92 The Science of Life. 



organisms to adapt themselves to conditions which 

 often seem to us most unpropitious. 



As living implies expenditure of energy, an income of 

 food is obviously essential to continued activity; and 

 yet we know that even food can be done without for 

 prolonged periods. Succi fasted for thirty days, and 

 the salmon seems to eat nothing for many months. 

 Some slow-going animals, like Amphibia, are said to be 

 able to fast for years, and there is no doubt of this in 

 the case of organisms which pass readily into the state 

 of latent life. 



Water is another essential, but again the limits of 

 necessity are wide. The desert plant, the spore in the 

 air, the dry seed, the encysted Protozoon, the desic- 

 cated wheel -animalcule and water- bear, show that 

 although water is necessary to keep the wheels of life 

 going, it may be in great part removed without irre- 

 trievably spoiling the mechanism. 



Since Priestley discovered oxygen and Mayow com- 

 pared living to slow combustion, it has been recognized 

 that an essential step in metabolism is the oxidation of 

 carbon compounds. Yet here again the necessity for 

 careful statement soon becomes obvious, and we learn 

 once more the lesson, that life does not readily admit of 

 being bound up in formulae. Pasteur and others showed 

 the existence of "anaerobic" micro-organisms, which 

 are able to live and flourish in media containing no 

 free oxygen, as the yeast-plant familiarly does in the 

 brewer's vat, the solution of the riddle being simply that 

 these ' ' anaerobics " obtain the oxygen they require by 

 splitting up the complex substances amid which they 

 live. Similarly, Bunge has shown that parasitic thread- 

 worms which live in the food-canals of many animals 

 may be quite lively for 4-5 days in media entirely free 

 from oxygen. As their store becomes exhausted, how- 

 ever, they sink into latent life. 



For most of the higher plants and animals the limits 

 of temperature consistent with life are comparatively 

 narrow, but this is far from being the case with all. 

 Over and over again, earth-worms, fishes, and frogs, 

 not to speak of simpler creatures, have been thawed out 



