Natural History Review/Series 2/Volume 1/Number 2/On the Species and Genera of Plants
Original Articles.
I say Species and Genera, rather than Genera and Species; for the whole system of classification depends, in the first instance, on a right understanding of what is meant by species.
The Species, in the ordinary traditional acceptation of the word, designates the whole of the individuals supposed to be descended from one original plant, or pair of plants. But this definition is practically useless—for we have no means of ascertaining the hereditary history of individual plants—and is considered theoretically incorrect by those who deny the original creation of a certain number of individuals, or pairs of individuals, forming each a parent stock, from which as many constantly distinct races have descended. It has, therefore, been proposed entirely to reject descent as an element in the definition of species, and to consider as such any set of individuals which present, either in their external form, or in their internal structure, or in their biological phenomena, any common character, or combination of characters, distinguishing them from all others. But in nature there are no two individuals exactly alike in every respect. In all collections of individuals, even when the immediate offspring of one parent, peculiarities will be found common to some, and not to all. The species or collection of individuals thus defined, becomes, therefore, as arbitrary as the genus or collection of species, and reduces the rules of classification in the one case, as in the other, to little more than rules of convenience.
Believing, however, as I do, that there exist in nature a certain number of groups of individuals, the limits to whose powers of variation are, under present circumstances, fixed and permanent, I have been in the habit of practically defining the species as the whole of the individual plants which resemble each other sufficiently to make us conclude that they are all, or may have been all, descended from a common parent. Their variations would be such only as we observe among individuals, which we know or believe to have had such a common descent. The specific identity of two or more individuals admits, therefore, but very rarely of positive proof; we must judge of it by inductive evidence, selecting by the careful consideration of what characters are known, especially in allied species, to remain permanent generation after generation, unaltered by change of soil, climate, or other circumstances, and what are the variations occasioned by causes which we can appreciate, or which are known to occur without assignable cause. The conclusions to be derived from such evidence will not, indeed, always be decisive, and different persons will often form different judgments ; but that is an unavoidable consequence of the imperfection of the human mind.
My own attention was first directed to the variations to which plants of the same species are liable, under different circumstances, in the year 1820. I had then become tolerably familiar with the common plants of France, in the West, in Upper Languedoc, and in the central Pyrenees; and, settling for some years in the neighbourhood of Montpelier, I was struck with the different aspect assumed by several of the same species in this very different soil and climate. In the first instance, I did indeed believe that many of these were representative, not identical species; but I could not but observe even then that, in many cases, species really the same underwent considerable modifications, through the influence of soil and climate. In 1823 I collected, with my friend, Dr. Arnott, a considerable number of Scotch specimens, which, two years later, after our Pyrenean tour, we had the opportunity of comparing with a similar vegetation grown in mountains of twice or thrice the elevation of the Scotch ones, but under a difference of latitude of 12 to 13 degrees; whilst, on the same Pyrenean chain, we were several times struck with the differences exhibited by plants of the same species growing in the cool northern, or the hot southern valleys. In 1821, on my father's estate near Montpelier, a considerable extent of the botanically rich waste lands, called garrigues, was walled in, to allow the natural wood to grow up; and, during the few succeeding years, I could observe a gradual, but in many instances very striking, change take place in the character and aspect of the wild plants protected by the enclosure. In 1837, when at Trieste, I visited a similar enclosure, on a larger scale, and of many years' standing, at Lippiza, near that town, and observed very marked differences in the individuals of some species, when growing within or without these walls. From the time, indeed, when I first began to collect notes on the vegetation of Southern Europe, some of which I embodied in my "Catalogue des Plantes des Pyrenées et du Bas-Languedoc," published in 1826, my attention has been much directed to the modifications of specific types, in all my herborisations in various parts of Europe, and, more especially, in the mountains of Scotland, the Pyrenees, Central France, and Tyrol; in the lower hills and plains of France, Britain, Sweden, Germany, Italy, and Sicily; and on the coasts of Britain, Western and Southern France, and various parts of the Mediterranean, Adriatic, and Black Seas. The preparation of large collections for distribution has given me opportunities of studying many American, African, and Asiatic species in large masses of specimens. In some of the more important monographs I have worked up, I have been enabled to compare the materials of the principal herbaria of Europe; and, since my working-stock has been transferred to Kew, the daily consultation of such a collection as that of Sir William Hooker has contributed very much to confirm my ideas as to the variability and limitation of species; and nothing more so than the extensive and highly instructive series brought from India by Drs. Hooker and Thomson, and the numerous accurate and judicious notes and memoranda so liberally communicated by Dr. Hooker. When, therefore, I speak of having observed a series of specimens collected in various parts of the geographical area of a species, I do not mean (as has been hinted) the examination of a few single specimens from different localities deposited in a herbarium, but the observation of a species in a living wild state in different countries, or the comparison of numerous specimens, either promiscuously collected, or selected, with notes, for the purpose of illustrating variations.
And here I would observe, that the use of herbaria in determining the extent of variability of species requires the greatest caution. Not only are the specimens preserved generally unaccompanied by any notes on the comparative frequency of the form gathered, and others closely resembling it, or on any other local circumstances affecting the question, but they are very likely to lead the botanist astray in these particulars. A collector is naturally struck by a plant differing in aspect from the generality of its species, and gathers it in preference to the forms more familiar to him. The consequence is, that it frequently happens that an accidentally abnormal variety, which may occur only once in the way in nature, having been cut up into a number of specimens, and distributed without notes to various botanists, has, from its presence in so many herbaria, all the appearance of a form abundant in the locality cited.
