No. 596] RAPIDITY OF EVOLUTION 



are over sixty such groups of allied genera in the endemic 

 flora of North America. Notable among these are the 

 alliances centering about Eriogonum in the Polygonacese ; 

 about Streptanthus and about Lesquerella in the Crucif- 

 eraB ; about Eschscholtzia in the Papaveraceae ; about Heu- 

 chera in the Saxifragaceae; about Cercocarpus in the 

 Rosacea?; about Godetia in the Onagraceae; about Cy- 

 mopterus in the Umbelliferae; about Pterospora in the 

 Pirolaceas; about Cryptanthe in the Boraginacere ; about 

 Trichostema and about A gust ache in the Labiatee; about 

 Pentastemon and about Castilleja in the Scrophulariaceae, 

 and about Brickellia, about Solidago, about Boltonia, 

 about Silphium, about Rudbeckia, about Hemizonia, about 

 Baeria, and about Microseris in the Compositae. Most of 

 these groups of genera have their center of distribution 

 in the southwestern United States or in northern Mexico. 

 The great majority of endemic genera in Europe are also 

 evidently " indigenous" rather than "relict" in charac- 

 ter. Centering chiefly in the Mediterranean region there 

 are seventy or more groups of closely allied genera, no- 

 table among which are the alliances dominated by Dian- 

 thus, Brassica, Alyssum, Lotus, Scandix, Asperula, Scabi- 

 osa, Anchusa, Anthemis, Carduus, and Cichorium. 



These "indigenous" endemic genera constitute a very 

 characteristic and important part of the present-day flora 

 of North America and Europe. Had they existed in any- 

 thing like their present numbers and importance at the 

 period when an easy exchange of plants was possible be- 

 tween the two northern continents, it is hard to believe 

 that they would not now be well represented in the floras 

 of both; particularly since herbs, of which we shall see 

 these genera to be almost exclusively composed, tend more 

 quickly than any other plant type to loose their endemic 

 character because of the power to migrate rapidly and 

 populate wide areas which is conferred by their ability to 

 produce seed from seed in a single generation. This is 

 well shown by the fact that in DeCandolle's famous list of 

 117 species, each of which at present occupies at least one 

 half of the land area of the globe, there are none but her- 



