26 The Field Naturalist's Quarterly Feb. 



a certain body temperature is necessary. Below a certain 

 point (which we have reason to beheve varies not a Httle 

 with different species) the organism is unable to lead an 

 active existence, though a still lower temperature is necessary 

 to abolish the capacity of the now comparatively inert tissues 

 to once more exhibit an obvious vitality with the return of 

 more favourable conditions. It is indeed somewhat doubt- 

 ful, as far as the lower organisms are concerned, whether a 

 simple reduction of temperature, even far below zero, is 

 adequate to destroy life apart from the mechanical injuries 

 which are generally produced by the formation of ice. 

 Animals such as mammals and birds can be active during 

 cold weather, for they have acquired the power of combat- 

 ing external variations of temperature of a considerable 

 range by appropriate alterations in the production and loss 

 of heat in their own persons, and are thus enabled to main- 

 tain their body temperature at a convenient working level 

 all the year round. Such invertebrates as snails no doubt 

 possess this power in some small degree, — experiment shows 

 that their bodies are a little cooler than the surroundings 

 in hot weather, and a trifle warmer in winter ; but the 

 mechanism of this heat regulation, especially on the side of 

 heat production, is far too poorly developed to enable them 

 to do more than lag a few degrees behind the changes in 

 general temperature in the direction of the mean coolness 

 which is their optimum. So snails have to be quiet in 

 winter and take what measures they can to protect them- 

 selves from the cold. 



It is to be hoped that ignorance spares them any pangs of 

 envy at the untasted delights which are enjoyed by such 

 Hyalinia and others as have learned the trick of living in 

 hothouses and similar places. Retirement into simple 

 cracks in a wall or to any sheltered corner where the air 

 is quiet, evades the cooling effect of a wind ; if, as is most 

 conspicuously done by the common brown garden-snail, 

 Cryptomphalus aspersus, many congregate close together, a 

 certain amount of warmth is collected for the common use 

 of the mass by multiple (and taken singly, useless) contribu- 

 tions from each individual. One of the soundest plans seems 

 to be that of burrowing a few inches into the ground, where 

 they are safe from anything but the severest frost. Many 



