XX viii INTRODUCTION. 



importance whatever appears to be constant in its nature among nearly 

 allfed species. Nothing which is thus constant can be considered unim- 

 portant for everything constant is dependent upon or connected with some 

 essential function. Therefore all constant characters, of whatever nature, 

 require to be taken into account in classifying plants according to their 

 natural affinities. Of this nature are the internal structure of stems and 

 leaves, the anatomical condition of tissue, the organisation of the anther, 

 pollen, and female apparatus, and the interior of the seed. 



On the Other band, whatever points of structure are variable in the same 

 species, or in species nearly allied to each other, or in neighbouring genera, 

 are unessential to the vital functions, and should be set aside, or be 

 regarded as of comparative unimportance. Hence the badness of the 

 tfonopetalous, Polvpetalous, and Apetalous divisions of Jussieu, depending 

 upon the mere presence or absence, and union or disunion, of petals. The 

 genus Fuchsia, for example, has petals highly developed ; but in F. excorticate 

 the) are absent, and yet the plant differs no otherwise from the rest of the 

 genus : the same is true of species of Rhamnus. Again, the Rue has the 

 petal- separate; and Correa, very nearly allied to it, has them combined. 



All classifications in which the foregoing principles are observed are 

 natural ; and that will he the most stable in which they are employed with 

 the greatest -kill. Some writers, indeed, maintain that there cannot be more 

 than one really natural system, any more than one planetary system ; and in 

 a certain sense this may he true, inasmuch as we must suppose that one plan 

 only bas been observed in the creation of living things, and that a natural 

 Bystem is the expression of that plan. But, on the other hand, it must 

 OOt be forgotten that such a plan may be represented in many ways ; and 

 that although the order of nature is in itself settled and invariable, yet that 

 human descriptions of it will vary with the mind of the describer. A 

 universal history is a collection of events ; but it is not necessary that all 

 universal histories should follow the same order of narration. The events 

 themselves are unalterable, but the way of combining them and causing 

 them to illustrate each other is manifold. 



In natural science, indeed, the mode of arranging the matter is suscepti- 

 ble of infinitely more variation than history : because in the latter subject 

 time is an inflexible leader who cannot be lost sight of. But in natural 

 science there is no beginning and no end. It is impossible, from the 

 nature of things, that any arrangement should exist which shall represent 

 the natural relations of plants in a consecutive series. It is generally 

 admitted by those who have tinned their attention to a consideration of 

 the manner in which organised beings are related to each other, that each 

 species is allied to others in different degrees, and that such relationship is 

 best expressed by rays "ailed affinities) proceeding from a common centre 

 (the species). Tn like manner, in studying the mutual relationship of the 

 several parts of the Vegetable Kingdom, the same form of distribution con- 

 stantly forces itself upon the mind : Genera and Orders being found to be 

 apparently the centre of spheres, whose surface is only determined by the 

 point- where the last traces of affinity disappear. But although the mind 

 may conceive such a distribution of organised beings, it is impossible that 

 it should be so presented to the eye, and all attempts at effecting that 

 object must of necessity fail. If in describing the surface of a sphere we 

 aiv compelled to travel in various directions, continually returning back to 

 the point from which we started ; and if in presenting it to the eye at one 

 fiance we are compelled to project it upon a plane, the effect of which is to 



