306 CONCLUSION 



of the world surface. The heavily forested plains of 

 Europe and North America have retained a small pro- 

 portion of woodlands : China has been cultivated for 

 countless centuries: all over the world, whole mountain- 

 chains have been left bereft of their plant covering and 

 turned into arid rocks : a good deal of the treelessness of 

 the Asiatic grass-lands is probably due to persistent 

 depredations : the tropical savana has been extended and 

 depleted by yearly grass fires at the expense of woodlands 

 and even of the tall forests. 



This interference with the balance of nature, some- 

 times justified by the very necessities of life, and 

 constituting legitimate improvements upon the original 

 conditions, from man's standpoint, has, in countless other 

 instances, been the result of ignorance and improvidence, 

 involving consequences disastrous for humanity. It is 

 somewhat sad to reflect that the activity of quite a con- 

 siderable portion of mankind is devoted to repairing 

 damages done by other portions, or by former generations. 



What has been said hitherto of the large masses of 

 vegetation obtains, and in a still larger measure, when 

 considering the various minor units of different orders 

 which compose them, and may be conveniently termed 

 here plant formations or communities. Ultimately, 

 among the primitive units, the growth forms or 

 vegetative types, which correspond to plant species 

 in the study of the flora, analogies of this sort are 

 innumerable. To mention only one or two of the most 

 striking instances : the candelabra cerei or cacti of 

 America correspond to the candelabra euphorbia of Africa; 

 the agaves of Mexico and Texas are equivalent to many 

 forms of African aloes, and are frequently mistaken for 

 them ; or again, analogous cushion-forms in arctic or 

 in alpine regions are drawn from the most diverse orders 



