H. BOTANY. 369 



have formed the connection between the beeches of the north 

 and south, just as we find points of contact between Chile 

 and Japan in other respects. An explanation of this is af- 

 forded by the fact that the beeches of Japan conform to 

 those of the miocene epoch. The centre of distribution of the 

 typical beeches must therefore be sought in a past geological 

 period, and from it they must have been dispersed in different 

 directions before the present distribution of land and water. 

 Besides the three principal centres alluded to, there are also 

 three secondary centres of distribution characterized by pe- 

 culiar genera and sub-genera. The principal groups, in pass- 

 ing beyond their respective regions and mingling with each 

 other, have produced regions of transition, in which forms ap- 

 pear which are the connecting links between the types from 

 the different centres. Various facts in regard to the distribu- 

 tion of plants in general are also peculiarly illustrated by 

 this family. Thus it exhibits most clearly the marked differ- 

 ence between the floras of Mexico and the Antilles, the oaks 

 being more numerous in Mexico than any other portion of 

 the world, while they are wholly wanting in the Antilles, al- 

 though the latter afford climatic conditions favorable to their 

 growth in many places. This contrast can be partially ex- 

 plained, as regards the oaks, by the fact that the seeds of the 

 latter soon lose their power of germination, and are not easily 

 transported by currents ; and, besides, the oaks occur in the 

 mountainous regions of Mexico, remote from the sea; and even 

 if the seeds were transported by the aid of rivers, they would 

 not find conditions favorable to their development on the 

 coast of the Antilles a fact in harmony with the general rule 

 that the larger number of the plants common to the Antilles 

 and the continent belong to the lowlands of the tropics, while 

 the plants of the mountains are generally endemic. The dis- 

 tribution of the cupuliferae also substantiates in a remarkable 

 manner the general rule that the floras richest in endemic 

 species are those where the physical obstructions to diffusion 

 of plants are greatest ; the ocean, high snow-covered mountain 

 ranges, especially those with their axes perpendicular to the 

 direction of the wind, forming sharply defined limits of floras. 

 Thus, while the white oak occurs all over Europe, the species 

 of cupuliferas in Sumatra and Java are entirely different from 

 each other. In like manner the characteristic cupulifera? of 



Q 2 



