118 SIZE AND SPACE [pt. ii, ch. xii 



suggested a grouping of families into classes based upon these 

 principles, for which he has suggested the title Rank and Range, 

 and it is clear that in all future systematic work, the question 

 of area must occupy some attention. 



Excellent examples of the application of the principle of Size 

 and Space may be found below, e.g. on pp. 132, 164, 165, 

 171-2, 174, 178, 187-8, 190, and 197. 



Summary 



If species spread in a country mainly in accordance with their 

 age, then it is clear that on the average some of those in the 

 genera represented by most species will have arrived before the 

 first of those in the genera represented by few. This principle 

 may be extended, and under the name Size and Space may be 

 thus expressed; on the whole, keeping to the same circle of 

 affinity, a group of large genera will occupy more space than a 

 group of small. The space occupied will A'ary more or less with 

 the number of species. 



Illustrations of the operation of this principle have already 

 been given in Chapter vii, and further examples are drawn from 

 the Helobieae, and from the flora of Britain, while a good instance 

 is also given by Prof. Small in the next chapter. IMany other 

 instances can be found, too, in later chapters. 



