vra BIOGENESIS AND ABIOGENESIS 239 



It is of great importance to apprehend Redi's 

 position rightly ; for the lines of thought he laid 

 down for us are those upon which naturalists have 

 been working ever since. Clearly, he held Bio- 

 genesis as against Abio genesis ; and I shall imme- 

 diately proceed, in the first place, to inquire how 

 far subsequent investigation has borne him out in 

 so doing. 



But Redi also thought that there were two 

 modes of Biogenesis. By the one method, which 

 is that of common and ordinary occurrence, the 

 living parent gives rise to offspring which passes 

 through the same cycle of changes as itself like 

 gives rise to like; and this has been termed 

 Homogenesis. By the other mode, the living 

 parent was supposed to give rise to offspring 

 which passed through a totally different series of 

 states from those exhibited by the parent, and 

 did not return into the cycle of the parent ; this 

 is what ought to be called Heterogenesis, the off- 

 spring being altogether, and permanently, unlike 

 the parent. The term Heterogenesis, however, 

 has unfortunately been used in a different sense, 

 and M. Milne-Edwards has therefore substituted 

 for it Xenogencsis, which means the generation of 

 something foreign. After discussing Redi's hypo- 

 thesis of universal Biogenesis, then, I shall go 



qucrce dalle punture delle mosche, in quella giusa stessa che 

 dalle punture d'ahri animaletti simiglievoli veggiamo crescera 

 do' tumori ne' corpi degli auimoli." 



