130 THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



III. The influence of constant temperatures, but at 

 different degrees. — We have seen that a fall below the optimum 

 of temperature or a rise above it is more or less injurious to the 

 animal, according as it ranges more or less near to the fatal 

 maximum or minimum. If there were a spot where the heat 

 was absolutely invariable, and if the degree of heat there exactly 

 corresponded to the optimum for the animal, if also by this 

 nies-ns the injurious influence of variation were excluded, abso- 

 lutely favourable conditions for the hfe and growth of the crea- 

 ture would prevail, so far as warmth was concerned. It is 

 evident, however, that very few animals Uve in such a peren- 

 nially equable climate ; none in fact but the parasites living 

 inside warm-blooded animals and the creatures at the bottom of 

 deep seas and lakes enjoy such an advantage. All others are 

 exposed more or less to the effects of great periodical variations. 

 Among these, however, those are evidently the most favoured 

 which are exposed to the smallest deviations above or below the 

 optimum. Hence a climate where the mean annual tempera- 

 ture differs in only a trifling degree from the winter or summer 

 mean temperature, or from the extremes of heat and cold, will 

 be the most favourable ; such an equable climate may occur in 

 high latitudes as well as between the tropics, since its existence 

 depends less on latitude than on the configuration of the land, 

 the vicinity of the sea, .and the predominant direction of winds 

 and currents. For example, the eastern half of both the old 

 and new continents are distinguished from the western by 

 havirg what is called an ' extreme ' clim.ate, in which the ex- 

 tremes of heat and cold lie far asunder, while England, and still 

 more Ireland, are characterised by an extraordinarily equable 

 climote as compai'ed with many more southerly parts of the 

 continent, which, like the countries of south-eastern Europe, have 

 a continental, i.e. an ' extreme,' climate, with great heat in the 

 summer and severe cold in the winter. 



Before entering on a discussion of the question as to how 

 far the theoretically assumed advantage of an equable climate is 

 practically real, I must anticipate one view which might other- 

 wise be regarded as the inevitable consequence of the foregoing 

 statement. It might seem as though it stood in trenchant 



