104 BIOGENESIS AND llOMOCiENESIS i.kss. 



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 

 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 : Euglenje 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 (Lesson XXII. Fig. 

 53), 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 Euglense or Infusoria 

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

 evolution of one form from another takes place by a series 



