284 HOMOGENESIS chap. 



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 Heterogenesis, 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 twenty 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 sub- 

 posed cases of heterogenesis is founded either upon errors 

 of observation or upon faulty inductions frorn correct 

 observations. 



It is obvious that the only way in which a case of hetero- 

 genesis could be proved would be by actually watching the 

 transformation, and this no heterogenist has ever done ; at 

 the most, certain supposed intermediate stages between the 

 extreme forms have been observed — say, between a Euglena 

 and a thread-worm — and the rest of the process inferred. 

 On the other hand, innumerable observations have been 

 made on these and other organisms, the result being that 



