CHAPTER XII 



SIZE AND SPACE 



We have already pointed out, on p. 71, that on the average 

 the larger famihes and genera in a country will probably be the 

 older there, inasmuch as it is highly improbable that the sinale 

 species of a genus represented only by one would always arrive 

 as soon as the first species of a genus represented by many The 

 tendency will be for the latter to arrive first, and if, as A^e and 

 Area mdicates, there is but little killing out of species once 

 established, one will expect that the first arrivals will have spread 

 the most. It is obvious, of course, that one must work with 

 averages of considerable numbers to ol^tain reliable results but 

 It seems to me that this extension, for which I propose the name 

 Size and Space, may be given to the original idea of Age and 

 Area. Under this supposition one will sav that on the whole 

 keeping to the same circle of affinity, the larger families and 

 genera mW be the older, and will therefore occupy the most 

 space. This, however, involves a break with the long current 

 idea, that the larger families and genera are the successful ones 

 the smaller the (comparative) failures. 



This principle obviously follows, once the central principle of 

 Age and Area is recognised, and it is further realised that destruc- 

 tion of species by natural selection takes place ^vhen tliey are 

 newly born and occupy minute areas of ground, and not when 

 they are once established on a reasonable area. Destruction 

 then, so far as we can see, will rarely happen, except in the case 

 of some great change of conditions, such as the secular drying 

 of climate, which (among other things) is apparently responsible 

 for the fact that Cupressus macrocarpa, etc., are now apparently 

 dying out (or rather not expanding) in California. 



One may get very good evidence in fa\'our of this view by 

 applying it to such a flora as that of Britain, for which there are 

 good statistics of distribution available. If we take the distribu- 

 tion of the plants by the number of Watson's "vice-counties" 

 that they reach (37) we get the table on p. 114. 



The diminution of the numbers in every line from left to right 

 of course means nothing, for the species are taken in order from 

 first to fifth most widely dispersed. But all the columns also 



