86 EX VIR'IBUS VIVIMUS. 



far, may destroy, but if moderated must extend, 

 length of life (at the expense of intensity). The 

 coldness of water, together with its diminished power 

 of oxygenation, as compared with the atmosphere, 

 is one of the direct causes of diminished expenditure 

 in aquatic animals, rendering their life necessarily less 

 intense than that of terrestrial forms, and so longer. 



In keeping animals in menageries, in rearing pets 

 and domesticated animals, man performs an experi- 

 ment by diminishing personal expenditure. He fre- 

 quently does the same in his own case, leading 

 a careless, labourless existence ; but there is in this 

 as in other experiments (which are rarely so good 

 in physiological enquiry in their results as natural 

 comparisons) a disturbing cause, for Luxury, 'the 

 fertile parent of a whole family of diseases,' as Galen 

 termed her, steps in and works against the diminished 

 expenditure. When man in his own person, or in the 

 organisms he interferes with, so far baulks Nature's 

 provisions that the organs become, as it were, rusty 

 through the suspension of that personal expenditure, 

 which is usually necessary to keep up the warmth by 

 oxygenation, and to obtain necessary food, then he 

 .shortens rather than increases the length of life, 

 disease attacks his victim, and death follows. This is 

 seen exemplified in the case of domesticated animals,^ 



' Mr. Darwin informed the writer that he did not know of any 

 reliable data admitting of a comparison between domestic animals and 

 their nearest wild representatives (their actual wild forms being unknown 

 in every case). 



