290 HOMOGENESIS CHAP. 



entrance of atmospheric germs, no micro-organisms 

 ever make their appearance. So that the last argument 

 for abiogenesis has been proved to be fallacious, and the 

 doctrine of biogenesis shown, as conclusively as observa- 

 tion and experiment can show it, to be of universal 

 application as far as existing conditions known to us are 

 concerned. It is also necessary to add that the presence 

 of microbes in considerable quantities in our atmosphere 

 has been proved experimentally. 



There is another question intimately connected with 

 that of biogenesis, although strictly speaking quite 

 independent of it. It is a matter of common observa- 

 tion that, both in animals and plants, like produces like : 

 that a cutting from a willow will never give rise to an 

 oak, nor a snake emerge from a hen's egg. In other 

 words, ordinary observation teaches the general truth 

 of the doctrine of homogenesis. 



But there has always been a residuum of belief in the 

 opposite doctrine of heter agenesis, according to which the 

 offspring of a given animal or plant may be something 

 utterly different from itself, a plant giving rise to an 

 animal or vice versa, a lowly to a highly organised plant 

 or animal and so on. Perhaps the most extreme case 

 in which heterogenesis was once seriously believed to 

 occur is that of the " barnacle-geese." Buds of a parti- 

 cular tree growing near the sea were said to produce 

 barnacles (p. 386), and these falling into the water to 

 develop into gsese. This sounds absurd enough, but 

 within the last forty years two or three men of science 

 have described, as the result of repeated observations, 

 the occurrence of quite similar cases among microscopic 

 organisms. For instance, the blood-corpuscles of the 

 silkworm have been said to give rise to fungi, Euglense 

 to thread- worms, and so on. 



It is proverbially difficult to prove a negative, and it 



