132 THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



species is capable of modification, the greatest alternations in 

 the conditions of existence will offer the most favourable oppor- 

 tunities for its transformation and improvement — thus, in the 

 case in question, the utmost variations of temperature — since 

 it is in this way that the most stringent selection is effected 

 between the weak and the strong. 



Among the numerous instances which here offer themselves 

 for discussion, want of room compels me to select only a few of 

 the most significant. One of the first in importance seems to 

 me to be a case of which the full value was first recognised by 

 Mobius, who also, and apparently with perfect accuracy, referred 

 it to its efficient cavise. He mentions ■" the fact that the same 

 species of Mollusc, living on the coast of Greenland or in the 

 Baltic Sea, was in the former instance very large, and in the 

 latter small and thin-shelled ; and he attempted to explain this 

 difference by saying that the animals in the Baltic are exposed 

 to very considerable variations in temperature between the 

 summer and winter, and are sometimes even frozen up, while 

 on the coast of Greenland they live in a temperature which, 

 though of course low, varies but little from winter to summer. 

 Hence they are enabled to carry on the assimilation of food at 

 an equable rate all the year round, never being disturbed by 

 too great a heat in summer or too severe a cold in winter, while 

 those in the Baltic are every year exposed to the liability of 

 being checked in their growth by a too high or too low tempera- 

 ture in constant alternation. Although the mean temperature 

 of the Baltic is higher than that of the sea off Greenland, the con- 

 stant low temperature, i.e. the equable climate, of the latter is 

 infinitely more favourable to the growth of the animal than 

 the higher mean temperature with constant' variations of the 

 Baltic. 



The numerous cases of successful acclimatisation lead us to 

 the same conclusion. So far as I know, the old system is now 

 given up in every zoological garden, by which foreign animals 

 were kept in houses or cages in which an attempt was made to 

 keep up by artificial means such a climate as they were used 

 to ; a contrary plan is now introduced, by which the animals 

 are accustomed as quickly and thoroughly as possible to the 



