PEINCIPLES OE CLASSIFICATION. 159 



There is another fact of daily experience which is of primary im- 

 portance in reference to this point ; that is, the circumstance that 

 plants produced from seeds most commonly resemble in all impor- 

 tant respects the parent plant from which the seeds were derived, 

 and this through an indefinite number of generations ; from which 

 it follows that kinds or species of plants are regularly reproduced 

 by their seeds. 



The definition of a species can only be considered as arbitrary ; 

 but for practical purposes it may be said that a species consists of 

 those individual plants which agree in all their important and con- 

 stant characters, in the same way as do individuals of analogous 

 structure, which we know to have descended through a number of 

 generations from a common stock, and which therefore may be 

 assumed to have been produced through seed from an original 

 individual, or pair of individuals, of a distinct kind. To these 

 may be added the assertions that individuals of the same species 

 may be cross-fertilized, to the improvement rather than the detri- 

 ment of the fertility of their seeds, and that they are affected in 

 a generally similar manner by external agencies. 



Diversity of opinion still exists amongst naturalists as to the origin and 

 fixity of species. On the one hand it is assumed that every distinct 

 species originated in a distinct creation of that form, which has been 

 perpetuated, with its essential characters unchanged, through succeeding 

 generations. It is usually added by the same school that, as regards 

 plants, every species originated from a single prototype, or a pair of 

 parents where the plant is dioecious. 



On the other hand, it is contended by most modern naturalists that 

 species were not necessarily created as we now see them, but that existing 

 species are the lineal descendants of those that have gone before, and 

 more or less modified in course of time by varying circumstances, such as 

 inherent tendency to vary, the effect of external agencies, and the com- 

 petition of other forms. This notion involves the conclusion that species 

 are not absolutely invariable. 



Varieties. Species are distinguished by those characters which 

 under present circumstances are constant so long as the conditions 

 under which they exist remain unchanged ; but individuals may possess 

 other additional characters of less importance, which are inconstant. 

 Even as in the human species we find every individual possessing cer- 

 tain peculiarities, so even in almost to the lowest of created beings 

 do we find what is called an idiosyncrasy, and individual character, 

 chiefly depending, in the vegetable kingdom, upon the conditions under 

 which they have grown up. We often find seeds from the same parent 

 producing individual plants differing in the colour, size, and number of 

 their flowers and of their vegetative organs, according to the conditions 

 of climate and soil to which we submit them. Very often, moreover, we 

 find these differences displaying themselves under what appear to us 

 identical conditions, as is particularly the case with many of the favourite 

 " florist's flowers " such as the Pelargonium, Fuchsia, Pinks, Asters, &c., 



