AND ITS INHABITANTS 93 



in the free state for the processes of life and for putrefaction 

 and fermentation of organic matter. The treatment to which 

 Spallanzani subjected his infusions might well have so changed 

 the organic matter, they argued, that it was incapable of pro- 

 ducing life. This objection was met by a series of experiments 

 by various investigators during the first half of the last 

 century, which showed conclusively that thoroughly sterilized 

 infusions never developed living organisms when air was 

 admitted which had been rendered sterile by heat or by having 

 all suspended matter removed, while the studies of Schwann 

 and Cagniard de la Tour intimated that fermentation and also 

 putrefactive changes in organic infusions are themselves the 

 result of micro-organisms. 



So far it was clear that organic infusions in which no life 

 is present do not develop living organisms except when con- 

 taminated with material from an extraneous source, which is 

 ordinarily the atmosphere, but it remained then to be shown 

 that living organisms are practically ubiquitous in the atmos- 

 phere and thus are available for the initiation of the micro- 

 flora and fauna of infusions. This was put upon a broad 

 empirical basis chiefly by the labors of Pasteur. Tyndall, 

 who himself made a notable contribution to the solution of 

 the problem by researches in experimental physics, confidently 

 asserted that there seems to be no flaw in the reasoning, and 

 it is so simple as to render it unlikely that the notion of life 

 developing from dust can ever again gain currency among the 

 members of a great scientific profession. 



We thus reach the general conclusion that, so far as human 

 observation and experimentation go, no form of life arises 

 today except from preexisting life. But since life is present on 

 the earth now and the paleontological record indicates that it 

 has been present without interruption for some hundreds of 

 millions of years, we have to consider the following alterna- 

 tive: Either life was transported to this planet from some 



