ginal species; because a tendency to this has been found to take place both in plants and animals. Varieties. A variety differs from a race, in not being susceptible of propagation by seed, at least with any thing like constancy and certainty. For example, the jargonelle pear may be continued by seed; but a jargonelle pear with variegated foliage could not be so propagated with certainty. We allow, however, that, if a great many seeds taken from the fruit of a jargonelle pear with variegated leaves were sown, some of the plants so raised would probably show variegation in their foliage. The same thing, we think, would take place in the case of sowing the seed of variegated hollies, or of fastigiate or pendulous-branched plants, but to what extent it is impossible to say. It certainly would not take place to such an extent as to confound varieties with races, or to render it desirable to propagate varieties in this way; and, consequently, varieties are always propagated by some modification of division, such as by cuttings, layers, grafting, &c. Variations differ from varieties in not being transmittible by any mode of propagation. They are always produced by local circumstances operating on the individual; and the moment these circumstances are changed, the variation disappears. For example, plants grown in the dark will have their leaves white; other plants with hairy leaves, when grown in water, will have their leaves smooth; and the hydrangea, grown in a certain description of soil, will have its flowers blue: but, remove the plant with white leaves to the light, and place the plant grown in water in dry soil, and the hydrangea in common soil, and it will be found that the leaves of the first will become green, and those of the second hairy, and that the flowers of the hydrangea will resume their natural pinkish hue. Mules and Hybrids. — Some confusion exists as to the use of these terms, when applied to plants. The term mule, we think, ought to be limited to such hybrids as are raised between different aboriginal species, and which it is believed are not susceptible of propagation by seed: such, for example, as the Pyrus spùria, which seems to be a hybrid between Pyrus Sórbus or A`ria and Pyrus Chamaméspilus. The term hybrid, on the other hand, we think, ought to be limited to the produce, by cross fecundation, of different races and varieties of the same species. Every one knows that this is one of the most important elements of culture, having given rise to the most valuable garden flowers, table fruits, culinary vegetables, and agricultural plants. Botanical Species. It will be seen, from the preceding remarks, that we follow De Candolle in denominating what Dr. Lindley and other British botanists distinguish as botanical species, races, or varieties. It is not to be supposed, however, that we undervalue botanical species, or that we either deny the distinctness of many that exist, or the propriety of having different names for them, and keeping them distinct. On the contrary, to compare plants with men, we consider aboriginal species as mere savages, and botanical species, or, according to De Candolle's classification, races and varieties, as civilised beings. What, then, it may be asked, is our object in endeavouring to show that many of our botanical species are only varieties? We have two objects in view; and both, we think, are very important ones. In the first place, by confounding varieties or garden or botanical species with aboriginal ones, a beginner, ignorant of the extent to which this is done, cannot make a judicious selection; and while, in the case Fráxinus, for instance (of which there are, in reality, as we think, only three species known, exclusive of O'rnus), if he wished to select, perhaps, three sorts, he might, instead of selecting the three really distinct species, which would give him a complete idea of the genus, fix on three of the varieties of F. americana or of F. excélsior, which would only give him an idea of one species. In the second place, we wish to prevent beginners, in the study or cultivation of trees, from puzzling themselves unnecessarily to make out the minute differences which distinguish what are called botanical species; believing, as we do, that it is impossible to make out many of these from the specific charac ters given of them in botanical works. The nicety of these distinctions has we know, deterred numbers from the study of practical botany; and has prevented others, who have had the courage to proceed, from ever hoping to attain any satisfactory result. It has also (and this we consider to be the most important part of the evil) prevented many persons from forming collections of trees and shrubs, by inducing them to believe that such collections could never be made anything like complete, without incurring an expense greatly beyond what is really necessary. Instead of this being the case, the number of hardy trees and shrubs is so small, when compared with that of hardy herbaceous plants, or stove or green-house plants, that there cannot be the slightest difficulty in becoming acquainted with all the species, provided these and the varieties are only seen together; and the cost of as complete a collection of species as can be procured in the London nurseries is such as to be within the reach of every planter of the grounds of a villa of a single acre in extent. The mode by which we propose to attain these objects is very simple. We shall retain the botanical species and varieties in the catalogues, so far as we believe them to exist; but we shall, in every case, place before them the name of the aboriginal species to which they belong: for example, in the case of the genus Fraxinus, which, in our Hortus Britannicus, appears to consist of 41 species and 12 varieties, we shall rank 30 of the species under the head of F. americàna, two of them under the head of F. lentiscifòlia, and the remainder under the head of F. excélsior. It may be asked, whether it would not be better at once to make distinct genera of these three species? To which we answer, that it would not; because they are all so obviously of the same general appearance, as evidently to belong to the same family. There would be the same objection to separating the oak family into different genera; though we think it highly probable that there are not a dozen aboriginal species of oak in the world. Every division, or conglomeration, in botany, that can assist the mind to generalise, at the same time assists it in particularising; and it will be found much more easy, after throwing all the races or varieties of Fraxinus americàna into one group, to distinguish them from each other, than by leaving them as distinct species, and having the trouble of distinguishing them, not only from other races or varieties of F. americàna, but also from all the races or varieties of F. excélsior. Such are