Chapter 14

Thus far we have an ordinary case of reversion. But the important side of the phenomenon was, that each plant exactly "recollected" from which parent it had sprung. All of those in my garden reverted to the carmine florets with white tips, and all of those in the nursery to the pale orange color and the other characteristics of the "Surprise" variety.

It seems absolutely evident, that no simple loss can account for this difference. Something of the character of the parent-varieties must have remained in the plant. And whatever conception we may formulate of these vestigial characters it is clear that the simplest and most obvious idea is their preservation in a dormant or latent state. Assuming that the distinguishing marks have only become inactive by virescence, it is manifest that on returning each will show its own peculiarities, as recorded above. Our second point was the incomplete loss of [232] the distinguishing quality in some varieties. It is of general occurrence, though often overlooked. Many white varieties of colored flowers give striking instances, among them many of the most stable and most prized garden-flowers. If you look at them separately or in little bouquets they seem to be of irreproachable purity. But if you examine large beds a pale hue will become visible. In many cases this tinge is so slight as to be only noticeable in a certain illumination, or by looking in an oblique direction across the bed; in others it is at once evident as soon as it has been pointed out. It always reminds the observer of the color of the species to which the variety belongs, being bluish in violets and harebells, reddish in godetias and phloxes, in _Silene Armeria_ and many others. It proves that the original color quality of the species has not wholly, but only partly disappeared. It is dormant, but not entirely obliterated; latent, but not totally concealed; inactive, but only partially so. Our terminology is an awkward one; it practically assumes, as it so often does in other cases, a conventional understanding, not exactly corresponding to the simple meaning of the words. But it would be cumbrous to speak always of partial inactivity, incomplete latency or half awakening qualities. Even such words as sub-latent, [233] which would about express the real state of things, would have little chance of coming into general use.

Such sub-latent colors are often seen on special parts in white varieties of flowers. In many cases it is the outer side of the petals which recalls the specific color, as in some white roses. In violets it is often on the spur that the remains of the original pigment are to be seen. In many instances it is on the tips of the petals or of the segments of the corolla, and a large number of white or yellow flowers betray their affinity to colored species by becoming red or bluish at the edges or on the outer side.

The reality of such very slight hues, and their relation to the original pigment of the species may in some cases be proved by direct experiment. If it is granted that latency is not an absolute quality, then it will be readily accepted, that even latency must be subjected to the laws of gradual variation or fluctuating variability. We will deal with these laws in a later lecture but every one knows that greater deviations than the ordinary may be attained by sowing very large numbers and by selecting from among them the extreme individuals and sowing anew from their seed. In this way the slightest tinge of any latent color may be [234] strengthened, not indeed to the restoration of the tinge of the species, but at least so far as to leave no doubt as to the identity of the visible color of the species and the latent or sublatent one of the variety.

I made such an experiment with the peach leaved harebell or _Campanula persicifolia_. The white variety of this species, which is often met with in our gardens, shows a very pale bluish hue when cultivated in large quantities, which however is subject to individual variations. I selected some plants with a decided tinge, flowered them separately, sowed their seeds, and repeated this during two generations. The result was an increase of the color on the tips of the segments of the corolla in a few individuals, most of them remaining as purely white as the original strain. But in those few plants the color was very manifest, individually variable in degree, but always of the same blue as in the species itself.

Many other instances could be given. Smooth varieties are seldom absolutely so, and if scattering hairs are found on the leaves or only on some more or less concealed parts, they correspond in their character to those of the species. So it is with prickles, and even the thornless thorn-apple has fruits with surfaces far from smooth. The thornless horse-chestnut [235] has in some instances such evident protuberances on the valves of its fruits, that it may seem doubtful whether it is a pure and stable variety.

