Ill HETEROGENESIS 285 



each species investigated has been found to go through a 

 definite series of changes in the course of its development, 

 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 Euglenas 

 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 IV.), 

 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 development : 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 gave rise sometimes to 

 polypes, sometimes to infusoria, sometimes to cuttle-fishes, 

 and all without any regular sequence — that would be 

 heterogenesis. 



It is perhaps hardly necessary to caution the reader against 

 the error that there is any connection between the theory of 

 heterogenesis and that of organic evolution. It might be 

 said — if, as naturalists tell us, dogs are descended from 

 wolves and jackals, and birds from reptiles, why should not, 

 for instance, thread-worms spring from Euglenas or Infusoria 

 from Bacteria ? To this it is sufficient to answer that the 

 evolution of one form from another takes place by a series 

 of slow, orderly, progressive changes going on through a 

 long series of generations (p. 222); whereas heterogenesis 

 presupposes the casual occurrence of sudden transformations 

 in any direction — i.e., leading to either a less or a more highly 

 organised form — and in the course of a single generation. 



