IX HETEROGENESIS 105 



of slow, orderly, progressive changes going on through a 

 long series of generations (see Lesson XIII.); whereas 

 heterogenesis presupposes the casual occurrence of sudden 

 transformations in any direction — i.e., leading to either a less 

 or a more highly organized form — and in the course of a 

 single generation.^ 



^ Apart from such continuous variations, others, which may be 

 described as discontinuous, do sometimes appear with apparent 

 suddenness, but not to the extent which would lie required by the 

 theory of heterogenesis. — IV. IV. P. 