The experience I have thus obtained has gradually produced in my mind a conviction of the truth of the following axioms:—
Every species has certain determinate limits of variation, which it only exceeds under exceptional circumstances.
The exceptionally abnormal forms thus produced are few in individuals, and are not reproduced; or their race becomes gradually extinguished, when the causes which produce them cease.
Within these limits of variation, a species will, in some countries, or under certain circumstances, produce an indefinite number of individual, or more or less permanent varieties, often passing into each other by almost imperceptible gradations; whilst, in other countries, or under other circumstances, a certain number of these varieties or races will remain, generation after generation, marked by positive, distinctive characters, having at first sight the appearance of real species.
Plants of the same species often breed freely together, the crossbreeding of different individuals sometimes producing a more vigorous offspring than those sprung from a single flower, and being, perhaps, occasionally even necessary in plants apparently hermaphrodite. In other species, cross-breeding between individuals or races is rare and exceptional, and apt to be unfruitful.[2]
Plants of distinct species breed together only under exceptional circumstances.
The hybrids thus produced are constitutionally more or less imperfect. They seldom produce a second generation, unless fertilized by an individual of one of the parent species, to which they then gradually return. They, therefore, do not establish permanent races, but disappear in nature, unless reproduced by a fresh cross-breeding between the parent species.
Setting aside, in the first instance, these hybrids, and accidentally abnormal extreme variations, monstrosities, and diseases, the variations of a species may be generally referred to two classes.
1. Variations resulting from the direct influence of soil, climate, food, or other external circumstances, such as luxuriance from a rich soil, fleshiness from a maritime exposure, &c. These act upon the individual; they may disappear in that individual when the exciting causes are removed, or they may become so engrafted on the constitution as to last through life, after removal of the causes; they may even become more or less hereditary through one or more generations. Seeds of a plethoric kitchen-garden vegetable, originally the result of a peculiar treatment in a rich soil, will, even under a different and uncongenial treatment, to a certain degree reproduce the same variety for some generations.
2. Variations which, arising from causes unknown to us, we consider as constitutional,—variations in the colour of the flower, in the form of particular parts, in the production or non-production of wings or other appendages to fruits, seeds, peduncles, &c. These, like variations in the features of animals, are often hereditary, and in plants under cultivation will last, or may be made (by selection of seed, &c.) to last almost indefinitely; and, in a wild state, they may, in particular localities, result in apparently permanent races. These races, however, generally breed readily with the typical forms of the species, and, although permanent and distinct in some localities, will generally, in some part of the area of the species, or under certain circumstances, be found to break out occasionally into a return to the typical form, or to be connected with it by numerous intermediates. Generally speaking, such of these aberrant races as have spread to the limits of the geographical area of the species, or have become introduced into distant countries, and have thus been adapted to a change of condition, will there be found more disposed to maintain their peculiarities, or even to diverge still more from their types. It is where the species is most at home, where it accommodates itself most readily to a variety of soils and exposures, where the stations it affects show a most ancient domicile, that the connecting links between the varieties it has produced must generally be sought for. And this is one great reason why permanence of form is so little conclusive as evidence of specific difference, unless observed in a considerable portion of the area of the species.
The investigation of the connecting links between two forms, with the view of determining whether they are distinct species or marked races of one species, is attended with great difficulty in the due appreciation of what are intermediates—of the difference between one or two definitely limited, though apparently intermediate species, and a chain of intermediate forms connecting the two extreme varieties of one species. The Allsike clover, in the colour of its flowers and mode of growth, has been looked upon as intermediate between the Dutch and the common red clover (T. repens and pratense), and some such idea suggested to Linnæus the name of T. hybridum. Yet the evidences of its specific distinctness from both are very strong. I have observed it with care in a living state over a great part of its natural area in Sweden and Central Europe. From T. pratense it is separated by characters among the most constant in the genus, without, in this instance, any tendency to variation. It is nearer to T. repens; and Professor Buckman, at the meeting of the British Association at Cheltenham, in 1856, stated that he had found it degenerate into that species. I cannot but think, however, that here there must have occurred one of those mistakes so common in botanical and experimental gardens—that a plant or its seeds have accidentally perished, and its place has been taken by someubiquitous species, so nearly allied as to escape observation when young, such as, in this instance, T. repens. I never could detect, either in those places where I have seen T. hybridum wild in the greatest abundance, nor yet in the fields where it is cultivated, any tendency to assume the creeping stems, the peculiar inflorescence, and other characters of T. repens.
Take, again, Hypericum linariifolium. The cursory inspection of a few herbarium specimens of this plant, of certain varieties of H. perforatum, and of H. humifusum, might suggest the idea that the former constituted a connecting link between the two latter. In this instance the characters are less decided, and of a less constant nature than in that of the three Clovers above quoted; yet, so far as my experience goes—and I have observed H. linariifolium living in parts of Western France, where it grows in the greatest abundance, besides numerous dried specimens from the greater portion of its area, and the two others living in a great variety of stations and countries—I have seen no real tendency of H. linariifolium to pass into H. humifusum, still less into H. perforatum.
