It is laid down as an axiom by some eminent modern Botanists, 'that nature only creates species and knows nothing of a natural system of Botany.' This, it is added, is entirely a human contrivance, founded on a consideration of the resemblances existing between the parts, properties and qualities of plants, according to which their affinities are determined, and species having the greatest degree of relationship, in all these particulars, placed next each other, in such a way, that the qualities or intimate structure of an imperfectly known plant may be inferred from a careful consideration of the known qualities and structure of those nearest to which its Botanical characters would place it in a system constructed on these principles. Whether or not this theory be just, I will not step to enquire, for, so long as the object is obtained, I feel quite satisfied that the method by which we attain it, whether a mere human contrivance or the work of nature herself, is so infinitely superior in its fitness for the supply human wants, and for furnishing matter for philosophical enquiry, to all the artificial ones that have gone before, that there can be no hesitation as to the propriety of its instant adoption, whatever may be the difficulties to be, in the first instance, surmounted in acquiring a knowledge of its principles. These in the present instance are not by any means so great as they have been represented, for, as Jussieu well observes, whatever trouble is experienced in remembering or applying the characters of natural orders is more than compensated for in the facility of determining genera the characters of which are simple in proportion as those of orders are difficult. The reverse takes place in arbitrary arrangements, where the distinction of classes and sections are simple and easy to remember while those of the genera are in proportion numerous and complicated. On this question, therefore, as there can scarcely be two opinions, it now only remains for me very briefly to explain, the principles of the arrangement or distribution of the orders adopted both in this work and in our Prodromus.
The arrangement followed is very nearly that of Jussieu as modified by DeCandolle and adopted in his Systema,and Prodromus. According to this system all plants are first distributed under two principal classes Cellular and Vascular, the former comprehending all the plants destitute of spiral vessels and of Cotyledons — Cellulares; the latter, including all the flowering plants which are furnished with both these organs — Vasculares. The vascular plants, of which only we have as yet treated, are again divided into two classes Dicotyledons or Exogenous plants; and Monocotyledons or Endogenous plants. To the former all trees and shrubs which increase in thickness from the centre towards the circumference by a succession of concentric layers of wood belong, as well as nearly all those herbaceous annuals, the leaves of which, have reticulated or anastomosing vessels. To the latter the various kinds of grasses, Lilies, Orchidiae, Palms, Plantains, &c. belong, the vessels, of the leaves, of which pass either in straight lines from the base to the apex, or from the midril to the margin, and the leaves are sheathing in place of being attached by a joint to the stem. These distinctions are not without exceptions, but the exceptions are so few as scarcely in practice, to affect the value of the rule. These primary divisions though thus based on the most obscure and difficult portions of vegetation to investigate, the minute structure of the seed and organization of the stem, are in fact the easiest, generally speaking, of determination. The simple circumstance of a plant having a flower proving that it has spiral vessels, while the practised eye of a Botanist can, almost invariably tell, at a glance by merely inspecting the distribution of the vessels of leaves and structure of the stem