288 BIOGENESIS CHAP. 



into the wound and again attack the red corpuscles, and 

 thus spread the infection. Thus the mosquito is not a 

 mere germ-carrier, like the common house-fly. 



The study of the foregoing living things and especially 

 of Bacteria, the smallest and probably the simplest of all 

 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 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 circumstances 

 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 sometimes called spontaneous or 

 equivocal generation, according to which fully-formed 

 living organisms sometimes arise from not-living matter. 



In former times the occurrence of abiogenesis was 

 universally 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 



