either now in operation or dependent on a previous geological state of our globe, which have produced the actual distribution of plants on its surface.

This, indeed, was the only course by which he could hope to carry conviction into the mind of the student. Our object, however, being to give the general reader a short sketch of the stage to which the science has been brought by his work, we shall adopt a somewhat different plan, exhibiting first the ideas entertained as to the original creation of species; secondly, the principal theories as to their diffusion up to the commencement of the present era or geological period, and the causes now in operation which oppose or facilitate their further dispersion; and, thirdly, the history of the changes which species are supposed to undergo. We shall treat rather as admitted axioms than as propositions to be proved, such general facts as we may have to adduce.

But, at the outset, it is necessary that we should endeavour to affix some definite idea to the word species which, in modern days, has given rise to so much vague controversy.

The traditional idea to which botanists have given the name of a species, is the same as that commonly attached to the human race as a species, or to any other species of animal-the whole of the individuals descended from one original parent or pair of parents. But we can in no instance obtain any direct evidence of the pedigree of plants beyond a few generations; and the very limitation of the original creation to one individual or pair of each kind is denied by many; a denial which cannot be refuted any more than it can be supported by any convincing proof. The above definition, taken rigorously, becomes therefore practically useless; and various have been the attempts so to modify it, as to convey a precise idea without prejudging these doubtful questions.

Some botanists, especially of the more modern German and French schools, appear to have abandoned all idea of attaching to the word any definite meaning at all, and content themselves with calling species any collection of individuals which resemble each other more than they do any other set of individuals, without any limitation as to the degree of resemblance or difference which shall determine whether they belong to the same or to distinct species. But the more rational definition adopted in substance by all the great masters of the science of the present day is that of a collection of individuals which, by their resemblance to each other, or by other circumstances, we are in'duced to believe are all descended or may have descended from

' one individual or pair of individuals.' The various modifications of this definition enumerated by De Candolle, consist chiefly in the more or less detailed enumeration of the other 'circumstances,' such as hereditary constancy of character, facility of intermixture of breeds, &c., which are taken into consideration in estimating the probability or possibility of the original common descent, and serve as checks to the conclusions drawn from resemblances and differences alone. In the practical application of such a definition to individual cases, in the determination of the specific identity or distinctness of two given plants, there is, indeed, among botanists the most deplorable diversity of opinion; but this depends on the different views which different minds always entertain in weighing circumstantial evidence.

Such being the general idea which we attach to the species, we must also bear in mind some of its universally admitted properties. All the individuals of a species, all the descendants of a common parent, resemble each other in certain general features of outward form, internal structure, and physical constitution, which we call the characters of the species, but may differ from each other in many minor points of size, colour, hairiness, &c. The characters of a species are (in the opinion of all except the very few advocates of progressive development) fixed, and remain the same from generation to generation. Individual variations are sometimes confined to a single individual, sometimes common to a considerable number, which we collectively term a variety of the species. These variations have a greater or less tendency to heredity, which, if very decided through a number of generations, converts the variety into a race. Hereditary races descended from one common species are now admitted by nearly all botanists. Our garden vegetables afford a familiar example of them.

Plants are endowed with the means of multiplication, dissemination, and migration of the species by the dispersion of seeds, or of the individual by means of suckers, runners, &c., in consequence of which they will spread in every direction unless checked by physical obstacles or other counteracting influences which they cannot surmount. This has been going on for a sufficient length of time for every species once planted in any spot of our globe to have spread by this time over its whole. surface were it not for these obstacles and counteracting influences. So great, indeed, is the effect of these obstacles that the greater number of species of plants only occupy a very limited district; there are but few that are found equally in the different large continents of which the surface of the earth is

composed, or even spread over the whole of any one continent, and there is no plant known to grow indiscriminately in every part of the globe. The district thus occupied by a plant is technically called its area.

In accounting for these phenomena, our attention is first drawn to the great diversity in the vital properties or constitutional idiosyncracies of the several species which enable them to live only each in its own particular climate, soil, or station. The date palm would perish as rapidly in the cold regions of Northern Europe as would the Alpine vegetation of the Arctic zone, or of our mountain summits, if transported to the burning deserts of Africa. It would be as hopeless a task to cultivate the waterlily on a dry rock as to grow the caper plant in a pond. There are many plants, however, which will accommodate themselves to a great variety of climates and stations where yet they are not to be found. This leads us to the consideration of the physical obstacles which prevent their dispersion. The most important are the intervention of a tract of country unsuited to the plant, too broad for its seeds to traverse by their ordinary means of transport, and the preoccupation of a district by a vegetation already vigorous enough to choke any intruders. We can easily imagine that the intervention of the ocean should prevent the interchange of species inhabiting similar climates and stations in Europe and North America; and we daily see how seeds dropped under a dense forest, or in a thick meadow, if even they germinate, are soon choked or destroyed by the preexisting vegetation.

