ORGANIC ADAPTATION 



319 



ings and assume a resting condition in which the metabolic proc- 

 esses are reduced to the lowest terms. In this spore or encysted 

 state they are immune to extremes of temperature and of dry- 

 ness to which they succumb during active life. Thus some 

 Bacteria can withstand nearly — 200° C. for six months, and 

 about — 250° C. for shorter periods, while others can endure 

 140° C. for a short time. (Fig. 200.) 



-.41- 



3Hfc 



m 



m 



" f '-'-C 



••• 





® 



/C\ 



tx% 



•. 



A B C D E 

 Fig. 200. — A-E, Bacillus biifschlii: A, cell structure; B, C, spore forma- 

 tion; D, E, germination of a spore; F, various types of spore formation occur- 

 ring among bacilli. (From Smith and others; A-E after Schaudinn.) 



It is clear that the great majority of organisms are at the mercy 

 of environmental temperatures. This is true of all except the Birds 

 and Mammals. These homothermal animals possess a complex 

 mechanism which maintains their body temperature practically 

 constant; e.g., in Man at 37° C. 



The heat regulatory mechanism represents, so to speak, the final 

 result of the assembling and elaborating, throughout Vertebrate 

 evolution, of elements that appear in the Fishes. In the Mammals 

 it comprises insulation by the skin, a closed blood vascular system, 

 power of rapid oxidation, endocrinal and other glandular products, 

 evaporation surface of the lungs and skin, ' trophic ' and ' tempera- 

 ture' nerves, coordinating centers, etc., — the whole complex 

 rendering its possessors largely independent of the surrounding tem- 

 perature and making possible the carrying on of the various func- 

 tions with such nicety as the life of these forms demands. Indeed, 

 this mechanism makes it possible for a man to stay in a hot dry 

 chamber sufficiently long to see a chop cooked. It is hardly prob- 

 able that the human brain could have developed to function as it 

 does if its cells were subject to wide temperature variations. 



