CH. XIII] D. GEOGRAPHICAL DISTRIBUTION 163 



TEST CASE XXXIV. OVERLAP OF LARGEST 

 GENERA IN A FAMILY 



If the differentiation explanation of the origin of a family be the 

 correct one, the first two genera of a family, the largest upon the 

 whole, should overlap in their distribution, as one of them sprang 

 from the other, but there is no reason why this should be so under 

 natural selection. Geological or other changes may, of course, at 

 times have rendered this impossible. Upon examination, we find 

 that in the majority of families this overlap does occur, though 

 there are a number of families like the Apocynaceae with a large 

 genus in each of the continents, or in the Old and New Worlds. 

 Exceptions are frequent among the families of the southern 

 hemisphere, with their broken areas of distribution, but in the 

 greater number of families the rule holds. Among the small 

 genera in a large family, this is rarely the case, but in a small 

 family it usually occurs. It is not impossible for a grouping like 

 this to have been produced by natural selection, but there must 

 have been something upon which it could get a grip, and one can 

 scarcely ever find anything of this kind. 



II-2 



