Besides this there is another form of latent characters, in which this awakening power is extremely limited, or wholly absent. It is the systematic latency, which may be said to belong to species and varieties in the same way as the ordinary latency belongs to individuals. As this individual latency may show itself from time to time during the life of a given plant, the first may only become active from time to time during the whole existence of the variety or the species. It has no regular period of activity, nor may it be incited by artificial stimulation.

[220] It emerges from concealment only very rarely and only on its own initiative. Such instances of atavism have been described in previous lectures, and their existence has been proved beyond doubt.

Systematic latency explains the innumerable instances in which species are seen to lack definite characteristics which ordinarily do not fail, either in plants at large, or in the group or family to which the plant belongs. If we take for instance the broom-rape or _Orobanche_, or some other pale parasite, we explain their occurrence in families of plants with green leaves, by the loss of the leaves and of the green color. But evidently this loss is not a true one, but only the latency of those characters. And even this latency is not a complete one, as little scales remind us of the leaves, and traces of chlorophyll still exist in the tissues. Numerous other cases will present themselves to every practical botanist.

Taking for granted that characters, having once been acquired, may become latent, and that this process is of universal occurrence throughout the whole vegetable and animal kingdom, we may now come to a more precise and clear conception of the existing differences between species and varieties.

For this purpose we must take a somewhat [221] broader view of the whole evolution of the vegetable kingdom. It is manifest that highly developed plants have a larger number of characters than the lower groups. These must have been acquired in some way, during preceding times. Such evolution must evidently be called a process of improvement, or a progressive evolution. Contrasted to this is the loss, or the latency of characters, and this may be designated retrogressive or retrograde evolution. But there is still a third possibility. For a latent character may rea.s.sume its activity, return to the active state, and become once more an important part of the whole organization. This process may be designated as degressive evolution; it obviously completes the series of the general types of evolution.

Advancement in general in living nature depends on progressive evolution. In different parts of the vegetable kingdom, and even in different families this progression takes place on different lines. By this means it results in an ever increasing divergency between the several groups. Every step is an advance, and many a step must have been taken to produce flowering plants from the simplest unicellular algae.

But related to, and very intimately connected with this advancement is the retrogressive [222] evolution. It is equally universal, perhaps never failing. No great changes have been attained, without acquiring new qualities on one side, and reducing others to latency. Everywhere such retrogressions may be seen. The polypetalous genera _Pyrola_, _Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkable instance of this. The whole evolution of the monocotyledons from the lowest orders of dicotyledons implies the seeming loss of cambial growth and many other qualities. In the order of aroids, from the calamus-root or sweet flag, with its small but complete flowers, up to the reduced duckweeds (_Lemna_), almost an unbroken line of intermediate steps may be traced showing everywhere the concurrence of progressive and retrogressive evolution.

Degressive evolution is not so common by far, and is not so easy to recognize, but no doubt it occurs very frequently. It is generally called atavism, or better, systematic atavism, and the clearest cases are those in which a quality which is latent in the greater part of a family or group, becomes manifest in one of its members. Bracts in the inflorescence of crucifers are ordinarily wanting, but may be seen in some genera, _Erucastrum pollichii_ being perhaps the [223] most widely known instance, although other cases might easily be cited.

For our special purpose we may take up only the more simple cases that may be available for experimental work. The great lines of evolution of whole families and even of genera and of many larger species obviously lie outside the limits of experimental observation. They are the outcome of the history of the ancestors of the present types, and a repet.i.tion of their history is far beyond human powers. We must limit ourselves to the most recent steps, to the consideration of the smallest differences.

But it is obvious that these may be included under the same heads as the larger and older ones. For the larger movements are manifestly to be considered only as groups of smaller steps, going in the same direction.

Hence we conclude, that even the smallest steps in the evolution of plants which we are able to observe, may be divided into progressive, retrogressive and degressive ones. The acquisition of a single new quality is the most simple step in the progressive line, the becoming latent and the reactivating of this same quality are the prototypes of the two other cla.s.ses.

