INTRODUCTION. XXV 
of flowering plants in the Flora of India yield nearly 37 per cent. of the total number of 
species. In Mexico 4:6 per cent. of the genera comprise 39 per cent. of the species; and 
in Australia the amounts are about 4°64 and 37 per cent. Itis unnecessary to add that 
most of the above genera are widely spread, and many of them common to the New 
World. 
Genera common to the widely separated areas of India and Mesico. 
From the relations of the average number of species to a genus in the areas under 
consideration to the average for the whole world, it follows that the average area of a 
genus must be at least double one of these areas. But there is no necessary relation 
between the area a genus covers and the number of species it contains, though, speaking 
generally, monotypes have a restricted area. Indeed, if we exclude aquatic and sea- 
coast plants, and such as have probably been dispersed through human agency, it is 
difficult to multiply instances of monotypes with a large area. The curious Cressa cretica 
may be cited as a possible exception *. On the other hand, the species of some large 
genera are concentrated in one region, as Eucalyptus in Australia, and Miconia in 
America. | 
Disregarding exceptions, it may be stated that the genera and species of the northern 
Floras have the widest range; those of the tropics an intermediate one, and those 
characteristic of the southern Floras, excluding that of the coldest zone, the most 
restricted range. Probably not less than 75 per cent. of the genera of the Flora of 
Eastern America, north of Mexico, are represented in the Old World, for in some 
statistics on the vegetation of the north-eastern part of the United States, drawn up 
by the late Dr. A. Gray more than thirty years ago}, it is shown that 63 per cent. of 
the then known genera were common to America and Europe, or America and eastern 
temperate Asia. Since that date many others have proved to be common to America 
and the Old World, and the rich collections made by Dr. A. Henry within the last three 
years in Hupeh, one of the central provinces of China, have added several conspicuous 
genera to the number. 
A careful comparison of the generic composition of the Mexican and Indian Floras 
reveals the fact that 581, or 25°58 per cent., of the Indian genera are likewise repre- 
sented in Mexico. Our own table (vol. iv. pp. 207, 208) shows that more than a third 
of the Mexican genera are widely dispersed, that is to say, they occur as well in two or 
more of the large divisions of the Old World; and only 11 per cent. are endemic. 
Engler} finds that only about an eighth of the tropical dicotyledonous genera inhabit 
both America and some part or parts of the Old World. On the other hand, 30-5 
per cent. of the Australian genera and 35:5 per cent. of the South-African are endemic. 
* Grisebach (Symb. ad Fl. Argent. p. 266) refers a second species to this genus with an extended diagnosis. 
+ Silliman’s ‘Journal of Science and Art,’ 2nd series, xxii. (1856) pp. 204-231. 
+ Versuch einer Entwicklungsgeschichte der Florengebiete, ii. p. 174. 
BIOL. CENTR.-AMER., Bot. Vol. I., October 1888. d 
