INTRODUCTORY ESSAY. v 



from these would also be applicable to the whole vegetable kingdom. I shall arrange these results 

 successively under three heads; viz. facts derived from a study 'of classification; secondly, from 

 distribution ; thirdly, from fossils ; after which I shall examine the theories with which these facts 

 should harmonize. 



§ 2. 

 On the General Phenomena of Variation in the Vegetable Kingdom. 



1 . All vegetable forms are more or less prone to vary as to their sensible properties, or (as it 

 has been happily expressed in regard to all organisms) , " they are in a state of unstable equili- 

 brium."* No organ is exactly symmetrical, no two are exact counterparts, no two individuals are 

 exactly alike, no two parts of the same individual exactly correspond, no two species have equal 

 differences, and no two countries present all the varieties of a species common to both, nor are 

 the species of any two countries alike in number and kind. 



2. The rate at which plants vary is always slow, and the extent or degree of variation is gra- 

 duated. Sports even in colour are comparatively rare phenomena, and, as a general rule, the best- 

 marked varieties occur on the confines of the geographical area which a species inhabits. Thus the 

 scarlet Rhododendron (R. arboreum) of India inhabits all the Himalaya, the Khasia Mountains, the 

 Peninsular Mountains, and Ceylon ; and it is in the centre of its range (Sikkim and the Khasia) that 

 those mean forms occur which by a graduated series unite into one variable species the rough, rusty- 

 leaved form of Ceylon, and the smooth, silvery-leaved form of the North-western Himalaya. A 

 white and a rose-coloured sport of each variety is found growing with the scarlet in all these locali- 

 ties, but everywhere these sports are few in individuals. Also certain individuals flower earlier than 

 others, and some occasionally twice a year, I believe in all localities. 



3. I find that in eveiy Flora all groups of species may be roughly classified into three large 

 divisions : one in which most species are apparently unvarying ; another in which most are conspicu- 

 ously varying; and a third which consists of a mixture of both in more equal proportions. Of 

 these the unvarying species appear so distinct from one another that most botanists agree as to their 

 limits, and their offspring are at once referable by inspection to their parents; each presents several 

 special characters, and it would require many intermediate forms to effect a graduated change from 

 any one to another. The most varying species, on the contrary, so run into one another, that botanists 

 are not agreed as to their limits, and often fail to refer the offspring with certainty to their parents, 

 each being distinguished from one or more others by one or a few such trifling characters, that each 

 group may be regarded as a continuous series of varieties, between the terms of which no hiatus 

 exists suggesting the intercalation of any intermediate variety. The genera Rubus, Rosa, Salix, and 

 Saxifraga, afford conspicuous examples of these unstable species ; Veronica, Campanula, and Lobelia, 

 of comparatively stable ones. 



4. Of these natural groups of varying and unvarying species, some are large and some small ; 

 they are also very variously distributed through the classes, orders, and genera of the Yegetable 

 Kingdom ; but, as a general rule, the varying species are relatively most numerous in those classes, 

 orders, and genera which are the simplest iu structure. f Complexity of structure is generally ac- 



* Essays: Scientific, Political, and Speculative; by Herbert Spencer: p. 280. 



f Mr. Darwin, after a very laborious analysis of many Floras, finds that the species of large genera are relatively 

 more variable than those of small ; a result which I was long disposed to doubt, because of the number of variable 



