xv.J SPONTANEOUS GENERATION. 353 



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 lias "been termed Ilomogcnesis. 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 he 

 called Heterogenesis, the offspring being altogether and 

 permanently unlike the parent. The term Het oroge- 

 nesis, however, has unfortunately been used in a differ- 

 ent sense, and M. Milne-Edwards has therefore sub- 

 stituted for it Xenogenesis, which means the generation 

 of something foreign. After discussing liecli's hypothesis 

 of universal Biogenesis, then, I shall go on to ask how 

 far the growth of science justifies his other hypothesis 

 of Xenogenesis. 



The progress of the hypothesis of Biogenesis was 

 triumphant and unchecked for nearly a century. The 

 application of the microscope to anatomy in the hands 

 of Grew, Leeuwenhoek, Swammerdam, Lyonet, Vallis- 

 nieri, Reaumur, and other illustrious investigators of 

 nature of that day, displayed such a complexity of 

 organisation in the lowest and minutest forms, and 

 everywhere revealed such a prodigality of provision 

 for their multiplication by germs of one sort or another, 

 that the hypothesis of Abiogenesis began to appear 

 not only untrue, but absurd ; and, in the middle of 

 the eighteenth century, when Needham and Buffon 

 took up the question, it was almost universally dis- 

 credited. 1 



1 ISTeedham, writing in 1750, says : — 



" Les naturalistes modernes s'accordcnt unanimcnicnt a 6taLlir, comme 

 nne v6rite certaine, que toute plante vient dc sa semence specifiqne, tout 

 animal d'un ccuf ou dc quelque chose d'analogue preexistant dans la plante. 



