stalks into flat and simple phyllodes. But many of them are liable to recall this primitive form [177] when very young, in the first two or three, or sometimes in eight or ten primary leaves. These leaves are small because of the weakness of the young plant and therefore often more or less reduced in structure. But they are usually strictly bipinnate and thereby give testimony as to their descent from species which bear such leaves throughout their life.

Other similar cases could be given, but this will suffice. They once more show how necessary it is to separate the different cases, thrown together until now, under this general name of atavism. It would be far better to give them all special names, and as long as these are not available we must be cautious not to be misguided by the name, and especially not to confuse different phenomena with one another, because at the present time they bear the same names.

Taking into consideration the relatively numerous restrictions resulting from this discussion, we will now make a hasty survey of some of the more notable and generally acknowledged cases of atavism by bud-propagation. But it should be repeated once more that most of the highly cultivated plants, grown as vegetables, or for their fruit or flowers, have so many crosses in their ancestry, that it seems better to exclude them from all considerations, in which purity of [178] descent is a requisite. By so doing, we exclude most of the facts which were until now generally relied upon. For the roses, the hyacinths, the tulips, the chrysanthemums always have furnished the largest contributions to the demonstrations of bud-variation. But they have been crossed so often, that doubt as to the purity of the descent of any single form may recur, and may destroy the usefulness of their many recorded cases of bud-variation for the demonstration of real atavism.

The same a.s.sertion holds good in many other cases, as with _Azalea_ and _Camellia_. And the striped varieties of these genera belong to the group of ever-sporting forms, and therefore will be considered later on.

So it is with carnations and pinks, which occasionally vary by layering, and of which some kinds are so uncertain in character that they are called by floriculturists "catch-flowers." On the other hand there is a larger group of cases of reversion by buds, which is probably not of hybrid nature, nor due to innate inconstancy of the variety, but must be considered as pure atavism. I refer to the bud-variations of so many of our cultivated varieties of shrubs and trees. Many of them are cultivated because of their foliage. They are propagated by grafting, and in most cases it is probable that all the numerous specimens [179]

of the same variety have been derived in this way from one primitive, aberrant individual. We may disregard variegated leaves, spotted or marked with white or yellow, because they are too inconstant types.

We may next turn our attention to the varieties of trees with cut leaves, as the oakleaved _Laburnum_, the parsley-leaved vine and the fern-leaved birch. Here the margin of the leaves is deeply cut and divided by many incisions, which sometimes change only the outer parts of the blade, but in other cases may go farther and reach, or nearly reach, the midvein, and change the simple leaf into a seemingly compound structure. The anomaly may even lead to the almost complete loss of all the chorophyll-tissue and the greater part of the lateral veins, as in the case of the cut-leaved beech or _f.a.gus sylvatica pectinata_.

Such varieties are often apt to revert by buds to the common forms. The cut-leaved beech sometimes reverts partially only, and the branches often display the different forms of cut-leaved, fern-like, oak-leaved and other variously shaped leaves on the same twigs. But this is merely due to the wide variability of the degree of fissure and is to be considered only as a fluctuation between somewhat widely distant extremes, which may even apparently include [180] the form of the common beech-leaves. It is not a bud-variation at all, and it is to be met with quite commonly while the true reversions by buds are very rare and are of the nature of sports appearing suddenly and remaining constant on the same twig. a.n.a.logous phenomena of wide variability with true reversion may be seen in the variety of the European hornbeam called _Carpinus Betulus heterophylla_. The leaves of this tree generally show the greatest diversity in form. Some other cases have been brought together by Darwin. In the first place a subvariety of the weeping-willow with leaves rolled up into a spiral coil. A tree of this kind kept true for twenty-five years and then threw out a single upright shoot bearing flat leaves. The barberry (_Berberis_) offers another case; it has a well known variety with seedless fruit, which can be propagated by cuttings or layers, but its runners are said always to revert to the common form, and to produce ordinary berries with seeds. Most of the cases referred to by Darwin, however, seem to be doubtful and cannot be considered as true proofs of atavism until more is known about the circ.u.mstances under which they were produced.

