94 TEST CASES [ch. x 



the production of genera in such a way that the result must be a 

 hollow curve. 



The result of this first test is thus clearly in favour of dif- 

 ferentiation. 



TEST-CASE II. THE SIZE OF THE LARGEST 

 GENUS IN A FAiMILY 



On the theory of natural selection, the parent of a new species 

 will tend to become a relic, ultimately disappearing, but on that 

 of differentiation, there is no necessary reason why this should 

 happen. The parent may survive, probably does, long after the 

 throwing of offspring that may be specifically or even generically 

 distinct. As time goes on, the mutations in any one line seem to 

 tend to become perhaps less marked, so that generic mutations 

 perhaps become less frequent in proportion. It is possible that at 

 first, when considerable divergence is more easy, all or most of the 

 divergences may be what we should consider as generic. But on 

 the whole, it is evident that in any case the earlier members of a 

 family should be larger than the later ones — in numbers of 

 species if genera, in area occupied if species. They started first, 

 and on the average they should keep in front, so long as one con- 

 siders only related forms growing in similar conditions, as already 

 fully explained in Age and Area. The oldest genus in a family, 

 therefore, should in general tend to be the largest genus in it, and 

 the older and larger the family, the larger should its largest genus 

 be. But we have no absolute test of age, and must not try to 

 make comparisons of age, except between close relatives in 

 similar conditions. To say that the largest genus in a quickly 

 reproducing, mainly herbaceous family like the Compositae is older 

 than, or even as old as, the (far smaller) largest genus in the slowly 

 growing and reproducing giant trees of the Dipterocarpaceae, is 

 to make a statement which has nothing whatever to back it. The 

 latter, though only 5 per cent of the size of the first, may even be 

 very much the older genus. All kinds of accidents also interfere 

 with arithmetical regularity in these matters, so that it is really 

 very astonishing to see how regular the figures are, in spite of all 

 the geological or climatic changes, or other outside interferences. 

 None the less, as has already been shown in Age and Area, 

 p. 188, the supposition that the size of the largest genus goes with 

 the size of the family (a fact which could not be predicted by the 

 aid of natural selection) is borne out when one takes averages. 



