102 BIOGENESIS AND IIOMOGENESIS less. 



same tale : they prove conclusively that in properly sterilized 

 putrescible infusions, adequately protected from the entrance 

 of atmospheric germs, no micro-organisms ever make their 

 appearance. So that the last argument for abiogenesis has 

 been proved to be fallacious, and the doctrine of biogenesis 

 shown, as conclusively as observation and experiment can 

 show it, to be of universal application as far as existing 

 conditions known to us are concerned. 



It is also necessary to add that the presence of microbes 

 in considerable quantities in our atmosphere has been 

 proved experimentally. By drawing air through tubes 

 lined with a solid nutrient material Prof. Percy Frankland 

 showed that the air of South Kensington contains about 

 thirty-five micro-organisms in every ten litres, and by ex- 

 posing circular discs coated with the same substance he was 

 further able to prove that in the same locality 279 micro- 

 organisms fall upon one square foot of surface in one 

 minute. 



There is another question intimately connected with that 

 of Biogenesis, although strictly speaking quite independent 

 of it. It is a matter of common observation that, in both 

 animals and plants, like produces like : that a cutting from 

 a willow will never give rise to an oak, nor a snake emerge 

 from a hen's egg. In other words, ordinary observation 

 teaches the general truth of the doctrine of Ho7nogenesis. 



But there has always been a residuum of belief in the 

 opposite doctrine of Heterogenesis^ according to which the 

 offspring of a given animal or plant may be something 

 utterly different from itself, a plant giving rise to an animal 

 or vice versa, a lowly to a highly organised plant or animal 

 and so on. Perhaps the most extreme case in which hetero- 

 genesis was once seriously believed to occur is that of 