Systematic latency may betray itself in different ways, either by normal systematic marks, or by atavism. With the latter I shall deal at length on another occasion, and therefore I will give here only one very clear and beautiful example. It is afforded by the common red clover. Obviously the clovers, with their three leaflets in each leaf, stand in the midst of the great family of papilionaceous plants, the leaves of which are generally pinnate. Systematic affinity suggests that the "three leaved" forms must have been derived from pinnate ancestors, evidently by the reduction of the number of the leaflets. In some species of clover the middle of the three is more or less stalked, as is ordinarily the case in pinnate leaves; in others it is as sessile as are its neighbors. In a subsequent chapter I will describe a very fine variety, which sometimes occurs in the wild state and may easily be isolated and cultivated. It is an ordinary red clover with five leaflets instead of three, and with this number varying between three and seven, instead of being nearly wholly stable as in the common form. It produces from time to time pinnate leaves, [236] very few indeed, and only rarely, but then often two or three or even more on the same individual. Intermediate stages are not wanting, but are of no consequence here. The pinnate leaves obviously constitute a reversion to some prototype, to some ancestor with ordinary papilionaceous leaves. They give proof of the presence of the common character of the family, concealed here in a latent state. Any other explanation of this curious anomaly would evidently be artificial. On the other hand nothing is really known about the ancestors of clover, and the whole conception rests only on the prevailing views of the systematic relationships in this family. But, as I have already said, further proof must be left for a subsequent occasion.

Many instances, noted in our former lectures, could be quoted here. The systematic distribution of rayed and rayless species and varieties among the daisy-group of the composites affords a long series of examples. Accidental variations in both directions occur. The Canada fleabane or _Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some others may at times be seen with ray-florets, and according to Murr, they may sometimes be wanting in _Aster Tripolium_, _Bellis perennis_, some species of _Anthemis_, _Arnica montana_ and in a number [237] of other well-known rayed species. Another instance may be quoted; it has been pointed out by Grant Allen, and refers to the dead-nettle or Lamium album. Systematically placed in a genus with red-flowering species, we may regard its white color as due to the latency of the general red pigment.

But if the flower of this plant is carefully examined, it will be found in most cases not to be purely white, but to have some dusky lines and markings on its lower lip. Similar devices are observed on the lip of the allied _Lamium maculatum_, and in a less degree on the somewhat distant _Lamium purpureum_. With _Lamium maculatum_ or spotted dead-nettle, the affinity is so close that even Bentham united the two in a single species, considering the ordinary dead-nettle only as a variety of the dappled purple type. For the support of this conception of a specific or varietal retrograde change many other facts are afforded by the distribution of the characteristic color and of the several patterns of the lips of other labiates, and our general understanding of the relationships of the species and genera in this family may in a broad sense be based on the comparison of these seemingly subordinate characteristics.

The same holds good in many other cases, and systematists have often become uncertain [238] as to the true value of some form, by its relationship to the allied types in the way of retrogressive modification. Color-differences are so showy, that they easily overshadow other characters. The white and the blue thorn-apple, the white and the red campion (_Lychnis vespertina_ and _diurna_) and many other illustrative cases could be given, in which two forms are specifically separated by some authors, but combined by others on the ground of the retrograde nature of some differentiating mark.

Hitherto we have dealt with negative characters and tried to prove that the conception of latency of the opposite positive characteristics is a more natural explanation of the phenomenon than the idea of a complete loss. We have now to consider the positive varieties, and to show that it is quite improbable that here the species have struck out for themselves a wholly new character. In some instances such may have been the case, but then I should prefer to treat these rather as elementary species. But in the main we will have to assume the latency of the character in the species and its reassumption by the variety when originating, as the most probable explanation.

Great stress is laid upon this conception by the fact, that positive varieties are so excessively rare when compared with the common occurrence [239] of negative ones. Indeed, if we put aside the radiate and the color-varieties of flowers and foliage, hardly any cases can be cited. We have dealt with this question in a former lecture, and may now limit ourselves to the positive color-varieties.

The latency of the faculty of producing the red pigment in leaves must obviously be accepted for nearly the whole vegetable kingdom. Oaks and elms, the beautiful climbing species of Ampelopsis, many conifers, as for instance _Cryptomeria japonica_, some brambles, the Guelder-rose (_Viburnum Opulus_) and many other trees and shrubs assume a more or less bright red color in the fall. During summer this tendency must have been dormant, and that this is so, is shown by the young leaves of oaks and others, which, when unfolding in the spring show a similar but paler hue. Moreover, there is a way of awakening the concealed powers at any time. We have only to inflict small wounds on the leaves, or to cut through the nerves or to injure them by a slight bruising, and the leaves frequently respond with an intense reddening of the living tissues around and especially above the wounds. _Azolla caroliniana_, a minute mosslike floating plant allied to the ferns, responds to light and cold with a reddish tinge, and to shade or warmth with a pure green. The foliage [240] of many other plants behaves likewise, as also do apples and peaches on the insolated sides of the fruits. It is quite impossible to state these groups of facts in a more simple way than by the statement that the tendency to become red is almost generally present, though latent in leaves and stems, and that it comes into activity whenever a stimulus provokes it.

