246 



HEAT AND PROTOPLASM 



[Ch. VIII 



Summarizing these conditions, we find that the following 

 organisms, even when thawed out carefully, are killed by sub- 

 jecting to a temperature of between 0° and — 5° for 60 minutes: 

 Amoeba, swarm-spores, white blood corpuscles of Amphibia, 

 planarians, pulmonate embryos, small leeches, certain entomos- 

 tracans, and some insects and spiders. These organisms are all 

 either soft bodied or of small size, and, excepting animals which, 

 like the bees, live in protected situations, they do not winter 

 over in our northern temperate countries. The following 

 species, on the other hand, resist — 10° for at least 60 minutes : 

 some snails (possibly), the beetles Melolontha (perhaps) and 

 Phytonomus, the Vanessa larva, Pelopseus chrysalis, and the 

 crayfish, — all protected by a thick covering, or of rather large 

 size. The reason why large size and thick covering should 

 increase resistance is not far to seek, — both conditions tend 

 to prevent the rapid loss of heat, — to defend the body from 

 freezing through and through. 



Another cause of variation in resistance to cold is, doubtless, 

 the amount of water in the protoplasm. I have already referred 

 to this cause as explaining the fact that dry seeds and spores 



