544 DR. p. CHALMERS MITCHELL ON LONGEVITY AND 



cold, but that even rapid and consideiuble changes of temperature 

 are a necessary stimulus to their via.bility. 



On the other hand, there are cases where the change to the 

 climate of London is certainly trying, but these occur not amongst 

 tropical but tempei'ate or arctic animals. The production of a thick 

 coat against the onset of winter cold, and its doffing when summer 

 approaches, appear not to be direct reactions to temperature but 

 organic rhythms adapted to the seasons in the natural habitat of 

 the animals. Thus animals from the far North, accustomed to the 

 short arctic summer, retain their thick coats in this climate long 

 after their possession is cumbei-some. Still worse is the case of 

 animals brought from South temperate zones, which begin to 

 moult their thick coats when our winter, theii' summer, approaches. 

 I have not information as to how soon if ever these rhythms 

 readjust themselves to the reversed conditions. It is clear, on 

 the other hand, that part at least of the mechanism by which 

 coats become warmer, is direct stimulation from the surrounding 

 teoiperature, and such different creatures as Carnivora and 

 Baboons rapidly get better coats when exposed to the open air. 

 The seasonal rhythms connected with breeding are also a cause 

 of mortality to young or adults, when the conditions of climate 

 are reversed. Birds from the Southern hemisphere, if they 

 survive, appear to readjust themselves in this respect : it is more 

 doubtful if mainmals do so. 



(8) The idea that it is a fundamental necessity to protect healthy 

 adult mammals and birds from cold by providing them with 

 artificial heat is fallacious. The supreme necessity is free access 

 to open air. In most cases this should be combined with shelter 

 from rain and wind, and in some cases the shelter should be 

 supplied with ai'tificial heat — perha,ps often even in excess of what 

 is now customaiy — but only so far as it can be arra,nged withoiit 

 any detriment to fresh air. I think this is probably specially 

 important in the case of nocturnal a.nimals ; as we are accustomed 

 to see these asleep all day in the warmest corner given them, we 

 are disposed to forget that at night they move about? actively often 

 in great cold. 



For all mammals and birds stendy exposure to an even tem- 

 perature is unnatui'ai and unhealthy; change is a necessaiy 

 stimulus, and permanent existence indoors is the worst possible 

 condition for viability and longevity.* 



* [When this mpinoiv was read my friend iJr. Leiper called my attention to the 

 nndoulited fact that the prohlem of the duration ot life of animals in captivity is com- 

 plicated by the ett'ects of parasiti's. In the course of my paper (see pp. 470 and 540) 

 I have inade reference to this subject, pointing out that some of the evil caused by 

 confinement in warmed houses is undoubtedly- not the direct result of absence of 

 fresh air, or of artificial heat, but comes about mediately inasmuch as the conditions 

 are specially favourable to parasites. It is true of course that any argument as to 

 the potential longevity of animals based on maximum durations in cajitivity is' 

 subject to the effects of parasitism. I hope that the work of Mr. Plimmer, Mr. Beddard, 

 Dr. Leiper, Dv. Nicnll, and Mr. Coventry, who are now systematically examining 

 the parasites at the Gardens, will bei'ore long enable us to eliminate some at least of 

 these evils.] 



