290 IIOMOGENESIS CHAP. 



conditions known to us are concerned. It is also 

 necessary to add that the presence of microbes in consider- 

 able 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 observation that, in both 

 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 hetero- 

 genesis was once seriously believed to occur is that of the 

 " barnacle-geese." Buds of a particular tree growing 

 near the sea were said to produce barnacles, and these 

 falling into the water to develop into geese. This sounds 

 absurd enough, but within the last thirty 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 might 

 not be easy to demonstrate, what all competent naturalists 

 must be firmly convinced of, that every one of these sup- 

 posed cases of heterogenesis is founded either upon errors 

 of observation or upon faulty inductions from correct 

 observations. 