In our own days these physical obstacles are occasionally overcome by the direct or indirect agency of man. Immense tracts of land are devoted by him expressly to the cultivation of plants which did not previously exist there. In cultivating these, he has unintentionally sown others which come up as weeds; seeds borne by these cultivated plants, or accidentally carried by man with his goods, &c., in the course of his migrations, have sown themselves and established their species in any stations suited to their constitutions where they may have been dropped. In some few instances we may observe, even now, the area of species extended by causes purely natural; but in the great majority of instances, where man has not exercised his influence, the area of species appears to have remained the same for a period going far beyond our historical times, and commencing probably long previous to the creation of man.

Hence the division of the plants of a district into cultivated plants, either intentionally by the express will of man, or as weeds of cultivation, cultivated in spite of the will of man: '

and wild plants, which are termed naturalised if they have established themselves after their introduction by the agency of man direct or indirect, indigenous if we believe them to have existed in the country previous to the operation of human agency. In tracing the origin of these indigenous species, if we suppose that the majority of them occupied the same area at the commencement of the present geological species that they now do (where not interfered with by man), there arises the question: What changes had taken place previously to that 'period?' We here enter upon the field of the wildest and most varied conjectures, hypotheses, and theories, and the scene of most animated discussions.

It is universally admitted as a fact, founded on revelation, tradition, or a kind of innate conviction in our minds, that inorganic matter existed before any organised beings; and that all species of plants, as well as animals, had a commencement in consequence of an act which we call creation. This commencement, or creation, which every form of religion teaches us to be the work of an all-powerful Supreme Being, is inexplicable to the atheist. For whether we suppose, with Lamarck and the author of Vestiges of the Creation,' that all species are derived from the successive development of one originally created 'monade' or cell; or, with some modern theorists, that the earth, previously barren, was simultaneously clothed with a mass of vegetation such as it now bears; still there is the original first creation, equally inexplicable by any process analogous to the physical laws now in operation, or within the sphere of our observation or comprehension.

The two extreme doctrines as to the mode of creation, which we have just mentioned, are equally rejected by all our greatest naturalists. That of successive development is purely hypothetical, and unsupported by a single ascertained fact. Against that of simultaneous creation over the whole surface of the earth, we have strong arguments derived from the mass of facts accumulated by modern geologists. A third hypothesis, adopted by Linnæus, and founded on a narrow interpretation of the book of Genesis, that all species of plants were created at one time, and on one spot, whence they have been gradually dispersed, is equally incompatible with the geological and physiological facts now known.

In the chapter devoted to these questions (pp. 1165. et seq.), M. De Candolle very clearly exhibits the arguments in refutation of the above, and some minor hypotheses, and in support of the doctrine, that species of plants were successively created at different geological periods, and in different parts of the earth; that whilst

some species have survived through several geological periods, others have disappeared with the great changes that have occurred in the configuration of the surface of our globe. He tells us, moreover, that of the species now existing, whilst the great majority belong evidently to the earlier geological periods, there is reason to believe that the creation of others dates only from the epoch of those phenomena which produced the present geographical conformation. But there is no evidence, nor any plausible ground, to suppose that any species has been added to the vegetable kingdom since the creation of man.

On the other hand, it is well known that within our historic times, certain species of plants have been gradually restricted in their area, and have even finally disappeared, either from natural causes depending on geological changes, or by the direct or indirect agency of man. This total destruction of species, as evidenced particularly in such island floras as that of St. Helena, is more insisted on by Hooker than by De Candolle, who believes that many which have apparently disappeared for a time, may be one day restored to us by the germination of seeds hoarded in the earth; a question to which we shall presently revert.

Another question, much debated in modern days, and which as yet affords no conclusive arguments in support of either of the theories propounded, relates to the number of individuals of each species originally created. Was each species originally represented by a single individual, or pair of individuals, or by a number of individuals connected together by those characters which, as above explained, constitute our idea of a species? and, if so, were these individuals all placed in immediate proximity to each other, or dispersed in different parts of the earth, separated by intervals which they could not cross by the ordinary means of dissemination with which they were endowed? latter part of the question, the unity or multiplicity of centres of creation for each species, is indeed the main point. For the prolific power of most species of plants is so great, that the lapse of a very few years, after a single creation, would suffice to explain the appearance of thousands or myriads of contiguous individuals.

The

The theory of the unity of centres of specific creation has been ever the most popular, but has been strongly combated, chiefly on the ground of the frequent existence of one and the same species in regions separated by barriers unsurmountable by any means of transport we know it to possess. Primulas, saxifragas, and other Arctic or Alpine species, which cannot spread their seeds beyond a few yards at a time, inhabit chains of mountains separated by plains or seas of immense extent.