104 THE INFLUENCE OF INANIMATE SUEROUNDINGS. 



does not assimilate its food — that is to say, digest and grow — 

 till the water has attained a warmth of from 14° to 15° centi- 

 grade, and it entirely ceases to do so when the water has 

 reached 30° to 32° Hence it is plain that the pond-snail is 

 for a long time less favourably circumstanced than a bird or 

 mammal living near the pond or in it, and close to the snail ; 

 for they, like all warm-blooded animals, can carry on the pro- 

 cess of digestion even when the temperature falls below the 

 freezing point or rises above 36° or even higher ; the Lymnaea 

 meanwhile is materially checked in its growth, or even killed. 

 Nevertheless the calculated mean daily temperature may be 

 identical with that which would afford the most favourable con. 

 ditions for the pond-snail. Hence it is evident that a classifi- 

 cation of animals according to the climate in which they live 

 in fortuitous community — as those of the tropical, temperate, 

 and frigid zones — has no real value, and is simply an expression 

 of the fact that different animals live in different climates. All 

 such divisions founded on the terminology of meteorology serve 

 only to conceal the true relations of animal life to the tempe- 

 rature that influences it, and consequently cannot be regarded 

 scientifically as either accurate or useful. I therefore shall 

 suggest another method of classification which agrees better 

 with the nature of the relations that can be proved to subsist 

 between animals and those variations of temperature to which 

 they are subjected.^' 



Almost all animals are exposed within the course of a day 

 to more or less considerable changes of temperature. If we 

 assume, as we are justified in doing by certain observations 

 that shall presently be communicated to the reader, that there 

 is a certain degree of heat (which need by no means be identical 

 with the meteorological mean temperature of the day) which is 

 most favourable to the well-being of one or of several species 

 of animals, obviously every rLse or fall of temperature above or 

 below this favourable point must be to a certain extent injurious 

 to the creature. The interval between the daily extremes may 

 be great or small without any alteration in the daily meteoro- 

 logical mean ; moreover, the favourable temperature— the opti- 

 mum of temperature for the animal — may either coincide with 



