282 BIOGENESIS chap. 



But there have always — until comparatively recently, at any 

 rate — been upholders of the view that the lower forms of life, 

 bacteria, monads, and the like, may under certain circum- 

 stances originate independently of pre-existing organisms : 

 that, for instance, in a flask of some organic infusion 

 boiled so as to kill any living ^things present in it, fresh 

 forms of life may arise de novo — may in fact be created 

 then and there. 



We have therefore two theories of the lower organisms, 

 the theory of Biogenesis, according to which each living 

 thing, however simple, arises by a natural process of bud- 

 ding, fission, spore-formation, or what not, from a parent 

 organism : and the theory of Abiogenesis, or as it is some- 

 times called Spontaneous or Equivocal Generation, accord- 

 ing to which fully formed living organisms sometimes 

 arise from not-living matter. 



In former times the occurrence of abiogenesis was uni- 

 versally believed in. The expression that a piece of meat 

 has " bred maggots " ; the opinion that parasites such as the 

 gall-insects of plants or the tape-worms in the intestines of 

 animals originate where they are found ; the belief still held 

 in some rural districts in the occurrence of showers of frogs, 

 or in the transformation of horse-hairs kept in water into 

 eels ; all indicate a survival of this belief. 



As accurate enquiries into these matters were made, the 

 number of cases in which equivocal generation was supposed 

 to occur was rapidly diminished. It was not surprising 

 however, considering the rapidity with which Bacteria and 

 Monads were found to make their appearance in organic 

 substances and infusions, that many men of science imagined 

 them to be produced abiogenetically. The rapid multiplica 

 tion of these forms means, of course, that a certain amount 

 of fresh living protoplasm has been formed out of the 