On the other hand, a very good example of really intermediate forms, erroneously (in my opinion) considered as constituting a distinct species, is afforded by the Daisy. In my early botanical days I was familiar with the two extreme forms—the large-flowered, long-leaved Bellis sylvestris of the south of Prance, of which I dried rather largely, selecting (as is usual with collectors), the most characteristic specimens, and our common, much smaller-flowered, and broader-leaved B. perennis, which I never particularly examined, and which is reckoned too common a plant to be frequently preserved in herbaria beyond a single specimen. The difference between the two was striking; and I adopted, without hesitation or consideration, their established specific distinctness. I subsequently received from Prof. Gussone his B. intermedia, which I laid in, on his authority, as a distinct species, the single specimen being quite insufficient to enable me to form any independent opinion on the subject. But when, in the autumn of 1846, I saw the neighbourhood of Constantinople abounding in daisies of various sizes, usually fully as large as the Montpelier ones, but sometimes much more like our northern ones, and equally variable in the form of their leaves, I felt much puzzled as to which species I should refer them to. In the following spring, in my Sicilian herborisations in Gussone's own country, I paid particular attention to these plants. The three supposed species there appeared to me to pass most gradually the one into the other, the intermediates being more abundant than either of the extremes; and since that time, in other parts of Europe, I have observed that where either of the extremes grows alone, its distinctive characters are not nearly so constant as they are supposed to be. I have thus been irresistibly led to the conviction that Bellis intermedia and sylvestris are mere varieties of B. perennis.
In the above instances, the evidences of specific diversity in the two first, and of identity in the third, are to my mind conclusive; and, as further examples of cases where a conviction of specific identity has, as it were, been forced upon me in opposition either to the views I had at first entertained, or to those of a large number of modern botanists, I would refer to Fumaria officinalis, Cerastium vulgatum, Rubus fruticosus, &c., which have all been the subject of long-contined observation, and endeavours to maintain as distinct species forms which I have, in my Handbook, reunited under the above names. There are, however, a number of cases where the evidences, as hitherto collected, are so insufficient or so conflicting, as to render any satisfactory decision hopeless, until carefully conducted experiments and observations shall have made us better acquainted with the hereditary permanence of certain apparently positive, but minute and unimportant characters.
It may be observed, in the first place, that there are frequently two nearly allied forms, of nearly the same geographical range, which are found more or less in company with each other, retaining over the whole of that range certain distinctive characters, of no great importance in their respective genera, yet apparently constant in that particular case. Such are, for instance, Viola odorata and hirta, Lychnis vespertina and diurna, Ulex Europæus and nanus, Sonchus oleraceus and asper, Senecio jacobæa and erucæfolius, Orchis maculata and latifolia, Juncus articulatus and obtusiflorus, and a number of others. In some of these cases, the balance of evidence has appeared to me to be in favour of their specific distinctness, in others of their identity, and I have so recorded them in my Hand-book, but often with great hesitation; and it is not improbable that further observation and experiment may induce a change of opinion in regard to some of them.
Again, there are sometimes two, three, or more forms, having every appearance of really distinct species, all common over an extensive area, or spreading into distant regions, and everywhere retaining their characters; and yet we are occasionally startled by the appearance of intermediate forms of various degrees, suggesting in some minds the specific identity of the whole series, in others a progressive development from one species to another; and in others, again, natural hybrids; whilst in some instances the observer may have been deceived by accidentally abnormal specimens, carefully preserved and occupying a conspicuous part in the herbarium, without any record of the attending circumstances which might have accounted for their production, but which forms in nature are very rare, and of mere temporary existence. Such occur, for instance, among some of the common species of Rumex, Mentha, &c. It is also frequently a matter of great nicety to determine what constitutes an intermediate form; for two plants, to be really intermediate, should not be so in one character only,, but in general habit and aspect, in a combination of all the characters which separate the two species it stands between. The species of Carduus (including Cirsium) for instance, have been artificially divided into species, with their leaves decurrent or not. When, therefore, a specimen of one which has usually sessile leaves is met with having them slightly decurrent, it has been, on that account alone, set down as intermediate between that and some other species to which it shows no approach in any other point; and thus figures in books as a hybrid, or a distinct species, according to the tendency of its describer.
One source of deception as to the real permanency of an abnormal form, even when observed without variation in a wild state in the greatest abundance, arises from the facility with which certain perennials, or shrubs, multiply by runners, suckers, bulbs, or other modes of division, especially in cool, moist, and comparatively sunless climates like our own. Individual peculiarities are thus propagated naturally in a wild state, as we do artificially in gardens, spreading over the country in such numbers, as to be mistaken by the cursory observer for races, if not for species. Seedling brambles, mints, creeping-rooted weeds, &c., are rare in our climate; the bulbiferous Alliums, the viviparous grasses, many introduced plants, such as the Periwinkles, Hypericum grandiflorum, &c., seldom produce any seed. Carduus arvensis may often be observed in great numbers for hundreds of yards along a roadside, all of one sex, evidently all from suckers, originating, perhaps, many years back in a single individual. In like manner, an individual bramble will, in the course of years, spread through a whole wood; a fragment of coltsfoot or couch grass infest large fields; or Elodea Canadensis fill our canals, though not a single seedling be raised.
