iv HETEROGEXESIS 291 



might not be easy to demonstrate, what all competent 

 naturalists must be firmly convinced of, that every one 

 of these supposed cases of heterogenesis is founded 

 either upon errors of observation or upon faulty 

 inductions from correct observations. 



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

 heterogenesis could oe proved would be by actually 

 watching the transformation, and this no heterogenist 

 has ever done ; at the most, certain supposed inter- 

 mediate 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 each 

 species investigated has been found to go through a 

 definite series of changes in the course of its develop- 

 ment, the ultimate result being invariably an organism 

 resembling in all essential respects that which formed 

 the starting point of the observations : Euglenae always 

 giving rise to Euglenae and nothing else, Bacteria to 

 Bacteria and nothing else, and so on. 



There are many cases which imperfect knowledge 

 might class under heterogenesis, such as the origin of 

 frogs from tadpoles or of jelly-fishes from polypes (see 

 Chapter V), but in these and many other cases the 

 apparently anomalous transformations have been found 

 to be part of the normal and invariable cycle of changes 

 undergone by the organism in the course of its develop- 

 ment : the frog always gives rise ultimately to a frog, 

 the jelly-fish to a jelly-fish. If a frog at one time 

 produced a tadpole, at another a trout, at another a 

 worm : if jelly-fishes (p. 314) gave rise sometimes to 

 polypes, sometimes to infusorians, sometimes to cuttle- 

 fishes (p. 411), and all without any regular sequence 

 that would be heterogenesis. 



U 2 