Red or brown-leaved varieties of trees and shrubs also occasionally produce green-leaved branches, and in this way revert to the type [181]

from which they must evidently have arisen. Instances are on record of the hazel, _Corylus Avellana_, of the allied _Corylus tubulosa_, of the red beech, the brown birch and of some other purple varieties. Even the red bananas, which bear fruits without seeds and therefore have no other way of being propagated than by buds, have produced a green variety with yellow fruits. The _Hortensia_ of our gardens is another instance of a sterile form which has been observed to throw out a branch with cymes bearing in their center the usual small staminate and pistillate flowers instead of the large radiate and neutral corollas of the variety, thereby returning to the original wild type. Crisped weeping-willows, crisped parsley and others have reverted in a similar manner.

All such cases are badly in need of a closer investigation. And as they occur only occasionally, or as it is commonly stated, by accident, the student of nature should be prepared to examine carefully any case which might present itself to him. Many phases of this difficult problem could no doubt be solved in this way. First of all the question arises as to whether the case is one of real atavism, or is only seemingly so, being due to hybrid or otherwise impure descent of the varying individual, and secondly whether it may be only an instance of the regularly [182]

occurring so-called atavism of the sporting varieties with which we shall deal in a later lecture. If it proves to be real atavism and rare, the case should be accurately described and figured, or photographed if possible; and the exact position of the reverting bud should be ascertained. Very likely the so-called dormant or resting buds are more liable to reversions than the primary ones in the arils of the leaves of young twigs. Then the characters of the atavistic branches should be minutely compared with those of the presumed ancestor; they may be quite identical with them or slightly divergent, as has been a.s.serted in some instances. The atavism may be complete in one case, but more or less incomplete in others. By far the most interesting point is the question, as to what is to be expected from the seeds of such an atavistic branch.

Will they keep true to the reverted character, or return to the characters of the plant which bears the retrograde branch? Will all of them do so, or only part of them, and how large a part? It is very astonis.h.i.+ng that this question should still be unsolved where so many individual trees bear atavistic branches that remain on them through long series of years. But then many such branches do not flower at all, or if they flower and bear seed, no care is taken to prevent [183]

cross-fertilization with the other flowers of the same plant, and the results have no scientific value. For anyone who cares to work with the precautions prescribed by science, a wide field is here open for investigation, because old reverted branches may be met with much less rarely than new ones.

Finally the possibility is always to be considered that the tendency to bud-reversions may be a special feature of some individuals, and may not be met with in others of the same variety. I have spoken of this before.

For the practical student it indicates that a specimen, once observed to produce atavistic buds, may be expected to do the same thing again. And then there is a very good chance that by combining this view with the idea that dormant buds are more apt to revert than young ones, we may get at a method for further investigation, if we recur to the practice of pruning. By cutting away the young twigs in the vicinity of dormant buds, we may incite these to action. Evidently we are not to expect that in so doing they will all become atavistic. For this result is not at all a.s.sured; on the contrary, all that we might hope to attain would be the possibility of some of them being induced to sport in the desired direction.

Many questions in scientific research can only [184] be answered by long and arduous work in well equipped laboratories; they are not to be attempted by every one. But there are other problems which the most complete of inst.i.tutions are not able to study if opportunity is not offered them, and such opportunities are apt to occur more often in fields, gardens, parks, woods and plains, than in the relatively small experimental gardens of even the largest inst.i.tution. Therefore, whosoever has the good fortune to find such sports, should never allow the occasion to pa.s.s without making an investigation that may bring results of very great importance to science.

[185]

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. d.u.c.h.esne 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. d.u.c.h.esne's definition was evidently a sharp and useful one, since it developed for the first time the idea of latent or dormant qualities, [186] 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 circ.u.mstances 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 circ.u.mstances 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, [188] 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, d.u.c.h.esne, 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 [189] 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 [190] 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." [191] And to avoid the repet.i.tion 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. [192]

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 [193] 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 [194] 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 circ.u.mstances.

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 ill.u.s.trative examples. From seeds of a yellow snapdragon, I got one red-flowered specimen among half a hundred [195]

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 quant.i.ties 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 [196] 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_.