174 ACCLBIATISATION. [Chap. V. 



same genus to inhabit hot and cold countries, if it 

 be true that all the species of the same genus are 

 descended from a single parent-form, acclimatisation 

 must be readilv effected during a long course of descent. 

 It is notorious that each species is adapted to the 

 climate of its own home : species from an arctic or even 

 from a temperate region cannot endure a tropical 

 climate, or conversely. So again, many succulent 

 plants cannot endure a damp climate. But the degree 

 of adaptation of species to the climates under which 

 they live is often overrated. We may infer this from 

 our frequent inability to predict whether or not an 

 imported plant will endure our climate, and from the 

 number of plants and animals brought from different 

 countries which are here perfectly healthy. We have 

 reason to believe that species in a state of nature are 

 closely limited in their ranges by the competition of 

 other organic beings quite as much as, or more than, by 

 adaptation to particular climates. But whether or not 

 this adaptation is in most cases very close, we have 

 evidence with some few plants, of their becoming, to a 

 certain extent, naturally habituated to different tempera- 

 tures ; that is, they become acclimatised : thus the pines 

 and rhododendrons, raised from seed collected by Dr. 

 Hooker from the same species growing at different 

 heights on the Himalaya, were found to possess in this 

 country different constitutional powers of resisting cold. 

 Mr. Thwaites informs me that he has observed similar 

 facts in Ceylon ; analogous observations have been made 

 by Mr. H. C. Watson on European species of plants 

 brought from the Azores to England ; and I could give 

 other cases. In regard to animals, several authentic 

 instances could be adduced of species having largely 



