WHY IMPEACTIOABLE. 219 



only exist under those peculiar conditions in which they have 

 been placed by nature. 



This explanation of a well known horticultural difficulty 

 seems better to agree with facts than the suggestion of Baron 

 Humboldt, to which I formerly assented, that diminished 

 atmospheric pressure was the cause. On this point 

 Dr. Hooker has remarked that "a comparison of Arctic 

 vegetation with that of elevations of 17,000 feet, where 

 literally fifteen inches of pressure are removed, shows no 

 difference in the characters or habits of such plants as are 

 common to both regions ; it certainly induces no peculiarity of 

 vegetation, or there would be a character common to the 

 Alpines of India and of America which the temperate and 

 Arctic regions should not share; but though the Alpine 

 floras of these tropical regions widely differ from each 

 other, they are both Arctic floras in the greatest degree, 

 generically." 



In bis Himalayan Journah this distiaguislied traveller has some 

 additional observations, which deserve quotation. " It has long been 

 surmised that an Alpine vegetation may owe some of its peculiarities to 

 the diminished atmospheric pressure ; and that the latter, being a con- 

 dition which the gardener cannot supply, he can never successfully 

 cultivate such plants in general. I know of no foundation for this 

 hypothesis ; many plants, natives of the level of the sea in other parts 

 of the world, and some even of the hot plains of Bengal, ascend to 

 12,000 and even 15,000 feet on the Himalaya, unaffected by the 

 diminished pressure. Any number of species from low countries may 

 be cultivated, and some have been for ages, at 10,000 to 14,000 feet, 

 without change. It is the same with the lower animals ; innumerable 

 instances may with ease be adduced of pressure alone inducing no 

 appreciable change, whilst there is absence of proof to the contrary. 

 The phenomena that accompany diminished pressure are the real 

 obstacles to the cultivation of Alpiae plants, of which cold and the 

 excessive climate are perhaps the most formidable. Plants that grow 

 in localities marked by sudden extremes of heat and cold, are always 

 very variable in stature, habit, and foliage. In a state of nature we 

 say the plants ' accommodate themselves ' to these changes, and so they 

 do withia certain limits ; but for one that siirvives of aU the seeds that 

 germinate in these inhospitable localities, thousands die. In our gardens 

 we can neither imitate the conditions of an Alpine cUmate, nor offer 

 others suited to the plants of such climates." (Vol. ii., p. 415.) 



