iS95- NOTES AND COMMENTS. 225 



the beginning of the experiment, the author thinks that the decrease 

 in the assimilation of nitrogen would be still greater. 



A second result was that the animals lost weight more quickly 

 under the sterilised conditions than under normal conditions, while, 

 at the same time, the excretion of nitrogen and of carbonic acid was 

 more than usual. 



A third result was still more remarkable. In a large number of 

 the experiments the animals died, sometimes a few minutes, more 

 often a few hours or a few days, after the beginning of the experiment. 

 No cause could be assigned for this. The possible causes were all 

 excluded, and the inexplicable fact remained. The novelty of the 

 idea that sterilisation of the air is fatal to life no doubt is attractive ; 

 but we agree with the writer of the paper, that even his careful and 

 laborious experiments are not sufficient to justify the belief that 

 microbes in the air are necessary to the life of air-breathing animals. 

 Physiologists will remember the experiments of Dr. Haldane at 

 Oxford, which showed that animals were not poisoned by their own 

 organic exhalations. It is clear enough that a very large amount of 

 work must be done before respiration is understood. 



Geographical Distribution and Temperature. 



In an address published in the American Nat. Geog. Mag. (vol. vi., 

 1894), Dr. Hart Merriam discusses temperature as a barrier to distri- 

 bution of animals and plants. In the northern hemisphere, animals 

 and plants are distributed in circumpolar zones which follow 

 isothermal lines rather than parallels of latitude. The main 

 divisions are known as the boreal, austral, and tropical belts ; and 

 each of these is broken up into minor zones. Thus, in North 

 America, the boreal belt is divided into the Arctic, Hudsonian, and 

 Canadian zones ; the austral belt into the transition, upper austral, 

 and lower austral zones. But the isotherms are computed for 

 arbitrary periods, for months, seasons, and years, and so cannot be 

 applied directly to fix the boundaries of the regions. 



Dr. Merriam suggests that it is not the temperature of the whole 

 year, but the temperature of the period of growth and reproductive 

 activity, that determines the distribution of terrestrial animals and 

 plants. In the tropics this period extends nearly throughout the 

 year, and hence, in the tropics, there is a close agreement between 

 the mean annual temperatures and the zones of life. Within the 

 Arctic circle, and on the summits of high mountains, the reproductive 

 and growing period lasts only about two months : hence in these 

 regions there is a wide discrepancy between mean annual tempera- 

 tures and the zones of life. 



Starting with the generally-accepted idea that a temperature of 

 6 degrees centigrade is the minimum that marks the inception of 

 physiological activity in plants and of reproductive activity in 