Having taken this theoretical point of view, it remains to inquire, how it concurs with the [224] various facts, given in former lectures and how it may be of use in our further discussions.

It is obvious that the differences between elementary species and varieties on the one hand, and between the positive and negative varieties as distinguished above, are quite comparable with our theoretical views. For we have seen that varieties can always be considered as having originated by an apparent loss of some quality of the species, or by the resumption of a quality which in allied species is present and visible. In our exposition of the facts we have of course limited ourselves to the observable features of the phenomena without searching for a further explanation. For a more competent inquiry however, and for an understanding of wider ranges of facts, it is necessary to penetrate deeper into the true nature of the implied causes.

Therefore we must try to show that elementary species are distinguished from each other by the acquisition of new qualities, and that varieties are derived from their species either by the reduction of one or more characteristics to the latent state, or by the energizing of dormant characters.

Here we meet with a great difficulty. Hitherto varieties and subspecies have never been clearly defined, or when they have been, it was [225]

not by physiological, but only by morphological research. And the claims of these two great lines of inquiry are obviously very diverging.

Morphological or comparative studies need a material standard, by which it may be readily decided whether certain groups of animals and plants are to be described or de-nominated as species, as subspecies or as varieties. To get at the inner nature of the differences is in most cases impossible, but a decision must be made. The physiological line of inquiry has more time at its disposal; it calls for no haste. Its experiments ordinarily cover years, and a conclusion is only to be reached after long and often weary trials. There is no making a decision on any matter until all doubtful points have been cleared up. Of course, large groups of facts remain uncertain, awaiting a closer inquiry, and the teacher is constrained to rely on the few known instances of thoroughly investigated cases. These alone are safe guides, and it seems far better to trust to them and to make use of them for the construction of sharp conceptions, which may help us to point out the lines of inquiry which are still open.

Leaving aside all such divisions and definitions, as were stamped with the name of provisional species and varieties by the great systematist, [226] Alphonse De Candolle, we may now try to give the proofs of our a.s.sertion, by using only those instances that have been thoroughly tested in every way.

We may at once proceed to the retrogressive or negative varieties. The arguments for the a.s.sumption that elementary species owe their origin to the acquisition of new qualities may well be left for later lectures when we shall deal with the experimental proofs in this matter.

There are three larger groups of facts, on which the a.s.sumption of latent characters in ordinary varieties rests. These are true atavism, incomplete loss of characters, and systematic affinity. Before dealing with each of these separately, it may be as well to recall once more that in former lectures we have treated the apparent losses only as modifications in a negative way, without contemplating the underlying causes.

Let us recall the cases of bud-atavism given by the whitish variety of the scarlet _Ribes_, by peaches and nectarines, and by conifers, including _Cephalotaxus_ and _Cryptomeria_. These and many other a.n.a.logous facts go to prove the relation of the variety to the species.

Two a.s.sumptions are allowable. In one the variety differs from the species by the total loss of the [227] distinctive character. In the other this character is simply reduced to an inactive or dormant state.

The fact of its recurrence from time to time, accompanied by secondary characters previously exhibited, is a manifest proof of the existence of some relation between the lost and the resumed peculiarity. Evidently this relation cannot be accounted for on the a.s.sumption of an absolute disappearance; something must remain from which the old features may be restored.

This lengthy discussion may be closed by the citation of the cases, in which plants not only show developmental features of a former state, but also reproduce the special features they formerly had, but seemingly have lost. Two good ill.u.s.trative examples may be given. One is afforded by the wheat-ear carnation, the other by the green dahlias, and both have occurred of late in my own cultures.

A very curious anomaly may from time to time be observed in large beds of carnations. It bears no flowers, but instead of them small green ears, which recall the ears of wheat. Thence the name of "Wheat-ear"

carnation. On closer inspection it is easily seen how they originate.

