130 EVOLUTION AND DOGMA. 



that the various species observed are, at least in 

 this case, originated by derivation with modifica- 

 tions. 1 



The case just adduced is frequently appealed to 

 by evolutionists, not only because ft has been exhaus- 

 tively studied, but also because it tells so strongly in 

 favor of the theory of derivation. 



An equally striking instance, perhaps, is found 

 in the case of another group of mollusca belong- 

 ing to the paludina. At first, the six or eight 

 known gradational forms of this mollusc were reck- 

 oned as entirely distinct species. Subsequently, 

 however, numerous connecting forms were discov- 

 ered, so that now over two hundred varieties are 

 counted. But so gradual are the transitions of 

 one form into another, that shells which other- 

 wise would be considered as belonging to dif- 

 ferent genera are, by reason of the known con- 

 necting links, regarded as constituting but one and 

 the same species. 2 



Similar gradations have been shown by Cope to 

 exist among certain extinct mammalian forms, not- 

 ably among the species of the generalized family, 

 oreontita, but it is unnecessary to give further illus- 

 trations of this character, as those just instanced are 

 quite sufficient to exhibit the nature and force of 

 the argument which is based on the existence of 

 such gradational forms. 



1 Cf. A. Hyatt's "Anniversary Memoir of the Boston Society 

 of Natural History, 1880, on Genesis of Tertiary Species of 

 Planorbis at Steinheim." 



J Cf. Romanes' " Darwin after Darwin," vol. I, p. 19. 



