136 T O R R E Y A 



Requirements for the development of many species are either that the forms 

 are allopatric and have geographic isolation or, if sympatric, that they have 

 some form of genetic (internal) reproductive isolation. Regions in which there 

 are many closely related species are usually regions of habitat diversity, as 

 noted by Vavilov (1940). It is entireh' possible then that a phyletic stock that 

 has had its origin elsewhere may, through migration, encounter a region in 

 which there are numerous available ecological niches that are unsaturated — 

 that is, in which competition pressure is low. Such a region may provide a 

 variety of habitats with at least partial isolation. Under these conditions a phy- 

 letic stock may show a "burst" of evolutionary radiation. It is apparent that such 

 a region of polymorphism is not necessarily indicative of the original center of 

 origin nor of dispersal, but is a fortuitously derived secondary center of diiter- 

 entiation. Two more examples of this general t3^pe can be taken from Fernald's 

 (1926) criticism of age-and-area. He uses the conclusions of Schonland (1924) 

 concerning Erica, which has nearly one thousand species in South Africa. There 

 is not a single known fact that indicates that the genus arose in South Africa 

 where there are the most endemics and the greatest diversity (species and sec- 

 tions). Furthermore, Willis had concluded that the number of endemics in any 

 genus would rise gradually to a maximum at or near the point where the genus 

 entered a land area, or where a genus had its center of origin. Of this corollary 

 of age-and-area Schonland (1924) says, "Applying this prediction to the genus 

 Erica in South Africa, this point would be a part of Southwest Cape Colony 

 west of George, where not only a large number of endemics are massed, but 

 where, moreover, the greatest diversity owing to formation of subgenera and 

 derived genera is to be found ; but I fear no contradiction when I assert that it 

 is certainly not the place where the genus Erica entered South Africa, or where 

 it originated." 



Further evidence as to the care required in arriving at conclusions concern- 

 ing geographic problems is illustrated by Senecio. J. Small (In Willis, 1922) 

 localizes the evolution of the Composites through Senecio in the northern Andes 

 in Upper Cretaceous time, because of the present great expansion of that large 

 genus in the Andean region. Senecio in the mountains of tropical America is in 

 the most active stage of maturity, according to Greenman (1925), not because 

 it originated there, but because it is a region geologically young and diversified. 

 Small's and Willis' conclusion regarding Senecio rest on what Fernald (1926) 

 gleefully calls a "colossal geological error," because the present great elevation 

 of the Andes, where Senecio now has its magnificent development, did not oc- 

 cur until the close of the Tertiary (Pliocene) and the beginning of the Pleis- 

 tocene. From Schuchert's (1935) recent historical geology, however, it appears 

 that the Cordillera Occidental and the still more western and low Cordillera de 

 Choco of northern South America are more ancient elevated land masses than 



