80 Animal Life and Intelligence. 



room or subsistence for the many born. The tendency is 

 therefore held in check by elimination, involving the struggle 

 for existence. And the factors of elimination are three : 

 first, elimination through the action of surrounding physical 

 or climatic conditions, under which head we may take such 

 forms of disease as are not due to living agency ; secondly, 

 elimination by enemies, including parasites and zymotic 

 diseases ; and thirdly, elimination by competition. It will 

 be convenient to give some illustrative examples of each of 

 these. 



Elimination through the action of surrounding physical 

 conditions, taken generally, deals with the very groundwork 

 or basis of animal life. There are certain elementary 

 mechanical conditions which must be fulfilled by every 

 organism however situated. Any animal which fails to 

 fulfil these conditions will be speedily eliminated. There 

 are also local conditions which must be adequately met. 

 Certain tropical animals, if transferred to temperate or 

 sub-Arctic regions, are unable to meet the requirements of 

 the new climatic conditions, and rapidly or gradually die. 

 Fishes which live under the great pressure of the deep sea 

 are killed by the expansion of the gases in their tissues 

 when they are brought to the surface. Many fresh-water 

 animals are killed if the lake in which they live be invaded 

 by the waters of the sea. If the water in which corals live 

 be too muddy, too cold, or too fresh near the mouth of a 

 great river on the Australian coast, for example they will 

 die off. During the changes of climate which preceded 

 and followed the oncoming of the glacial epoch, there must 

 have been much elimination of this order. Even under 

 less abnormal conditions, the principle is operative. Darwin 

 tells us that in the winter of 1854-5 four-fifths of the 

 birds in his grounds perished from the severity of the 

 weather, and we cannot but suppose that those who were 

 thus eliminated were less able than others to cope with or 

 stand the effects of the inclement climatic conditions. My 

 colleague, Mr. G. Munro Smith, informs me that, in culti- 

 vating microbes, certain forms, such as Bacillus violaceus 