The normal flowers of the carnations are preceded by a small group of bracts, [228] which are arranged in opposite pairs and therefore const.i.tute four rows.

In this variety the flower is suppressed and this loss is attended by a corresponding increase of the number of the pairs of bracts. This malformation results in square spikes or somewhat elongated heads consisting only of the greenish bracts. As there are no flowers, the variety is quite sterile, and as it is not regarded by horticulturists as an improvement on the ordinary bright carnations, it is seldom multiplied by layering. Notwithstanding this, it appears from time to time and has been seen in different countries and at different periods, and, what is of great importance for us, in different strains of carnations. Though sterile, and obviously dying out as often as it springs into existence, it is nearly two centuries old. It was described in the beginning of the 18th century by Volckamer, and afterwards by Jaeger, De Candolle, Weber, Masters, Magnus and many other botanists. I have had it twice, at different times and from different growers.

So far as I have been able to ascertain reversions of this curious carnation to normal flowers have not yet been recorded. Such a modification occurred last summer in my garden on a plant which had not been divided or layered, but on which the slender branches had [229]

been left on the stem. Some of them remained true to the varietal type and bore only green spikes. Others reverted wholly or partially to the production of normal flowers. Some branches bore these only, others had spikes and flowers on neighboring twigs, and in still other instances little spikes had been modified in such manner that a more or less well developed flower was preceded by some part of an ear.

The proof that this retrograde modification was due to the existence of a character in the latent state was given by the color of the flowers.

If the reverted bud had only lost the power of producing spikes, they would evidently simply have returned to the characteristics of the ordinary species, and their color would have been a pale pink. Instead of this, all flowers displayed corollas of a deep brown. They obviously reverted to their special progenitor, the chance variety from which they had sprung, and not to the common prototype of the species. Of course it was not possible to ascertain from which variety the plant had really originated, but the reproduction of any one clearly defined varietal mark is in itself proof enough of their origin, and of the latency of the dark brown flower-color in this special case.

A still better proof is afforded by a new type of green dahlia. The ordinary green dahlia [230] has large tufts of green bracts instead of flowering heads, the scales of the receptacle having a.s.sumed the texture and venation of leaves, and being in some measure as fleshy. But the green heads retain the form of the ordinary flower-heads, and as they have no real florets that may fade away, they remain unchanged on the plants, and increase in number through the whole summer. The new types of green dahlia however, with which I have now to deal, are distinguished by the elongation of the axis of the head, which is thereby changed into a long leafy stalk, attaining a length of several inches. These stalks continue growing for a very long time, and for the most part die without producing anything else than green fleshy scales.

This long-headed green dahlia originated at Haarlem some years ago, in the nursery of Messrs. Zocher & Co. It was seen to arise twice, from different varieties. Both of these were double-flowered, one a deep carmine with white tips on the rays, the other of a pale orange tint, known by the name of "Surprise." As they did not bear any florets or seeds, they were quite sterile. The strain arising from the carmine variety was kindly given to me by Messrs. Zocher & Co., and was propagated in my garden, while the other was kept in the nursery. In the earlier cultures both remained true to [231] their types, never producing true florets. No mark of the original difference was to be seen between them. But last summer (1903) both reverted to their prototypes, bearing relatively large numbers of ordinary double flowerheads among the great ma.s.s of green stalks. Some intermediate forms also occurred consisting of green-scaled stalks ending in small heads with colored florets.

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. a.s.suming 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 G.o.detias 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 a.s.sumes, 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 c.u.mbrous 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 ident.i.ty 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 quant.i.ties, 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 p.r.i.c.kles, 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 const.i.tute 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 alb.u.m. 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 l.a.b.i.ates, 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 ill.u.s.trative 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 a.s.sume the latency of the character in the species and its rea.s.sumption 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 j.a.ponica_, some brambles, the Guelder-rose (_Viburnum Opulus_) and many other trees and shrubs a.s.sume 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 circ.u.mstances 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 b.u.t.tercups (_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 b.u.t.tercups.