One of the greatest difficulties in arriving at a just conclusion as to the value to be attached to intermediate forms, is owing to the doubts which still hang over the question of hybridity. The existence of hybrids in the vegetable kingdom, less perfect in their nature than true species, analogous to the mule among animals, has at all times been a popular notion; and wild plants, having some resemblance to cultivated or useful ones, but less perfect in respect of the qualities sought from them, have in most countries been stigmatised as bastards. Linnæus corrected many of these popular errors which had crept into the scientific nomenclature of the day; but he still gave his sanction to the idea of the hybrid origin of certain species, by adopting the term as the specific name in certain cases, without, however, probably having given the matter much consideration. Since his time it has been shown that his Chelidonium hybridum, Vioia hybrida, Campanula hybrida, Chenopodium hybridum, &c., are genuine, substantive species; and the existence of hybrids in a state of nature has been denied by several botanists, and admitted only with great reservations by some even of the most distinguished ones of the present day. Others, on the contrary, of our most acute observers, having acquired convincing evidence of natural hybridity in a few cases, have generalized their conclusions; they have supposed natural hybrids to be of constant and frequent occurrence; and they have ascribed to this cause alone the majority of variations from the supposed typical forms of species, or even attributed to original hybridisations the multitude of nearly allied, but constant species, in several of the largest genera.
That wild hybrids do exist, I had already convincing evidence from personal observation during the years 1825 and 1826, when my attention was specially directed to the search after them in the Pyrenees and the South of France; and the proofs brought forward by other observers are not to be resisted. But the cases are very few, and it requires great caution before we can attribute to this cause the appearance of individuals of a species showing some approach in their characters to some other species. In Western Europe, there are but six genera in which I have myself been able to collect satisfactory proofs of natural hybrids, viz., Cistus (including Helianthemum), Geum, Saxifraga, Gentiana, Verbascum, and Digitalis. We are also bound to admit on the authority of other observers, at least four more, viz., Epilobium, Carduus (including Cirsium), Salix, and Narcissus, and perhaps also Centaurea, Erica, Rumex, and Polygonum. The supposed hybrids in Viola, Medicago, Primula, if cross-breeds at all, are probably between varieties of one species, not between two species. The cases adduced in Serapias, Aceras, and Orchis, require much farther investigation, especially now that it is known how singular are the anomalies which occasionally break out in the flowers of some Orchideæ, where hybridity is quite out of the question. The wild hybrids described in Dianthus, Galium, Hieracium, and Stachys, appear to me to be exceedingly doubtful; and in the single alleged instance among Gramineæ, that of the hybrid between Ægilops and Triticum, one of the parents at least is in a cultivated state. We must also bear in mind the observation of C. F. Gaertner, how numerous are the genera, where several nearly-allied species grow together in the greatest abundance all over Europe, and are never known to hybridise. Such are Ranunculus acris, repens, and bulbosus; Brassica Sinapistrum and nigrum; Stellaria Holostea and graminea; Geranium molle, pusillum, and rotundifolium; Potentilla argentea, verna, reptans, and anserina, &c., &c.
Admitting, however, that in the extensive and diversified Flora of Europe, wild hybrids have been observed in some twenty to twenty- five genera, if we consider that the species in those genera which will hybridise are but few; that the individuals raised are always very few, and often isolated; that they are either not reproduced in a second generation, or their offspring is a further approach to the parent species; and that even two individuals sprung directly from the same two parent species generally differ quite as much from each other as from one of their parents; we shall find it very difficult to believe in the permanent establishment of wild hybrid intermediate races, distinguished by positive characters; and we cannot but reprobate the modern practice of introducing into Floras and systematic works so-called hybrid species, races, or varieties, with a pretended diagnosis, which are, in fact, nothing but descriptions of individuals. The reader is thus misled; for the chances are that the diagnosis will not apply to any fresh individual he may find of the same hybrid. A mere indication in the Flora or other work, under each parent species, of the existence or suspected existence of hybrids with such and such other species, is always sufficient for all legitimate purposes.
None of the above observations apply to artificial hybrids, the subject of so much careful experiment on the part of W. Herbert, C. F. Gaertner, A. Braun, Naudin, and others, whose labours have done much towards elucidating the physiology of hybrids in general. But the plants thus experimented upon were placed in exceptional circumstances; and the results obtained bear but indirectly on the evidences of wild hybridity, or are often indeed calculated in some measure to mislead. The fact that artificial impregnation between certain species can be effected with great facility, is no proof that these species, or others allied to them, are the more apt to produce hybrids in a wild state. It is well known, for instance, how numerous are our garden hybrids in the genus Erica. When I worked up that genus for the Prodromus, I had before me wild specimens from various collectors of almost every Cape species, and often in considerable numbers, including the original specimens of Masson, Niven, and others, from whom were obtained the majority of our garden forms; I examined them all with great care, as well as nearly complete sets of our then cultivated varieties, pure or hybrid, from four of our largest living collections, and thus acquired a tolerable idea of the characteristic features assumed by hybrids in this genus. Yet among the wild plants there was only one, in an old collection of Roxburgh's, that had the slightest appearance of a hybrid; and among European ones, the only instance I am aware of, is that mentioned by Hewett Watson, of the Cornish hybrids, between E. ciliaris and E. Tetralix. So in the genus Dianthus, according to C. P. Gaertner, artificial hybrids are very readily produced, and are more fertile than those of almost any other plants, and yet wild hybrids are very rare. Lecoq, it is true, speaks of hybrids between D. Monspessulasnus and D. Seguieri as being very abundant in the Montdore, and certainly these two species are, in that locality, very variable, but not more so than I have observed them in the Pyrenees, Provence, &c., when growing separately.
