CHAPTER XI 



THE FURTHER EXTENSION OF THE 

 APPLICATION OF AGE AND AREA 



In most of the work so far published, and in the first part of 

 this book, Age and Area is used only within narrow limits, as 

 applying to the flora of a single given country. But this is a 

 purely arbitrary limitation, and was adopted in order to render 

 less complex its application to the problems of distribution; 

 and in this second part of the book Age and Area will be 

 applied to genera as well as to species, and to the flora of the 

 world as a whole. 



Like Age and Area itself, its twin principle, to which I give 

 the name Size and Space, has also been used as yet in a limited 

 way, e.g. on p. 71, where it is pointed out that genera that are 

 represented in a country by several species are likely to be (on 

 the average) older in that country than genera that are only 

 represented there by one. The exact graduation of commonness 

 with number of species which is there shown indicated that this 

 principle was also capable of extension, and it is expanded in 

 Chapter xri into the more general proposition that within any 

 circle of afiinity, the larger genera will be the older, and when 

 taken in groups of ten allied genera will be older in rough pro- 

 portion to their numbers of species. 



This supposition is very strikingly confirmed by an examina- 

 tion of the British flora, which shows that the distribution in 

 Britain of the most widely distributed species of each genus (on 

 the average of the whole number) varies with the number of 

 species that the genus possesses in Britain. The same is the case 

 with the second, third, foiu'th, and so on to tenth, most widely 

 distributed species in each genus. Extension of the principle to 

 the whole world is then illustrated by aid of the Helobieae, by 

 refcrcnci^ to Prof. Small's work on the Compositae (in the next 

 chapter), and also to many other cases given below. The general 

 result, therefore, is to show that Age, Size, and Space (or 

 Area) go together. 



In the next chapter Prof. Small shows how Age and Area can 

 be applied Avith effect to the distribution of a single family, by 

 dealing with the Compositae. The average generic area is deter- 



