CHAPTER IV 



THE GENERAL CONDITIONS OF LIFE 



THE living substance of organisms forms a part of the mass of 

 matter that composes the earth. As has been seen, the differences 

 between it and lifeless substances are not fundamental in nature, 

 for the elements that constitute the former constitute also the 

 latter. The differences between organic and inorganic substance 

 are no greater than the differences between many inorganic sub- 

 stances, and consist merely in the mode of union of the elements. 

 It is important to familiarise ourselves with the thought of living 

 substance, not as something mystical, which has no connection with 

 and stands in contrast to all other substance, but as a part of 

 the matter that constitutes the earth's crust. It is evident that 

 life is conditioned wholly by the character of the environment, that 

 the evolution of living substance must be inseparably connected 

 with the evolution of the earth. Accordingly, the composition and 

 the form of the living substance that now covers the earth's surface 

 are to be considered from exactly the same point of view as, for 

 example, the composition of the present sea, i.e., as something that 

 has gradually become, and exists in its present state only because 

 the conditions are such as they are at the moment. Just as the sea 

 with its salt could not have existed as it does now before the water 

 had appeared upon the earth in a liquid state, so also living sub- 

 stance could not then exist with its present composition, for it 

 contains upon an average more than fifty per cent, of water. But 

 just as water was obliged to assume its present form when certain 

 conditions in the earth's evolution were fulfilled, so living substance 

 was obliged gradually to take on its present character to the extent 

 to which the present conditions of the earth's surface were per- 

 fected. The separation of living substance out of the mixture of 

 materials of the earth's crust is only one result of the earth's 

 evolution, like the separation of rocks, salts, or water. 



The same idea is arrived at from another starting-point, when, 

 not the elementary composition, but the, vital phenomena of living 

 substance are considered. It is an error easily conceived and due 