The apparent permanence given by cultivation to abnormal or intermediate races has afforded a plausible argument against the supposed constancy of the limits assigned to species in nature. The manner in which the Cape Pelargoniums, the South American Verbenas and Petunias, &c., have produced varieties without end, blending the original species together in inextricable confusion, is well known; and gardeners reckon with tolerable certainty on reproducing, by seed, the numerous varieties of our kitchen-garden annuals. But, as in the case of artificial hybrids, these plants are then placed in an anomalous condition, in which they are maintained by cultivation only. Restore them to the conditions of a wild growth, leave them exposed to all those obstacles which nature opposes to their multiplication, and they will soon yield to the more hardy or more favoured genuine forms, and gradually perish without being reproduced. This temporary character, when wild, may be observed in all the extraordinary aberrations from the common form, however healthy the individuals may appear, such as Orchis pyramidalis with spurless flowers, or Linaria vulgaris, with five spurs; Helianthemum vulgare, or Narcissus juncifolius, with linear or divided petals; or Stellaria Holostea, with no petals at all, &c.; they are none of them perpetuated; they cannot resist the immense chances there always are against the offspring of any one individual plant ever coming to perfection.[3]
To sum up the foregoing remarks:—When a plant is observed apparently allied to some known species, but differing in one or more characters hitherto unobserved or unrecorded in that species, before deciding whether it be a distinct species or a mere variety, the points to be considered, independently of direct experiment, will be chiefly the following:—
Are the distinctive characters such as can be accounted for by station, climate, or other known influences, of which I have enumerated several in my Handbook? (Introd. p. 31 and 32.)
Are the circumstances under which it was growing, and its general aspect, such as to suggest its being a hybrid between the allied and some other species?
Are the distinctive characters such as are known to occur in mere varieties of other species, more especially of such as are systematically allied to the one in question?
Is the plant in question, an isolated individual (including in the same category any number of individuals naturally propagated from a single one by runners, suckers, bulbs, &c.); or has it been observed in more or less abundance in any variety of stations over any considerable independent geographical area, or in any important part of the area of the allied species?
Is the distinctive character relied on confined to a single organ, or is it more or less accompanied by differences in other organs of the plant; and, if so, how far does the plant retain all the characters in all the different stations and localities where it has been observed?
Have intermediates between the plant and its allied species been sought for in any considerable portion of the area of the latter, and especially in those countries where it is most liable to variations? And, if such intermediates exist, what is their relative number, and how far do they vary, or pass one into another in all, or any, and which, of the points in which the plant in question differs from its allied species?
It is only in proportion as the evidence on all these points is full, satisfactory, and reliable, that our decisions on the value of a species can be fair, independently of any want of tact, experience, powers of observation or judgment, which we are all liable to; and not to mention the cases of but too frequent occurrence where ignorance, a false pride, vanity, a love of controversy, a desire of flattering, or even mercenary motives, have influenced the reckless splitting or over-hasty reunion of species.
With regard to direct experiment in aid of inductive reasoning, it has been said that cultivation is a sure and easy test of the identity or distinctness of species; and nothing is more common than to find as an argument in support of a "critical" species, that it has been growing for many years in a garden, always retaining its distinctive characters. But the results thus obtained are liable to very great fallacies, unless the experiment is followed out in all its bearings, with many precautions rarely attended to; and what is supposed to furnish irresistible proofs of permanency of character, when inquired into, will often be found to add nothing at all to the arguments derived from observation.
In the first place, it is a very common practice, in thus testing by cultivation the permanency of character in a plant, to remove it bodily to a garden, and there to propagate it by suckers, cuttings, or other modes of division—an experiment which may, indeed, show the immediate effects of soil, climate, or other extraneous influences on the individual—but, as a test of value between species and variety, it can be of no avail. It is the very method adopted by gardeners for perpetuating individual variations. The only mode in which the test can really bear upon the question, is by sowing the seed, and observing the results in future generations. And in this proceeding it is not enough to raise a few plants in one spot, for two or three generations; for such a course would prove our varieties of kitchen-garden annuals to be all distinct species, which we all know is not the fact; the cultivation must be on a large scale, under circumstances of soil, climate, &c., as varied as the plant will bear, and for many generations; and, after all, the proofs of distinctness can scarcely be absolute, for they consist, as it were, in proving a negative. The object is not to show how long a particular form can be made to endure, but that it will always endure, in spite of external influences or other accidents—that it will not vary under any circumstances, or at any time. The cultivation must be that of the gardener, whose object is to raise new varieties—not of the curator, desirous of keeping his botanic garden usefully cropped, and correctly named—still less of the botanist, who seeks to uphold a species he has set up. The former sows extensively in different localities, in order to have the greater chance of accidental aberration; he carefully watches his seedlings as they grow up, and selects his seeds for the next generation from such plants as show the slightest tendency to vary in the wished-for direction. The curator, on the contrary, anxious to keep his types true, if he selects the seed at all, takes it from the most healthy, normal, and characteristic individuals.
