CHAPTER VI.

OF SPECIES, VARIETIES, GENERA, ORDERS, AND CLASSES.

THE next point of consideration is the meaning that is attached to the words species, genus, order, and class, and the characters upon which those groups severally depend.

A species is a union of individuals agreeing with each other in all essential characters of vegetation and fructification, capable of reproduction by seed without change, breeding freely together, and producing perfect seed from which a fertile progeny can be reared. Such are the true limits of a species; and if it were possible to try all plants by such a test, there would be no difficulty in fixing them, and determining what is species and what is variety. But, unfortunately, such is not the case. The manner in which individuals agree in their external characters is the only guide which can be followed in the greater part of plants. We do not often possess the means of ascertaining what the effect of sowing their seed or mixing the pollen of individuals would be; and, consequently, this test, which is the only sure one, is, in practice, seldom capable of being applied. The determination of what is a species, and what a variety, becomes therefore wholly dependent upon external characters, the power of duly appreciating which, as indicative of specific difference, is only to be obtained by experience, and is, in all cases, to a certain degree, arbitrary. It is probable that, in the beginning, species only were formed; and that they have, since the creation, sported into varieties, by which the limits of the species themselves have now become greatly confounded. For example, it may be supposed that a Rose, or a few species of Rose, were originally created. In the course of time these have produced endless varieties, some of which, depending for a long series of ages upon permanent peculiarities of soil or

climate, have been in a manner fixed, acquiring a constitution and physiognomy of their own. Such supposed varieties have again intermixed with each other, producing other forms, and so the operation has proceeded. But as it is impossible, at the present day, to determine which was the original or originals, from which all the Roses of our own time have proceeded, or even whether they were produced in the manner I have assumed; and as the forms into which they divide are so peculiar as to render a classification of them indispensable to accuracy of language; it has become necessary to give names to certain of those forms, which are called species. Thus it seems that there are two sorts of species: the one, called natural species, determined by the definition given above; and the other, called botanical species, depending only upon the external characters of the plant. The former have been ascertained to a very limited extent of the latter nearly the whole of sytematic botany consists. In this sense a species may be defined to be "an assemblage of individuals agreeing in all the essential characters of vegetation and fructification." Here the whole question lies with the word essential. What is an essential character of a species? This will generally depend upon a proneness to vary, or to be constant in particular characters, so that one class of characters may be essential in one genus, another class in another genus; and these points can be only determined by experience. Thus, in the genus Dahlia, the form of the leaves is found to be subject to great variation; the same species producing from seed, individuals, the form of whose leaves vary in a very striking manner: the form of the leaves is, therefore, in Dahlia, not a specific character. In like manner, in Rosa, the number of prickles, the surface of the fruit, or the surface of their leaves, and their serratures, are found to be generally fluctuating characters, and cannot often be taken as essential to species. The determination of species is, therefore, in all respects, arbitrary, and must depend upon the discretion or experience of the botanist. It may, nevertheless, be remarked, that decided differences in the forms of leaves, in the figure of the stem, in the surface of the different parts, in the inflorescence, in the proportion of parts, or in the form

of the sepals and petals, usually constitute good specific differences.

A genus is an assemblage of species, agreeing with each other in the essential characters of fructification. It is an artificial means of condensing our ideas of the forms of plants, by sinking characters of minor importance in such as are of greater. While species depend upon the endless modifications of the organs of vegetation, genera depend upon the less numerous varieties of the organs of fructification, to which those of vegetation are subordinate. But as the value of the characters derived from the organs of fructification is uncertain, and dependent upon no fixed rules, so the limits of genera are arbitrary, and depend upon the caprice of botanists. Hence we find that some are disposed to excessive combination, and others to excessive division, in their genera. The only rule that can be given is this, that as genera are destined to analyse and simplify our ideas by reducing variable characters to those which are less variable, and characters which are common to many plants to those which are common to a few, care must be taken that this end is effected by a perfect analysis of the characters of fructification. And this being the case, an excessive multiplication of genera, which ought to be only the result of a very careful analysis, is infinitely better than an excessive reduction of them. The former leads to precision of ideas, the latter to confusion of them. In general, a genus should not be formed upon a solitary species, unless that species is so distinctly characterised that it cannot be referred to any other known genus; but wherever two or more species can be found agreeing in particular modifications of the structure of their fructification, by which they also differ from others, such two or more species are to be taken as the representatives of a genus.

Strong peculiarities in the vegetation of plants may be sometimes used as generic characters, but those of the fructification should always be preferred if they can be found.

An order is an assemblage of genera, agreeing with each other in the higher characters of vegetation and fructification. Orders, like genera, are a contrivance for analysing and sim

plifying our ideas, by reducing their number; and, like genera, they are also to a certain degree arbitrary. But as orders depend upon modifications of structure of a less variable kind than either genera or species, so are their limits better understood. Orders are characterised by all the parts both of vegetation and fructification, but their essential distinctions depend upon a few only, such as the presence or absence of stipula; the insertion of the leaves, whether opposite or alternate; the degree of division of the calyx and corolla, station of the stamens, structure of the fruit, figure of the embryo, presence or absence of albumen, &c. &c.

Classes are merely orders of a higher kind, combined by a few characters common to and distinctive of many.

BOOK IV.

GLOSSOLOGY; OR, OF THE TERMS USED IN BOTANY.

In order to comprehend the language of botanists, it is necessary that the unusual terms or words which are employed in writing upon the subject, and which are either different from words in vulgar use, or which are in botany employed in a particular sense, should be fully explained.

It is a very common plan to mix up Glossology with Organography, or to confound the definition and explanation of those characteristic terms of the science which are universally applicable, with the description of particular organs: but this plan is attended with many inconveniences, and is far less simple than to treat of the two separately. It was an error into which Linnæus fell, in composing his admirable Philosophia Botanica; and is the more remarkable, if the logical precision with which that work is otherwise composed be considered. Instead of distinguishing those terms which have a general application to all plants or parts of plants, according to circumstances, from such as have a particular application, and relate only to special modifications, he placed under his definition of each organ those terms which he knew to be applicable to it; but, as it was not his practice to repeat terms after they had been once explained, it frequently happened that beginners in the science, finding a given term explained once only, and with reference to a particular organ, fell into the mistake of supposing that that term was applicable only to the organ under which it was explained. To avoid this difficulty, other botanists have col

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