Now it must be granted that the energizing of such a propensity under ordinary circumstances is quite another thing from the origination of a positive variety by the evolution of the same character. In the variety the activity has become independent of outer influences or dependent upon them in a far lesser degree. The power of producing the red pigments is shown to be latent by the facts given above, and we see that in the variety it is no longer latent but is in perfect and lasting activity throughout the whole life of the plant.

Red varieties of white flowers are much more rare. Here the latency of the red pigment may be deduced partly from general arguments like those just given, partly from the special systematic relations in the given cases. Hildebrand has clearly worked out this mode of proof. He showed by the critical examination of a large number of instances that the occurrence of the red-flowered varieties is contingent upon the [241] existence of red species in the same genus, or in some rare cases, in nearly allied genera. Colors that are not systematically present in the group to which a white species belongs are only produced in its varieties in extremely rare cases.

We may quote some special rules, indicated by Hildebrand. Blue species are n the main very rare, and so are blue varieties of white species also. Carnations, Asiatic or cultivated buttercups (_Ranunculus asiaticus_), _Mirabilis_, poppies, _Gladiolus_, _Dahlia_, and some other highly cultivated or very old garden-plants have not been able to produce true blue flowers. But the garden-anemone (_Anemone coronaria_) has allies with very fine blue flowers. The common stock has bluish varieties and is allied to _Aubretia_ and _Hesperis_, and gooseberries have a red form, recalling the ordinary currant. In nearly all other instances of blue or red varieties every botanist will be able to point out some allied red or blue species, as an indication of the probable source of the varietal character.

Dark spots on the lower parts of the petals of some plants afford another instance, as in poppies and in the allied _Glaucium_, where they sometimes occur as varietal and in other cases as specific marks.

The yellow fails in many highly developed [242] flowers, which are not liable to produce yellow variations, as in _Salvia_, _Aster_, _Centaurea_, _Vinca_, _Polygala_ and many others. Even the rare pale yellowish species of some of these genera have no tendency in this direction. The hyacinths are the most remarkable, if not the sole known instance of a species having red and blue and white and yellow varieties, but here the yellow is not the bright golden color of the buttercups.

The existence of varietal colors in allied species obviously points to a common cause, and this cause can be no other than the latency of the pigment in the species that do not show it.

The conception of latency of characters as the common source of the origination of varieties, either in the positive or in the negative way, leads to some rules on variability, which are known under the names given to them by Darwin. They are the rules of repeated, homologous, parallel and analogous variability. Each of them is quite general, and may be recognized in instances from the most widely distant families. Each of them is quite evident and easily understood on the principle of latency.

By the term of repeated variability is meant the well-known phenomenon, that the same variety has sprung at different times and in different [243] countries from the same species. The repetition obviously indicates a common internal cause. The white varieties of blue- and red-flowered plants occur in the wild state so often, and in most of the instances in so few individuals that a common pedigree is absolutely improbable. In horticulture this tendency is widely and vexatiously known, since the repetition of an old variety does not bring any advantage to the breeder. The old name of "conquests," given by the breeders of hyacinths, tulips and other flower-bulbs to any novelty, in disregard of the common occurrence of repetitions, is an indication of the same experience in the repeated appearance of certain varieties.

The rule of parallel variations demands that the same character occasionally makes its appearance in the several varieties or races, descended from the same species, and even in widely distinct species. This is a rule, which is very important for the general conception of the meaning of the term variety as contrasted with elementary species. For the recurrence of the same deviation always impresses us as a varietal mark. Laciniated leaves are perhaps the most beautiful instance, since they occur in so many trees and shrubs, as the walnut tree, the beech, the birch, the hazelnut, and even in [244] brambles and some garden-varieties of the turnip (_Brassica_).

In such cases of parallel variations the single instances obviously follow the same rules and are therefore to be designated as analogous. Pitchers or ascidia, formed by the union of the margins of a leaf, are perhaps the best proof. They were classified by Morren under two heads, according to their formation from one or more leaves. Monophyllous pitchers obey the same law, viz.: that the upper side of the leaf has become the inner side of the pitcher. Only one exception to this rule is known to me. It is afforded by the pitchers of the banyan or holy fig-tree, _Ficus religiosus_, but it does not seem to belong to the same class as other pitchers, since as far as it has been possible to ascertain the facts, these pitchers are not formed by a few leaves as in all other cases, but by all the leaves of the tree.

