476 X. GENERAL REMARKS. 



and western Yorkshire, probably because humid ; while 

 they shrivel and sometimes die under the brighter sun and 

 drier atmosphere of Surrey. So likewise the hill plants 

 of the Azore isles are kept with difficulty in the same 

 county of England ; where the winters are found to be 

 too cold for them, and the summers too dry, and perhaps 

 also too warm. Thousands of other such examples might 

 be cited. It may probably be quite true, that every spe- 

 cies is fitted to bear some diversities of climate ; while 

 man^J^ are adapted to bear very wide diversities. 



Grave difficulties appear to beset all attempts to ascer- 

 tain the range of temperature and other climatal condi- 

 tions of species with any close approach to exactness. 

 The climates in which they do not grow, and on experi- 

 ment will not grow, might indeed be ascertained aj^proxi- 

 mately for a goodly number. But the climate in which 

 they are now seen to grow naturally, is too frequently an 

 imperfect indication of the range or diversities of climate 

 in which they could grow. We preserve numerous foreign 

 plants in the borders of our gardens with little more care 

 than the simple process of preventing their destruction 

 by the native and imported weeds which outgrow and 

 smother them. Without human aid most of those foreign 

 species would soon cease to grow there, prevented by 

 other circumstances than climate, or operating conjointly 

 with climate. Yet who can venture to assert that no 

 combination of natural circumstances ever did or ever 

 will occur, sufficient to enable those same species to 

 acquire hereditary hold of the ground here, for an inde- 

 finite time, without human assistance ? 



In what manner, then, are we to ascertain and express 

 the relations between plants and climate, more especially 

 the temperature necessary for their development ? These 

 must still be inferred almost entii'ely fi'om the existing 



