LESSON IX 



BIOGENESIS AND HOMOGENESIS : HOMOGENESIS AND HETERO- 



GENESIS 



THE study of the foregoing living things and especially of 

 Bacteria, the smallest and probably the simplest of all known 

 organisms, naturally leads us to the consideration 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 circum- 

 stances originate independently of pre-existing organisms : 

 that, for instance, in a flask of hay-infusion or mutton-broth, 

 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 



