100 TEST CASES [ch. x 



TEST-CASE VI. SIZE AND SPACE 



The hypothesis of Size and Space is more fully described in Age 

 and Area, p. 113 ; it follows from that hypothesis. " On the whole, 

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

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

 space." If adaptational improvement ceases to be the prime (or 

 even perhaps an important) factor in evolution, there is no special 

 reason why one species should spread more rapidly, or over a 

 greater area, than other species closely related to it. As an illustra- 

 tion the case of distribution of species in Britain was taken, and it 

 was shown that it increased with the size of the genus. 



"A good proof for the general correctness of Size and Space is 

 that . . . the further out we go among the islands, the larger on the 

 average do the genera become (in the number of species that they 

 contain in the world). Whilst the world average for a genus is 

 12-13 species, the non-endemic genera found in India contain on 

 the average about 50 species in the world, in New Zealand 

 about 75, and in the Hawaiian Islands about 100. 



"The smaller families usually occupy smaller areas than the 

 larger, and the question arises whether they should be con- 

 sidered of equal rank to the latter. Guppy has 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" (66). 



It is clear that the facts shown under Size and Space cannot 

 be explained by aid of the hypothesis of natural selection or of 

 gradual adaptation, and can at present only be easily explained 

 by that of differentiation. 



TEST-CASE VII. "SOME STATISTICS OF EVOLUTION 



AND GEOGRAPHICAL DISTRIBUTION, 



AND THEIR SIGNIFICANCE" 



To give details would simply be to repeat the paper of Mr G. 

 Udny Yule and the author, in Nature, vol. cix, 9 February 1922, 

 p. 177, and it will suffice to call attention to it. The general con- 

 clusion was that: "Inasmuch as all families, both of plants and 

 animals, show the same type of curve, whether graphic or loga- 

 rithmic, it would appear that in general the manner in which 

 evolution has unfolded itself has been relatively little affected by 

 the various vital and other factors, these only causing deviations 

 this way and that from the dominant plan." It follows that 

 evolution must have been by mutation, and that this must, at 

 times anyway, have been large, as demanded by the theory of 

 differentiation. 



