144 ACCLIMATIZATION. Chat. V. 



Acclimatization, 



Habit is hereditary with plants, as in the period of flower- 

 iii<T, in the amount of raui requisite for seeds to germinate, in 

 the time of sleep, etc., and tliis leads me to say a few words on 

 acclimatization. As it is extremely common for species of the 

 same genus to inhaljit hot and cold countries, if it be true that 

 all the species of the sanic g-cnus are descended from a single 

 parent-form, acclimatization must be readily 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 can- 

 not 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 im})ortcd 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 cli- 

 mates. But whether or not the adaptation be generally very 

 close, we have evidence, in the case of some few plants, of 

 their becoming, to a certain extent, naturally habituated to 

 diff(>rent temperatures ; that is, they become acclimatized: thus 

 the pines and rhododendrons, raised from seed collected by Dr. 

 Hooker from the same species gro^^^ng at different heights on 

 the Himalaya, were found in this country to possess different 

 constitutional powers of resisting cold. ^Ir. Thwaites informs 

 me that he has observed similar facts in Ceylon ; analogous 

 observations have been made by Mr. H. C. Watson on Euro- 

 pean species of plants brought from the Azores to England ; 

 and I could give other cases. In regard to animals, several 

 autlientic instances could be adduced of species within histori- 

 cal times having largely extended their range from warmer to 

 cooler latitudes, and conversely ; but we do not positively know 

 tliat these animals were strictly adapted to their native climate, 

 though in all ordinary cases we assume such to be the case ; 

 nor do we know that they have subsequently become specially 

 acclimatized to their new homes, so as to be better fitted for 

 lliiMU than they Avere at first. 



As we may infer that our domestic animals were originally 



