development of bacteria. But although it seems to us nowadays self- 

 evident and unavoidable that such, turbidity must owe its origin to the 

 development of the ubiquitous and invisible germs omne vivum ex ovo 

 this 'occurrence was, until not so very many years ago, a profound mystery. 

 It appeared absolutely certain that these living ' animalcula ' must have 

 arisen spontaneously from the non-living constituents of the liquid. ' Spon- 

 taneous Generation ' (generatio aeguivoca) seemed, on apparently good 

 evidence, an indisputable fact. 



The question as to the origin of life an older problem even than that 

 of evolution is the natural outcome of the Kant-Laplace theory of the 

 origin of the earth, according to which our planet was at first a molten mass 

 that had to cool down before living things could exist upon its surface. 



Whence came these first forms of life ? Were they precipitated from 

 other planets through space upon our globe, or did they arise de novo from 

 the non-living substances that then alone existed? The first hypothesis 

 has little to support it, and would only throw the question one step further 

 back. It seems much more probable that the first organisms did really 

 arise from inorganic matter by ' spontaneous generation,' and that, as the 

 evolutionary theory assumes, all other living things have been gradually 

 evolved from them in continued series. The doctrine of evolution is, 

 indeed, incomplete without the assumption of such a commencement. Its 

 possibility once granted, there is no a priori reason to deny that it still 

 takes place, and that side by side with the ceaseless evolution of new species 

 there may actually arise new organisms by spontaneous generation. Since 

 we can imagine only the very simplest forms of life to be produced in this 

 manner, it is natural that evidence for their occurrence should have been 

 sought among bacteria and other fermentative organisms. Experiment 

 showed that in some cases boiling for many hours did not prevent the 

 development of bacteria in nutrient solutions, and since it was contrary to 

 all experience that any living things could have survived such temperatures 

 spontaneous generation seemed to be beyond dispute. When, as frequently 

 happened, such organic infusions remained sterile after boiling, the advo- 

 cates of the theory explained matters by saying that conditions had been 

 changed, and the fluid was ' no longer suitable ' for the genesis of life. 

 They also showed that passing air through it apparently removed these 

 unsuitable conditions, bacteria appearing again. If, however, the air 

 had been raised to a high temperature first, or passed through sulphuric 

 acid, or even through cotton-wool, the sterility of the liquids sometimes 

 remained unimpaired, but not always. Contradictory results abounded 

 and difficulties arose both for partisans and opponents of the theory. It was 

 long before the controversy could be regarded as satisfactorily settled, but 

 settled it was at last, and we know now that at the present day a spontaneous 

 generation of organisms does not take place in this way. We know that 