In some cases pitchers are only built up of part of the leaf-blade. Such partial malformations obey a rule, that is common to them and to other foliar enations, viz.: that the side of the leaf from which they emerge, is always their outer side. The inner surface of these enations corresponds to the opposite side of the leaf, both in color and in anatomical structure. The last of the four rules above mentioned is [245] that of the homologous variability. It asserts that the same deviation may occur in different, but homologous parts of the same plant. We have already dealt with some instances, as the occurrence of the same pigment in the flowers and foliage, in the fruits and seeds of the same plant, as also illustrated by the loss of the red or blue tinge by flowers and berries. Other instances are afforded by the curious fact that the division of the leaves into numerous and small segments is repeated by the petals, as in the common celandine and some sorts of brambles.

It would take too long to make a closer examination of the numerous cases which afford proof of these statements. Suffice it to say that everywhere the results of close inspection point to the general rule, that the failure of definite qualities both in species and in varieties must, in a great number of cases, be considered as only apparent. Hidden from view, occasionally reappearing, or only imperfectly concealed, the same character must be assumed to be present though latent.

In the case of negative or retrogressive varieties it is the transition from the active into a dormant state to which is due the origin of the variety. Positive varieties on the contrary owe their origin to the presence of some character [246] in the species in the latent state, and to the occasional re-energizing thereof.

Specific or varietal latency is not the same thing as the ordinary latency of characters that only await their period of activity, or the external influence which will awake them. They are permanently latent, and could well be designated by the word perlatent. They spring into activity only by some sudden leap, and then at once become independent of ordinary external stimulation.

[247]

LECTURE IX

CROSSES OF SPECIES AND VARIETIES

In the foregoing lectures I have tried to show that there is a real difference between elementary species and varieties. The first are of equal rank, and together constitute the collective or systematic species. The latter are usually derived from real and still existing types. Elementary species are in a sense independent of each other, while varieties are of a derivative nature.

Furthermore I have tried to show that the ways in which elementary or minor species must have originated from their common ancestor must be quite different from the mode of origin of the varieties. We have assumed that the first come into existence by the production of something new, by the acquirement of a character hitherto unnoticed in the line of their ancestors. On the contrary, varieties, in most cases, evidently owe their origin to the loss of an already existing character, or in other less frequent cases, to the re-assumption of a quality [248] formerly lost. Some may originate in a negative, others in a positive manner, but in both cases nothing really new is acquired.

This distinction holds good for all cases in which the relationship between the forms in question is well known. It seems entirely justifiable therefore to apply it also to cases in which the systematic affinity is doubtful, as well as to instances in which it is impossible to arrive at any taxonomic conclusions. The extreme application of the principle would no doubt disturb the limits between many species and varieties as now recognized. It is not to be forgotten however that all taxonomic distinctions, which have not been confirmed by physiologic tests are only provisional, a view acknowledged by the best systematists. Of course the description of newly discovered forms can not await the results of physiologic inquiries; but it is absolutely impossible to reach definite conclusions on purely morphologic evidence. This is well illustrated by the numerous discords of opinion of different authors on the systematic worth of many forms.

Assuming the above mentioned principle as established, and disregarding doubtful cases as indicated, the term progressive evolution is used to designate the method in which elementary species must have originated. It is the [249] manner in which all advance in the animal and vegetable kingdoms must have taken place, continuously adding new characters to the already existing number. Contrasted with this method of growing differentiation, are the retrogressive modifications, which simply retrace a step, and the degressive changes in which a backward step is retraced and old characters revived. No doubt both of these methods have been operative on a large scale, but they are evidently not in the line of general advancement.

In all of these directions we see that the differentiating marks show more or less clearly that they are built up of units. Allied forms are separated from each other without intermediates. Transitions are wholly wanting, although fallaciously apparent in some instances owing to the wide range of fluctuating variability of the forms concerned, or to the occurrence of hybrids and subvarieties.

These physiologic units, which in the end must be the basis for the distinction of the systematic units, may best be designated by the term of "unit-characters." Their internal nature is as yet unknown to us, and we will not now look into the theories, which have been propounded as to the probable material basis underlying them. For our present purpose the empirical evidence of the general occurrence of [250] sharp limits between nearly related characters must suffice. As Bateson has put it, species are discontinuous, and we must assume that their characters are discontinuous also.