Evolution of Plants 547 



velopment of large and effective root systems may be traced in 

 the same way. 



It is quite impossible to account for these gradual and pro- 

 gressive changes in plants except on the basis of evolution. When 

 we understand that the geological record of evolution covers 

 a period of an estimated length of several hundred million years, 

 we should not become impatient at failing to see new genera and 

 families of plants arising during our own brief period of observa- 

 tion. The time that has elapsed since critical observations upon 

 the evolution of living plants have been made, when compared 

 with the time represented by the geological record, is like one 

 second for the observation of the events of a year. 



Plant geography. Closely related to the fossil record is the evi- 

 dence of evolution that is derived from the present distribution of 

 plant groups. Closely related species of plants are not scattered 

 haphazard over the earth. Many families bear evidence of having 

 originated on some particular continent, or part of a continent, 

 and of having spread from the center of origin as new species 

 appeared. Some species have not spread far from their point 

 of origin, while others have moved far from the place of their 

 first appearance because of characteristics which enabled them to 

 live in a variety of conditions. Families which, because of their 

 structures and the absence of a fossil record, are believed to be 

 very modern are usually restricted in their distribution. Ancient 

 families, on the other hand, often have species scattered over 

 several of the continents. 



The cactus family, represented by about 1500 species, is native 

 in North and South America only. In North America the 

 family is best developed in Mexico, but it has spread north- 

 ward and eastward into the United States and to the islands of 

 the West Indies. The geographic distribution of all the North 

 American species points to a common origin in the Mexican 

 plateau. The yucca family and the agave family also appear to 

 have originated there and to have spread in a similar way to the 



