THE BEGINNINGS OF II FE. 607 



may regard them as more or less recent products of 

 Archebiosis and Heterogenesis. 



We do not pretend to fix the limits of ephem^ro-' 

 morphism and to say, on all sides, where true 

 species begin — though we may expect that much 

 light will be thrown upon this subject by subsequent 

 workers. It is only natural, however, to suppose 

 that the transitions from heterogenesis and meta- 

 morphism to homogenesis and specific fixity should 

 be more or less gradual ; so that many forms will 

 have to remain, as it were, upon the border-land. 

 Ascending development amongst the ephemeromorphs 

 tends to lead to the production of more complex and 

 less variable forms. But even long after the fii-st rudi- 

 ments of a sexual generation have been arrived at, and 

 the origin of ^ species ' has commenced, we might ex- 

 pect that the comparatively unspecialized matter of 

 which such organisms are composed, and the com- 

 parative weakness of the internal conservative prin- 

 ciple (essentially based, as it is, upon heredity and 

 complexity of organization), would permit of their under- 

 going marked variations from time to time, either under 

 the influence of changed external conditions or by virtue 

 of ^ spontaneously ' initiated internal changes '. 



^ Mr. Danvin's view is directly opposed ^o this ; for instance, after 

 speaking of fresh-water productions, and the fact that many of them are 

 low in the scale of nature, he adds (loc. cit., p. 467) :- ' We have reason 

 to believe that such low beings change or become modified less quickly (ban 

 the high.' And to show that this is no mere casual expression of 

 opinion, we find Mr. Dai-win saying in the last page but one of his 



