232 



BOTANY: PRINCIPLES AND PROBLEMS 



example, are practically confined to North America, the eucalypts 

 to Australia, the tobaccos to the western hemisphere, and so on. 

 We can explain such localized distribution only by assuming that 

 these plants were evolved in the regions which they now inhabit, 

 and have been confined there ever since by barriers of various 

 sorts (Figs. 131, 132, and 133). The same phenomena occur 



Fig. 1.31. — The distribution of four closely related species l)elonging to the 

 same genus (Sabatia, section Pleienta). 1, Sahatia decandra. 2, .S'. foliosa. 

 3, ,S'. dodecandra. 4, S. Kennedyana. These four very similar but readily 

 distinguishable species have presumably all evolved from a common ancestor, 

 which once grew on the coastal plain of southeastern North America. {Data from 

 M. L. Fernald) . 



repeatedly throughout the animal kingdom, and it is certain that 

 the great mass of facts which we now possess on the geographical 

 distribution of organisms would be largely unexplainable if we 

 did not believe that each species, genus, and family of plants 

 and animals has had its place of origin and its own individual 

 evolutionary history. The facts of distribution are meaningless 

 on any other hypothesis. 



