The Evolution of Plants 323 



apparent that the species belonging to any particular group 

 are not scattered haphazard over the earth. Many families 

 bear evidence of having originated on some particular con- 

 tinent or part of a continent, and of then having spread 

 from the center of origin. Commonly, in the center of origin 

 there is or was the greatest variety of species, and away from 

 the center the species are fewer and less varied. For example, 

 the Cactus family, represented by more than a thousand 

 species, is native in North and South America only. In 

 North America the family is best developed in Mexico, but 

 it has spread northward and eastward into the United States 

 and to the islands of the West Indies. The geographic dis- 

 tribution of all the North American species points to a com- 

 mon origin in the Mexican plateau. The Yucca family and 

 the Agave (century plant) family also appear to have orig- 

 inated there and to have spread in a similar way to the United 

 States and the West Indies. 



Sometimes the record is incomplete because the families 

 are very ancient and most of the species have become extinct. 

 In other families the record is so complete that it is possible 

 from the distribution to trace the origin and relationship of 

 many of the species. The geography of plants, therefore, 

 furnishes a second line of evidence that existing plant species 

 have been derived from preexisting forms. 



Similarity of structures of plants belonging to diverse 

 groups. One of the most striking proofs of evolution is found 

 in the remarkable similarity of the cells, tissues, and organs 

 that make up plants belonging to very different groups. The 

 conductive system, for example, is not very different in the 

 ferns and in the Angiosperms. The bundles are arranged dif- 

 ferently in these groups, and their arrangement becomes in- 



