IX HETEROGENESIS 103 



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, the 

 protoplasm of the green weed Nitella (see Fig. 44) to 

 Amoebse and Infusoria (see p. 107), Euglenae 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. 



Let us take a particular case by way of example. Many 

 years ago Dr. Dallinger observed among a number of Vorti- 

 cellae or bell-animalcules (Fig. 25) one which appeared to 

 have become encysted upon its stalk. After watching it for 

 some time, there was seen to emerge from the cyst a free- 

 swimming ciliated Infusor called Amphileptus, not unlike a 

 long-necked Paramoecium (Fig. 20, p. 108). Many ob- 

 servers would have put this down as a clear case of hetero- 

 genesis : Dallinger simply recorded the observation and 

 waited. Two years later the occurrence was explained : he 

 found the same two species in a pond, and watched an 

 Amphileptus seize and devour a Vorticella, and, after finish- 

 ing its meal, become encysted upon the stalk of its victim. 



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 



