INTRODUCTORY ESSAY. vu 



not so absolutely as the former) , but they, on the contrary, consist of comparatively exceedingly well- 

 marked genera and species. Melanthacece and Scrophularinece, on the other hand, are not limitable 

 as Orders, and contain very many differently constructed groups ; but their genera, and to a great 

 extent their species also, are well-marked and limitable. The circumstance of a group being either 

 isolated or having complex relations, is hence no indication of its members having the same characters. 



Again, as with species, so with genera and orders, we find that upon the whole those are the 

 best limited which consist of plants of complex floral structure : the Orders of Dicotyledons are 

 better limited than those of Monocotyledons, and the genera of Dichlamydese than those of Achla- 

 mydeae.* 



Now my object in dwelling on this parallelism between the characteristics of individuals in 

 relation to species, of species in relation to genera, and of genera in relation to Orders, is because I 

 consider (Introd. Essay to Fl. N. Z.) that it is to the extinction of species and genera that we are 

 indebted for our means of resolving plants into limitable genera and orders. This view is now, I 

 believe, generally admitted, even by those who still regard species as the immutable units of the 

 Vegetable Creation ; and it therefore now remains to be seen how far we are warranted in extending 

 it to the limitation of species by the elimination of their varieties through natural causes. f 



6. The evidence of variability thus deduced from a rapid general survey of the prominent facts 

 elicited from a study of the principles of classification, are to a certain extent tested by the behaviour 

 of plants under cultivation, which operates either by hastening the processes of Nature (in rapidly 

 inducing variation), or by effecting a prolepsis or anticipation of those processes (in producing sports 

 i. e. better marked varieties, without graduated stages) , or by placing the plant in conditions to which 

 it would never have been exposed in the ordinary course of natural events, and which eventually either 

 kill it or give origin to a series of varieties which might otherwise have never existed. J 



* There are too many exceptions to this to admit of our concluding at once that it is attributable to any 

 simple and uniform law of variation ; but it may be explained by assuming that the degree or amount of variation is 

 aifferently manifested at different epochs in the history of the group. Thus, if a genus is numerically increasing, and 

 consequently running into varieties, it will present a group of species with complex relations biter se; if, on the con 

 trary, it is numerically decreasing, such decrease must lead to the extinction of some varieties, and hence result in the 

 better limitation of the remainder. The application of this assumption to the fact of the best limited groups being 

 most prevalent among the higher classes (i. e. among those most complicated in their organization), would at first 

 sight appear an argument against progression, were it not for the consideration that the higher tribes of plants have 

 in another respect proved themselves superior, in that they have not only far surpassed the lower in number of genera 

 and species, but in individuals, and also in bulk and stature. And lastly, as all the highest orders of plants contain 

 numerous species and often genera of as simple organization as any of the lower orders are, it follows that that phy- 

 sical superiority which is manifested in greater extent of variation, in better securing a succession of race, in more 

 rapid multiplication of individuals, and even in increase of bulk, is in some senses of a higher order than that repre- 

 sented by mere complexity or specialization of organ. 



f It follows as a corollary to the proposition (That species, etc., are naturally rendered limitable by the destruc- 

 tion of varieties), that there must be some intimate relation between the rate of increase and the duration of genera 

 (or other groups of species) on the one hand, and the limitability of their species on the other. Thus, when a genus 

 consists of a multitude of illimitable forms, we may argue with much plausibility that it is on the increase, because 

 no intermediates have as yet been destroyed, and that the birth of individuals and the production of new forms is pro- 

 ceeding at a greater proportional rate than in an equally large genus of which the species are limitable. 



X My friend Mr. Wallace treats of animals under domestication, not only as if they were in very different 

 physical conditions from those in a state of nature, inasmuch as every sense and faculty is continually fully exer- 

 cised and strengthened by wild animals, whilst certain of these lie dormant in the domesticated, but as if they were 



