CH. xiii] D. GEOGRAPHICAL DISTRIBUTION 155 



ground, as may easily be seen by looking at various recent 

 publications in systematic botany, where a great part of the 

 endemic species are now admitted to be new. The first conception 

 is also beginning to receive support. Systematists have in recent 

 years made important additions to the evidence for mutational 

 origin of species and genera, though themselves only trying to 

 place these species and genera nearest to those which appear 

 to be most closely related to them. 



If one take as illustrations some of the more recent mono- 

 graphs in Engler's "Pflanzenreich", one notices at once the great 

 geographical separations of closely allied species, genera, sub- 

 families, families. In Cardamine, for example, species no. 70 

 is in New Zealand and Polynesia, no. 71 in the Azores, no. 72 

 in Chile. In Euphorbia one finds allied species in Venezuela and 

 Cape Colony, in Persia and in Africa, in central Asia and in N. 

 America, and so on. If in the Drabeae (of Cruciferae) one join 

 the consecutive related genera by a line, one crosses the Atlantic 

 five times and the Pacific once, and usually goes well into the 

 continent also. In the Arabideae the crossings are seven and 

 six respectively, and in the Lepideae the whole map is covered 

 with a web of lines. 



Now with relationship like this, which is so much complicated 

 by the great separations over the surface of the globe, to get 

 an explanation by the method of accumulation of small differences 

 is an extraordinarily difficult matter, and it is much simpler to 

 call in the Unking genera that cover the enormous gaps than to 

 suppose that the related genera, say in Chile and Siberia for 

 example, once overlapped or nearly overlapped each other, and 

 that then destruction took place upon an enormous scale, and 

 through all varieties of conditions and climates. All three of 

 these subfamilies have various genera that cover the whole or 

 much of the range, and it is much simpler to regard these as 

 connecting links — as in fact the ancestors, directly or at times 

 indirectly through intermediate genera, of the small scattered 

 (though so often closely related) genera. One may take any view 

 one pleases as to how they were derived from these large and 

 widely distributed linking genera, though personally I hold to 

 the views expressed in 1907, when pointing out how all the 

 existing Dilleniaceae might have been derived, directly or in- 

 directly, from Tetracera, the most widespread and about the 

 simplest of the family. The only necessary thing is to get rid of 

 the idea that small genera and species of restricted area are 

 necessarily relics, and we have seen that this conception is now 

 definitely losing ground. 



If one suppose a genus to give off new species more or less 

 in proportion to the area that it covers^ (which again will be 

 more or less in proportion to its age among its peers), it is clear 



^ For the mathematical consideration of the question, cf. Yule in Phil. 

 Trans. B, 213, 1924, p. 21. 



