112 ACCLIMATISATION. [Chap. V. 



have ranged over both continents, and then have become extinct, 

 excepting in their present secluded abodes. Far from feeling sur- 

 prise that some of the cave-animals should be very anomalous, as 

 Agassiz has remarked in regard to the blind fish, the Amblyopsis, 

 and as is the case with the blind Proteus with reference to the 

 reptiles of Europe, I am only surprised that more wrecks of ancient 

 life have not been preserved, owing to the less severe competition to 

 which the scanty inhabitants of these dark abodes will have been 

 exposed. 



Acclimatisation. 



Habit is hereditary with plants, as in the period of flowering, in 

 the time of sleep, in the amount of rain requisite for seeds to germi- 

 nate, &c, and this leads me to say a few words on acclimatisation. 

 As it is extremely common for distinct species belonging to the 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 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 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 in- 

 ability 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 temperatures ; 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 extended, 

 within historical times, their range from warmer to cooler latitudes, 



