368 THE PEINCIPLES OF 



ever grade, are not realities, but ideas ; their consideration involves 

 questions, not of things, between which absolute distinctions might 

 be drawn, but of degrees of resemblance, which may be expected to 

 present infinite gradations. (3d.) Although the grades of affinity 

 among species are most various, if not wlioUy indefinite, the nat- 

 uralist reduces them all to a few, and treats his genera, tribes, &c. 

 as equal units, or as distinguished by characters of about equal value 

 throughout, — which is far from being the case. (4th.) The nat- 

 uralist in his works is obliged to arrange the groups he recognizes in 

 a lineal series ; but each genus, or order, &c. is very often about 

 equally related to three or four others ; so that only a part of the 

 relationship of plants can practically be indicated in the published 

 arrangement. 



721. The natural system as sketched by Bernard and A. L. Jus- 

 sieu, and improved by the labors of succeeding botanists, essentially 

 consists of an arrangement of the known genera according to their af- 

 finities under two hundred or more natural orders, and of these under 

 a few great types or classes. What is now most wanted to complete 

 the system is a truly natural arrangement of the orders under the 

 great classes, like that of the genera under their respective orders. 

 Until this is done, the series in which the orders follow one another 

 in botanical works must not be regarded as a part of the system of 

 nature. Different authors adopt different modes of arranging them ; 

 and all of them that a learner could use are avowedly more or less 

 artificial. 



722. Omitting all historical details and statements of more or less 

 conflicting views, we will briefly sketch the outlines of the principal 

 divisions of the vegetable kingdom, according to the natural system 

 as we now practically receive it. In explaining the principles of 

 classification, we proceeded from the individual to the class. In ex- 

 amining the actual construction of the system of botany, it is simpler 

 to regard the vegetable kingdom as a whole, and show how it is nat- 

 urally divided and subdivided. This is the course a student must 

 follow with an unknown plant before him, which he wishes to refer 

 first to its class, then to its order, and finally to its genus and 

 species. 



723. The long and complex series, stretching from the highest 

 organized vegetable down to the simplest and minutest of the Fungi 



. and Algte, is most naturally divided, as we have already seen, into 

 two parts, forming a higher and a lower grade or series (98), viz. 