To illustrate the very slender grounds upon which botanists of considerable and well-deserved reputation will occasionally adduce the results of cultivation as convincing proofs of specific distinctness, let us select from Grenier and Godron's flora an instance taken from a genus worked up with great care, by one of the most accurate observers of individual varieties and local races, and whose views as to their reception as a species M. Grenier entirely adopts. Under Galium spurium, he says, "Cette espéce se produisant invariablement de graines sans perdre aucun de ses caractères, ne saurait être confondue avec le G. Aparine." To justify so sweeping and positive an assertion, we must suppose that he, or some one on whose exactness he has implicit reliance, has sown in several successive years each of the three varieties he mentions of G. spurium, besides the smaller forms of what he considers the true G. aparine; that he has raised them in considerable quantities; that he has each year selected his seeds from such of his own seedlings as have shown any tendency to variation; and that this process has been carried on in different soils, in different situations, in different climates, and at different seasons. It is scarcely to be imagined that this has been done for so very uninteresting a plant; and yet, if any one of these precautions has been neglected, it cannot be said to be proved that the plant will never lose any of its characters. And, after all, what are these characters, so invariably reproduced? Not the want of hairs at the nodes, nor the narrowness of the leaves, for these he admits to be variable in his G. tenerum—besides, that such hairiness is often scarcely perceptible in the stoutest specimens of G. aparine—nor yet the glabrous or hispid fruit, for that is admitted to occur in both his species. There remain, first, the size of the plant, not more than a foot in G. spurium, often above three feet in G. aparine; but to which would he refer the numerous specimens occurring in some localities from 1 to 2 feet high? 2ndly. The articulations, swollen in G. aparine, but not in G. spurium, a mere result of the luxuriance of the former. 3dly. The size of the fruit, 4 to 5 millimetres in G. aparine, 3 or 4 times smaller, consequently 1 to 112 millimetres, in G. spurium. To verify this character, I have measured the fruits of numerous specimens, living and dried, of both forms, and I have never found the diameter quite so little as 2 millimetres; but from that size I have measured every intermediate from half to half millimetres, up to 5 millim., the largest I have met with. And 4thly. The hairs of the fruit, rising from a small tubercle in G. aparine, and no such tubercles in G. spurium. As to this point, if we take the hairy-fruited varieties of each form, I confess myself unable to discover any difference but what depends on size; the larger the fruit, and the larger the hairs, the more prominent are the tubercles at the base. Upon the whole, as far as my own experience goes, the results of cultivation constitute an item, but one item only, and that often a fallacious one, among the evidences on which the permanency of character is to be judged by inductive reasoning.
Even the proof of specific identity by cultivation is often liable to error. Such experiments are often several years in carrying on; it is not to be expected that they can be daily watched during the whole of that period, and all who have had the charge of gardens must be aware of the mishaps which may occur during a short absence, without being directly noticed—such as labels accidentally or intentionally destroyed or misplaced, or the sown seed failing, or the seedling perishing, and replaced accidentally by some common allied species or variety. The abnormal circumstances in which a plant under cultivation is placed, may also induce an apparent approach to some other species, without any real alteration of essential character. I have already instanced the Trifolium repens and hybridum, as one in which the supposed proof of identity by cultivation, notwithstanding my confidence in the experimenter, produces no conviction in my mind; and it is only with great hesitation that I have admitted the specific identity of the Primrose and the Cowslip, although several experimenters are stated to have raised the one from the other. In all cases, proof by cultivation seems to require some confirmation by the observation of wild nature.
With regard to genera and orders, I need not here repeat the views I laid before the Linnean Society on a former occasion ("Journ. Linn. Soc. Bot.," v. ii., p. 31), on the importance of maintaining, for the convenience of language and study, large genera and orders, in preference to breaking them up into small independent ones. But the opinions I have on that and other occasions expressed, that genera, as such, have no independent existence in nature, have been in some measure misunderstood. Far be it from me to deny that groups of species exist in nature, resembling each other more than they do the species of any other group—that some of these groups, consisting of two, three, or any number of species, are in nature distinguished from all others by a number of well-marked characters, or that a single species may be so isolated; whilst others can only be separated by single or un- important or variable characters—that these groups may be collected into groups of a higher order, consisting in like manner of two, three, or any number of smaller ones, similarly distinguished in nature by more or less marked or important characters—that this synthetical process, always following natural indications, may be carried on till we arrive at the two or three great primary divisions of the vegetable kingdom—and that in all the stages very great differences exist in nature in the definiteness of the groups established, and in the relative importance of the characters distinguishing them; but that, generally speaking, the characters of a large group are more important than those which only distinguish its minor subordinates; for on these principles,—on a nice, appreciation of affinities (or calculation of resemblances and differences), and of the importance of characters, as indicated in nature,—depends the whole value of a natural classification. What I meant to assert was, that nature has not assigned everywhere precise definite limits to the groups she has indicated; nor has she fixed upon two stages in the synthetical process more definite than any others, to be marked out, the one for genera, the others for orders. These are often selected and limited, arbitrarily, though necessarily, for the convenience of system, language, and reference. The characters of plants are, indeed, very different in importance; but such differences are relative, not absolute; we cannot say that certain characters are of ordinal importance, whilst certain others are only of tribual, generic, or sectional value. Nor does any one character retain the same importance throughout the vegetable kingdom. There is no test by which we can determine whether two groups formed in different parts of the field of classification are co-equal in value, or whether the one be of a higher grade than the other.
It becomes, therefore, necessary to consider what constitutes the relative importance in characters, how far we can safely be guided by it in the formation of genera, orders, or other groups, and when it is that we run the risk of being led astray by the too close adherence to the rules laid down.
The most important character, in plants, will always be that which in the greatest number of species (or groups of species) is the most constantly accompanied by the greatest general resemblances among those species, and differences from all others—that which collects into the same group species showing the greatest general conformity in the structure and economy of all their parts, and which may, therefore, be supposed to be the most uniformly influenced by or acting upon the specific constitution of plants.
This question of the relative importance of characters has been frequently discussed, especially by French botanists; and by none has it been so clearly put as by the elder De Candolle, in his admirable "Théorie Elémentaire." He there lays it down as a rule, that the value of a character is in the compound ratio of the importance of the organ it is derived from, and of the point of view in which that organ is considered.