the principles which we have adopted, to guide us in arranging species, races, and varieties, from a perfect conviction of their truth. If we had not had an opportunity of observing, for several years past, the collections of trees and shrubs in the neighbourhood of London, and of studying them at every season of the year, with a view to the production of this work, we should never have been able to arrive at these principles, or to adopt them from others, with any degree of satisfaction to our own minds. We are, however, perfectly satisfied that we are in the right path; and we feel convinced that all practical botanists who have had an opportunity of making similar observations, and who have made them, will approve of our arrangement. SECT. III. Of the Mode of describing Trees and Shrubs. It is foreign to the object of this work, to enter any farther into botanical science than becomes necessary to elucidate the reasons which have induced us to depart, in any particular, from general practice. It will readily be conceived, from what has been stated in the preceding section, that we attach no great value to what are called the specific characters of botanical species; that is, of what we shall distinguish as races in some cases, and varieties in others. The reason is, that we do not think it is often practicable to discover a species or race by such characters alone. The specific character of an aboriginal species we consider in a different point of view; for, as we think all aboriginal species must be decidedly distinct, so we think it practicable to render this distinctness so obvious, in the few words which constitute a specific character, that the name of a plant may be discovered by it. To recur to the genera Fraxinus and Cratæ`gus, we will ask any botanist, either practical or theoretical, whether, from the specific characters of the botanical species of F. americàna or of C. Oxyacántha, he could discover the individuals to which those names are intended to apply, without having recourse to dried specimens or engravings? We ask the same question with reference to most of the alleged species of Sàlix, U'lmus, Quércus, Pìnus, and Rùbus. We admit that many of these botanical species, or varieties as we consider them, may be made out from lengthened descriptions; but we deny the practicability of doing this, in many cases, from short specific characters. That we may not be misunderstood, we refer more particularly to the genera Quércus, Sàlix, Ulmus, and Rubus; and even to Tília. But, though we question the utility of specific characters to botanical species as such, we are of opinion that they may be of some use when applied to these species, as being varieties of an aboriginal species, and indicating that they are such. For example, in the case of the specific character of Fraxinus pubescens, caroliniana, láncea, &c., as absolute species, and to be compared with different botanical species of the same aboriginal species, and also of F. excélsior, we think it would be extremely difficult, if not impossible, to apply them; but, if it were known that these botanical species were only varieties of F. americàna, the difficulties of distinguishing them would be greatly diminished. For this reason we shall, in many cases, adopt the specific characters of botanical species given by botanists, adding to them such descriptive particulars as our own observation has enabled us to supply. We may here refer to two causes, which have not only contributed to the great imperfection of the specific characters of botanical species; but which have been the means of multiplying the number and descriptions of these species in books, to an extent which, we are persuaded, does not exist in nature. One of the practices to which we allude is, that of describing species from dried specimens only; and the other, that of mistaking varieties for species by collectors. We admit that the first of these practices is unavoidable in the infant state of botanical science; and that it must necessarily be continued, till botanists shall rise up in every country in such numbers, and of such acquirements, as to be able to describe the plants of every country from nature; or till all the species, or all the alleged species, of every genus of plants shall be assembled together in one spot, and what are really aboriginal species shall be determined, after observing them for a series of years. Happily, both these results are in progress of attainment: botanists are beginning to spring up in every civilised country, or to emigrate from old, and settle in newly discovered countries; and, in all the wealthiest governments of Europe, assemblages of plants are being made in botanic gardens. If the directors of these gardens were to cooperate, and each to undertake the collection and the study of one or more genera of hardy plants, we should, at no distant period, be able to say what are really species, and what are not. If botanic gardens were established in every country and climate of the world, and the whole of the directors of these gardens were to act in concert (which concert, being quite distinct from political associations, would not be objected to by any government), in each garden might be assembled all the alleged species or varieties of at least two or three genera, those being selected for which its climate, situation, soil, and extent were most suitable; and, after a few years, the aboriginal species, and the more prominent varieties, might be determined In the meantime, this process might be commenced in many of the botanic gardens already established in the temperate regions of the world; and we have already shown (p. 192.) how, in every country in these regions, the determination of species, and their nomenclature, might be effected, as far as respects hardy trees and shrubs. on. When the natural system of botany comes to be more generally understood and cultivated, there will not, we may presume, be that desire, which now seems to exist among botanists, to increase the number of species. Formerly, and more especially among the disciples of Linnæus, the great business of the botanist was to collect, name, and describe plants. These were then the highest departments of the science; but, now, the anatomical, physiological, and chemical studies of plants occupy that station; and the naming and describing of species is considered as comparatively mechanical. The other cause which has contributed to increase the number of supposed species is, the natural eagerness of botanical collectors, sent abroad in order to discover novelties, to find something new, in order to answer the end for which they were sent out. This is very natural: and where there is a strong desire for, and also an important interest concerned in, obtaining anything, either the thing sought for, or something like it, will be found. Hence the young and ardent collector