CH. IX] OBJECTIONS TO THE HYPOTHESIS 87 



seldom occur, comparisons on the basis of age will be impossible. 

 But a group of mixed trees and herbs may be compared with 

 another allied group of the same general composition. 



These considerations also dispose to a large extent of the 

 objection (19) that age is only one factor of many, and (20) that 

 enough is not allowed for the action of other factors. Age is, as 

 has been pointed out above, only one factor, but it is a factor 

 whose action can be shown in figures which no one as yet has 

 been able — has e^en indeed attempted — to explain upon any 

 other supposition. If one were dealing with individual species, 

 one would have to allow for each individual factor, and could 

 never, or very rarely, be in a position to say how much was due 

 to this, and how much to that. No one has yet been able to 

 reduce to figures the effects of any of these factors, and their 

 action is still accepted upon a priori considerations. The effect 

 of my work is to disentangle from among them the effect of 

 age, and to show that it is very considerable indeed; and this 

 should of itself make much easier the study of the effects of the 

 many other factors that take part in the dispersal of a species 

 about the globe. 



The next group of objections is to the general effect (21) that 

 endemic forms, Avhether species or genera, are local adaptations, 

 suited expressly to the spots in which they occur. In one sense 

 this objection is a truism, for if a species or genus were not suited 

 to the spot where it occurred, it would die out there, so that if 

 it were endemic to a very small locality, it might easily dis- 

 appear from the earth. But the general explanatory idea which 

 lies behind this objection is very hardly pressed when it comes 

 to explaining such a series of species, arranged in "wheels 

 within wheels," as those of Ranunculus in New Zealand (p. 156), 

 or Doona in Ceylon (p. 153), and breaks down altogether when 

 it is once realised that endemic species and genera, as will be 

 more fully shown below, represent only a special case of species 

 and genera in general. It is not possible to explain upon any 

 theory of adaptability the varying areas occupied, and occupied 

 in a way that can be reduced to statistics which agree for each 

 family and area. One cannot suggest conditions that will overlap 

 like the rings in a shirt of chain mail, as do the genera and 

 species (p. 56). Nor will this view explain the increase of 

 cndemism as one goes sonthAvards, or outwards from the con- 

 tinental areas. Nor will it enable us to do any prediction about 

 geographical distribution whatever, though Age and Area has 