But, in regard to the first element, how are we to determine the relative importance of organs? De Candolle indicates two modes: à priori, by the consideration of the functions they perform, or the part they take in the vital phenomena; à posteriori, by the observation of the extent to which they prevail, the number of species in which they exist. The former mode has been the one eagerly pursued or attempted by the greater number of generalizing botanists; the latter is that which, after all, has practically led to the best classifications; and though characterized by De Candolle as "très ingénieux mais peu applicable," is really that which he has himself followed in the best parts of his systematic works.
These two modes of argument correspond to those arguments from final causes, and from observation of facts, which have divided zoologists. But in plants we are much less able even than in animals to trace the modifications of form and structure to any final causes. The animal goes after, and selects his food; and the whole economy of his structure is modified according to the nature of that food, and where and how it is to be obtained. The plant is stationary and must take what food comes within its reach; and that food, and the mode of absorbing it, is very similar in all species; nor can we discover any other final cause why one set of plants, for instance, should always have alternate and another opposite leaves—why in Digitalis purpurea there should be on an average 1200 seeds fecundated and ripened for every two pair of stamens, whilst in several Acacias there should be 10,000 stamens to every head of flowers, which sets and ripens some half-dozen or a dozen seeds only. And yet characters like these are, in some instances, so constantly accompanied by so many general resemblances as clearly to distinguish natural groups of several thousand species.
The importance of organs, also, in another way, admits of two distinct qualifications, not always concurrent: physiological importance and systematic importance. The compounding these two applications of the word is apt to lead into some fallacies. De Candolle, for instance, after showing the impossibility of establishing any comparison of relative importance in the functions of the organs of reproduction and those of vegetation, but explaining why it is that the former practically supply better characters than the latter, lays down the following scale of progression in the importance of these reproductive organs:—
1. L'embryon qui est le but de tout;
2. Les organes sexuels, qui en sont le moyen;
3. Les enveloppes de l'embryon;
4. Les enveloppes des organes sexuels;
5. Les nectaires ou organes accessoires.
But, in the first place, the embryo in its perfect state can no longer be called an organ of the parent plant. Until it is fully formed, it supplies no characters. When once formed, it has no function to perform till it commences life as a new independent being. And this is the great reason of the importance of the characters it then supplies. It is a whole plant, not an organ of a plant.
Secondly, the same arguments which show the impossibility of comparing the importance of the functions of the reproductive and vegetative organs, would apply to the flower (or 'les organes sexuels') and the fruit (comprised in 'les enveloppes de l'embryon')—the apparatus for producing the embryo and the apparatus for bringing it to perfection—and, again, in the flower, between the male and the female organs; for all these are equally essential for completing the series of vital phenomena which continue the species. It is true that, exceptionally, embryos may be formed and brought to perfection without normal fertilisation, but so also the whole series may be dispensed with, and plants are reproduced by buds without passing through the embryo state; but in all phanerogamic species, for their normal reproduction, the whole series, the male organs, the female organs, and the organs of maturation, are equally essential.
Perhaps all that can be said of the relative importance of organs with reference to their functions is this: That the so-called essential organs, the sexual organs, and the organs of maturation among the reproductive, and the perfect leaves or foliaceous surfaces, and the root-fibres among the nutritive, stand first; the protective organs, such as floral and fruit envelopes, bud-scales, &c., occupy the second rank; and accessory organs, including epidermal scales, are the lowest. But here again the relative importance of these organs is not proved by à priori arguments, derived from the necessity of their presence for performing those functions, but from the observation of the degree of constancy of their being so employed. Cryptogamic plants have sometimes none, sometimes not all, of the organs of the first degree; yet nutrition, fertilisation, and reproduction, take place, but by other means, with another class of organs. And had we observed that, in phanerogamous plants, fertilisation of the ovule never took place unless the sexual organs were enclosed in floral envelopes, we should have classed the latter among the essential organs of the first class.
The importance of characters, in as far as derived from the importance of the organs they relate to, would follow the same gradation,—observation (not theory) teaching us, however, to place those derived from the reproductive organs of each degree before the corresponding ones derived from the nutritive organs; and those derived from the embryo or young plant, more especially at the moment of germination, above all.
But the second element in the ratio of value of characters, the point of view in which the organs are considered, is one which experience shows to be often far more important than the nature of the organ itself, and the neglect of which contributes more than anything to the degeneracy of an apparently natural classification into a purely artificial one. The principal characters which an organ, or set of organs, can thus supply, and their relative importance, are admirably expounded by De Candolle, in his Taxonomie, div. I., chap. 3. He there establishes the following scale of gradation, in which I have ventured to make some slight modification in expression, but which I think should never be lost sight of by the systematist who has any pretension to establish natural groups.
1. The real presence or absence of organs (parts of organs, or sets of organs), independent of adherence or accidental abortion.
2. Their arrangement, or relative position, and numbers, as affecting or indicating the general plan upon which the plant is constructed.
3. Their external form, relative size, continuity or articulation, &c., all subordinate to the preceding class, only acquiring importance when indicative of a result from general arrangement.
