116 MASSACHUSETTS HORTICULTURAL SOCIETY. 



Lastly come the tropical floras, characterized by great abundance 

 of all kinds of vegetation and with a preponderance of succulent 

 moisture-loving plants. Of these there are three, markedly dis- 

 tinct, if "we consider the relationships of the plants of which they 

 are composed ; very similar if we consider their general appearance. 

 They are 



The American, reaching its highest development in the Valley of 

 the Amazon. It is excessively rich in species and luxuriant in 

 growth. Secondly comes 



The African, still imperfectly known but also very rich. Many 

 ■of its general physical characteristics have been vividly sketched 

 for us in Mr. Stanley's latest work. And third is 



The Indo-Malayan, including India and all the East Indian 

 Archipelago with the Malay Peninsula, and extending even to 

 Australia and Japan. 



The relationships of these three are sufficiently well-known to 

 indicate that while they differ much from each other, they are 

 more alike than are the floras of the South Temperate Zone which 

 we have considered, — a fact tending to show that the former may 

 have been connected with each other more recently than the latter. 

 From this the step is but a short one to the theory some years ago 

 propounded by an English naturalist, that all the floras and, there- 

 fore, all the plants of the earth have originated in the northern 

 hemisphere, and that they have spread southward in successive 

 great waves, the more southern and older floras being the most 

 ancient of these, and the more northern floras later in time. 



And in conclusion, to sum up the whole matter, I hope I have 

 shown you that while the causes controlling the Distribution of 

 Plants are many and rendered exceedingly complex b}' their 

 interaction, we can separate out the principal agencies and trace their 

 effects ; that the agencies limiting distribution are the needs of 

 plants with reference to the amounts of heat and moisture to which 

 the}" have respectively become adapted, and also by the past 

 history of each plant placing it in some one region, and not in all 

 to which it is adapted ; and finally that the active distributing 

 agent, aside from the comparatively feeble efforts of the plants 

 themselves and man's insignificant effects, has been the successive 

 migrations compelled by past climatic and geological changes, and 

 that these migrations have resulted in the distribution of all plants 

 over the earth's surface into those definite groups or floras which 



