EUEYTHBEMAL AND STENOTHERMAL. 105 



the meteorological mean or lie nearer to one of the extremes — 

 the maximum or -the minimum — than the other. We have here 

 assumed that the optimum is the same for all animals; hut they 

 may nevertheless be very differently affected by the variations in 

 temperature according to the degree of the variations themselves. 

 Then those animals which can endm'e the greatest variation in 

 the direction of either extreme evidently exhibit a certain 

 contrast to others which can only thrive under very small 

 departures from the optimum, and their distribution must 

 depend essentially on this characteristic. Certainly the distinc- 

 tion thus indicated cannot be regarded as absolute; but we 

 shall nevertheless do well to avail ourselves of it as a means 

 of classification, and to designate animals, according to Mobius,'^ 

 the former as eurytliermal, the latter as stenothermal. 



We also know that the optimum of temperature may be 

 extremely different for different animals, since some exist near 

 the poles and others on the equator, some live on ice and some 

 in hot- water springs. Nevertheless, a rise of temperature above 

 the optimum must in either locality influence the animal in an 

 analogous manner, whether it dwell on the ice of the North Pole, 

 or at the summit of a high peak, or on the scorching plains of the 

 tropics ; and a fall of temperature below the optimum must 

 likewise produce analogous, though not identical, phenomena. 

 Hence we may, in a certain sense, assume the optimum of tem- 

 .perature as being the same for all animals as a basis for our 

 discussion of the question : How do variations of temperature 

 affect the animal? and thus we may divide this chapter into 

 three sections, the first dealing with the effects of a falling 

 temperature, the second with those of a rising temperature, 

 and the third with those of an equal temperature, and at the 

 same time with the dependence of animal life on these three 

 'conditions — the term 'equal temperature' excluding any great 

 changes whether of rise or fall. 



I. The influence of a falling temperature on animal 

 life.- — This influence may exhibit itself in many different ways. 

 A small fall in temperature may be as injurious to one animal 

 as a great fall to another, while a third species may be wholly 

 unaffected by either. Animal life is often destroyed before the 



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