4. Their functions and sensible qualities,—the results, rather than the causes, of the preceding modifications.
By combining this scale of relative importance with that derived from the nature of the organs themselves, it might be possible to frame a general scale of relative importance of characters, which, with other rules suggested by the observation of the comparative prevalence of particular characters, might assist in judging of the expediency of describing as a new genus or order any newly-discovered plant which does not come precisely within the limits previously fixed for any known genus or order. But, in the grouping together any number of species or genera already known, the relative value of the characters relied upon should be tested, at every step, by a comparison with all the other features of the plants. The blind adherence to a pre-established scale, in distributing into genera the species of a large order, renders such a classification purely artificial. It had been ascertained that the relative arrangement of the radicle and cotyledons in the embryo of Cruciferæ, the relative prominence of the ribs of the fruit, and the number and arrangement of the vittas in Umbelliferæ, the various modifications of the pappus in Compositæ, were in many cases remarkably constant, not only in species, but in many very natural genera. But by taking these characters as absolute, and considering every slight modification of them as of generic importance, many of the most natural groups in those orders have been broken up, and split down almost to single species, classed into purely artificial tribes and sub-tribes. So, also, a character generally important may, in some instances, separate a single species from a large order with which it may agree in every other respect. The dismemberment of such exceptional species from that order,—as, for instance, that of Phryma from Verbenaceæ,—becomes then purely artificial, and contrary to all principles laid down for natural classification.
This introduction of artificial arrangements, under the disguise of a strict adherence to the rules of the natural system, is much promoted by a tendency to which we systematists are all very liable. It has happened to most close observers to have on some occasion brought forward some character till then comparatively neglected, but which has proved to be eminently useful for establishing natural groups in particular genera, orders, or classes. Such a character is then apt to assume an undue importance in the observer's mind, and to be applied by him indiscriminately throughout the vegetable kingdom. The arrangement of the parts of the floral whorls with relation to the main axis of inflorescence, the aestivation of the floral envelopes, the relative attachment of the floral whorls, and consequent modifications in form of the torus, disk, or floral receptacle; the numbers absolute and relative of the parts in the several floral whorls, the position of the ovary with relation to the rest of the flower, that of the ovules with relation to the ovary, the structure of the fruit, and even the most important of all, the relation of the embryo to the seed, and, the seat of deposit of starch for supplying the first nutriment to the growing embryo—whether as albumen around it, or in its cotyledons, or in the intermediate point (the collet) between the radicle and cotyledons—all characters which more or less generally mark out large and highly natural orders, have nevertheless, each in their turn, on some occasion or other, been applied too strictly, so as to dissever groups otherwise most natural.
On the other hand, however closely we follow natural indications, our system must be to a certain degree artificial. A purely natural method of arranging species and genera is impossible; at least, none has ever been brought forward. The affinities and cross-affinities of plants are so complicated and intertwined, that we have no method of representing them either by a linear series, or by mapping them out on a plane surface. Many of the most natural groups have no definite limits; and yet, to form any clear idea of them for the purpose of study, we must assign limits. The truly German idea of taking one species or genus as a normal type of a genus or order, and grouping others around it as more perfect, or reduced, or collaterally aberrant forms, leads to no practical results. However well it may read in chamber speculations, it produces nothing but confusion when applied to the actual grouping of species. There is no plant which arguments like those usually brought forward may not show equally well to be an aberrant form of almost any number of different types. The absurdity of such a system appears to me never to have been so fully exemplified as in an elaborate work received whilst writing out these pages, in which, for instance, Begoniaceæ, Melastomaceæ, Gesneriaceæ, Burmanniaceæ, and Orchideæ, are collected into one series, whilst Memecyleæ, Bignoniaceæ, and Iri- deæ, are far removed from them.
It appears to me, therefore, that whilst in an artificial or analytical system for finding out the name of a plant, one prominent character is selected to mark out each division; in a natural or synthetical system, on the contrary, for the arrangement and study of plants, the affinities according to which they are grouped should be judged of by the combination of as many and as constant characters as possible, derived from all parts of the plants; but that, in both cases, characters must be assigned. "Character non facit genus," it is true; but a genus without a character is of no assistance to the mind of the naturalist.
- ↑ The great length to which this paper, read at three different meetings of the Society, extended, prevented its immediate publication, and the subsequent appearance of Mr. Darwin's work rendered obsolete the short allusions I had made to the theories advanced on the origin of species. The present extract, however, is purely practical, relating to species as they now exist, and have existed within historical periods, quite independently of their theoretical origin.
- ↑ Since the above was written out, and when on the point of reading it to the Society, I observed in the "Gardener's Chronicle" of the 13th Nov., 1858, a very important communication from Mr. Darwin, in which he states his conviction that this cross-breeding between different individuals of the same species is universal. I admit readily that the vast number of curious observations he has made, most of them hitherto unpublished, tend to show that this cross-breeding is very much more general than we had supposed, and perhaps indispensable in certain species, or, at any rate, under certain climatological conditions; but I think there are numerous facts which argue strongly against its universality. On the other hand, Naudin, in a still more recent number of the "Annales des Sciences Naturelles," in which he gives an account of some highly instructive experiments connected with hybridity, may have been led too far in his doubts as to the frequency of cross-fecundation in some of the genera he has experimented upon.
- ↑ As a familiar instance of the disproportionate chances against the success of any individual seed in a wild plant, take the foxglove (Digitalis purpurea). It will often ripen 200 capsules, and even above twice that number have been counted on one plant, and the number of good seeds I have found in one capsule have varied from 800 to 1200. Taking, however, the average number of good seeds shed by every plant as only 100,000; as the average number of foxgloves in a given district remains the same year after year and century after century, we have only one plant coming to perfection for every 99,999 that perish either as seeds or young plants. It is often very curious to observe the luxuriant crops of crowded seedlings of various plants in autumn, which totally disappear before the following flowering season; and year after year, an attentive examination of the moors and heaths in many parts of Western England will disclose a profusion of seedling oaks, one, two, or three years old, not one of which ever attains the size and age even of a bush.