CH. x] A. NUMERICAL 95 



The table given there shows this clearly, and some later figures 

 show it equally well: 



The requirement of differentiation, that the size of the largest 

 genus of the family shall go up with that of the family itself, is 

 fully borne out, while no theory of natural selection or of gradual 

 adaptation can offer any explanation of the facts. 



TEST-CASE III. THE RELATIVE 

 SIZES OF GENERA 



We may now consider the relative sizes of the genera in a family 

 or other group. Upon the theory that they were formed by 

 gradual adaptation one cannot say more about their probable 

 relative sizes than that some (the ''successful" ones) will pro- 

 bably be large, and some (the "failures'" or "relics'") small. Nor 

 can one give even an indication of what their relative numbers 

 will be. Further, one will also be inclined to expect to find some 

 kind of distinction shown between the successes and the failures. 

 But if differentiation be the more correct view to take, evolution 

 is no longer of necessity a direct expression of continually im- 

 proving adaptation, nor is the geographical distribution of plants. 

 It is clear that if that be so, there would be little reason for one 

 plant to spread, on the average, faster than its near relatives. 

 All in a related group would tend to spread at a more or less 

 uniform speed. But the speed of spread would depend upon 

 many factors, and to average these out, as already explained in 

 Age and Area, plants should only be taken in groups of say ten 

 allied forms, which should only be compared with other tens 

 allied to the first. Plants of systematic affinities that were widely 

 different might spread at completely different speeds, or plants 

 that differed in habit, like trees and herbs, or in speed of repro- 

 duction or other things. But on averages, with groups of allies 

 growing in fairly similar conditions, the oldest genus of a family 

 should be the largest, whilst the others should show a continually 