will seize upon every variation produced by climate, soil, situation, age, or even accident, to add another specimen to his herbarium; which enables the botanist at home to add another name to the number of his species. This we believe to be much more frequently done from practical inexperience, than from any intention to deceive; so different is the appearance which plants present in a wild state and in a state of cultivation, and, often, in one country from what they do in another country; and so difficult is it to judge of an entire tree by a dried specimen, perhaps only a few inches in length. This state of things, in the comparative infancy of botanical science, is perhaps unavoidable; and it is, doubtless, erring on the safe side, to collect and bring home every thing that can be at all considered as distinct, leaving it to cultivators and botanists to determine afterwards whether it is really so. It is proper, however, to notice this state of things, to aid in accounting for the present state of confusion and uncertainty in the names and characters of trees and shrubs; and to show the little faith that is to be placed in botanical descriptions drawn up from dried specimens of any kind, and more especially from those procured by inexperienced collectors. If this may be considered as applicable to plants generally, it is more particularly so in the case of trees and shrubs; which, from the long period which they require to attain maturity, naturally assume very different appearances under different circumstances: and which, therefore, require to be studied, not only in the same locality, but in different localities, for a number of years, before any decided opinion can be pronounced respecting which are species and which are varieties. It will not, we trust, be supposed, from these observations, that we intend to set ourselves up as a model for imitation, in determining species and describing them; on the contrary, we value the Arboretum part of this Encyclopædia much more, as containing only the names of such things as we know to be really distinct, and actually in existence in England, than for its pretensions in a purely botanical point of view. CHAP. III. TREES AND SHRUBS CONSIDERED WITH REFERENCE ΤΟ THEIR THE large proportion which the ligneous vegetation of the earth's surface bears to its herbage, and the immense extent of the forests in comparison with that of the meadows, pastures, or plains, which it contains, seem to indicate that trees and shrubs act an important part in the economy of our globe. In countries uninhabited by man, the influence of forests must be on the climate, on the soil, and on the number of wild animals and herbaceous vegetables. In civilised countries, to these influences must be added the relation in which trees and shrubs stand to man. It is not our intention to enter farther into these subjects here, than may be necessary to show to what circumstances, in the economical history of trees, we ought chiefly to direct our attention, in composing the history of each particular species. The subject may be divided into two sections. SECT. I. Trees and Shrubs considered with Reference to uncultivated Nature. Ir appears highly probable, that the greater part of the surface of our globe has been, at one time, covered with wood; because, among other reasons, coal is found in almost all countries; at all events, it is certain that this has been the case with the greater part of the temperate regions of the world at no very distant period. North America was, till lately, almost entirely covered with trees and shrubs, and presented few naked surfaces, except those of the alluvial deposits on the banks of its larger rivers; and what was so recently the state of America must, we may reasonably suppose, have once, at least, been that of every other part of the world. The influence which a predominance of forest must have in a country uninhabited by man must have extended to the animals, the herbaceous vegetables, the soil, the waters, and the climate. To wild animals of every kind, especially to those of the more ferocious description, forests have, in all countries, furnished shelter, and, in a great measure, food: birds, insects, and reptiles are the more common inhabitants of forest scenery. Herbaceous plants are, for the most part, destroyed by dense forests; but some kinds, such as epiphytal lichens, mosses, and, in some cases, Orchideæ, are encouraged by the thickness of the shade, and the moist heat which prevails among the trunks and branches of the trees. But the great influence of forest scenery in a wild state is on the soil; and, in this point of view, natural forests may be regarded as a provision of nature for preparing the earth's surface for the cultivation of corn, and of the other plants which constitute the food of man, and of domestic animals. It is unnecessary to show how the soil is furnished with that organised matter, on which alone perfect plants can live, by the decay of leaves, and, ultimately, by the decay of trunks and branches. The waters of a country, the rivers and lakes, are necessarily affected by the state of the woods of that country. These woods must, in all cases, act more or less as a sponge in retaining the water which falls on them; and water must thus be supplied more gradually to the rivers, in countries covered with wood, than in countries which are cleared, and regularly drained. The influence of forest scenery in increasing the moisture of the atmosphere, and in preventing a climate from being so hot in summer, and so cold in winter, as it would otherwise be, is well understood, and, in such a slight outline as the present, requires only to be mentioned. The use of studying the influence of trees in an uncultivated country is, to afford useful hints with reference to the planting or thinning of them in countries which are civilised. That which takes effect on a grand scale, where forests cover many thousand acres, must operate more or less in the same manner where they extend only to hundreds, or even tens, of acres; and, consequently, this influence must be kept in view in the formation of plantations, both useful and ornamental. If the forests and plantations of Britain are no longer of such an extent as to afford a shelter for wolves and hyenas, they still harbour foxes, polecats, snakes, and other noxious animals, and several kinds of carnivorous birds, such as the hawk. The forests in France and Germany still contain wolves and wild boars; and, on most parts of the Continent, the forest is the place of refuge to which man flies for concealment after the commission of crime. (See Gautieri Dello Influsso dei Boschi, &c.) If forests in a wild state supply food to birds and insects, in a civilised country birds and insects may be expected to abound more or less wherever there are trees and shrubs to supply them with food and shelter. |