THE GENERAL CONDITIONS OF LIFE 273 



to superficial impressions to consider the organism as a closed 

 system, independent of its environment. The fact of metabolism 

 shows this at once ; for, if the organism lives only so long as it takes 

 in matter from the outside and gives off matter to the outside, it 

 stands in the closest dependence upon the external world ; the latter 

 conditions its life. 



Thus arises the conception of conditions of life, i.e., conditions 

 that must be fulfilled in order that the life of the organism can 

 exist. It is evident that every change of such conditions must 

 exercise an influence upon the life of the organism. Hence, in 

 order to complete a picture of the mutual relations of the organic 

 world and its conditions, it is necessary not only to investigate the 

 latter as they are now, but, so far as possible, as they were in the 

 earlier periods of the earth's evolution. A few fixed points 

 may thus be obtained for the consideration of the question of the 

 origin, the descent and the evolution of life upon the earth. 



I. THE PRESENT CONDITIONS OF LIFE UPON THE EARTH'S 



SURFACE 



All the conditions of life are not equally necessary for all 

 organisms living at the present time. What is absolutely necessary 

 for the existence of one organism may even endanger the life of 

 another. Marine animals when brought into fresh water soon die, 

 and fresh -water animals placed in sea- water experience the same 

 fate. This principle holds good not only for large groups of 

 organisms but for every individual form as well. Every individual 

 organism requires for its existence definite special conditions, 

 without the fulfilment of which it cannot continue to live. These 

 special conditions of life are as manifold as the innumerable forms 

 of organisms themselves. -To describe them is to describe the 

 natural history of every organism, and their investigation belongs 

 to the field of special physiology. But in contrast to them there 

 are other requirements that must be fulfilled for all organisms if 

 the latter are to live, and these must, therefore, be termed general 

 conditions of life. General physiology deals with the latter. In the 

 following pages we shall be able to glance at the special conditions 

 only momentarily, when they are of particular interest and present 

 peculiar adaptations of living substance to peculiar circumstances. 



It is usual to consider under conditions of life only external 

 factors, such as food, water, oxygen, temperature, etc. But in 

 contrast to these external conditions there are internal con- 

 ditions, which are inherent in the composition of the organism, 

 and the absence of which, like that of the external factors, is 

 followed by death. 



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