PART IV 



ORGANISMS IN THEIR EXTERNAL 

 RELATIONS 



CHAPTER LXIV 

 OBSTACLES TO LIFE 



389. Life and the environment. To live means to do. Pro- 

 toplasm tends to be active. But the activities of protoplasm de- 

 pend not alone on its own structure or composition ; they depend 

 in part, as we have learned, upon external conditions as well as 

 upon the opportunity to obtain various materials from without. 

 While many of the external conditions are favorable to the ac- 

 tivities of live matter, others are just as decidedly unfavorable. 



390. Temperature and life. Observations on various plants 

 and animals show that the activities of life are dependent upon 

 temperature. 



Warm-blooded animals can endure a wide range of tempera- 

 ture, but the protoplasm of such animals can really endure a 

 rather narrow range only. When such protoplasm is exposed 

 to a temperature several degrees below the normal, or several 

 degrees above, it ceases its activities and may even be killed. 

 On the other hand, the cells of the so-called cold-blooded 

 animals can actually endure extremes of temperature. Many 

 animals can be frozen and then thawed out again without 

 being appreciably injured. 



In careful experiments fish have been frozen in blocks of ice to 

 a temperature of 5 F. (27 degrees below the freezing point), kept 

 this way for some time, and then slowly thawed out without being 



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