288 BIOGENESIS CHAP. 



known organisms (p. 257), naturally leads us to the con- 

 sideration of one of the most important problems of Biology 

 the problem of the origin of life. 



In all the higher organisms we know r that each individual 

 arises in some way or other from a pre-existing individual : 

 no one doubts that every bird now living arose by a process 

 of development from an egg formed in the body of a 

 parent-bird, and that every tree now growing took its origin 

 either from a seed or from a bud produced by a parent- 

 plant. But there have always until quite 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 origin of the lower 

 organisms, the theory of Biogenesis, according to which each 

 living thing, however simple, arises by a natural process of 

 budding, 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. 



