THE CONCEPTION OF SPECIES 



959 



round-whorled species, is the characteristic form, while the highest 

 heds are characterized by P. hoerncsi (Fig. k), an angular-whorled, 

 strongly bicarinate type, which had been separated under the dis- 

 tinct generic name of Tulotoma. Certainly these end-forms are 

 widely separated, yet from the intermediate beds individuals con- 

 stituting a complete gradational series from one to the other have 

 been obtained. This discovery led many to reconsider the classifica- 

 tion of these forms and to group them all as varieties under one 

 species. 



The belief in the mutability of species was gradually accepted 



Fig. 252. Series of Paludinas (Vivipara) from the Lower Pliocenic deposits 

 of Slavonia. (After Nenmayr.) a. Palndina ncumayri, k. P. 

 {Tulotoma) hocniesi from the highest i)eds. h-i. intermediate 

 forms, showing gradation, from the intermediate beds. 



by naturalists after the publication of "The Origin of Species" in 

 1859. To-day there is scarcely a naturalist of prominence who does 

 not unhesitatingly afifirm his belief in the mutability of species. 

 N'^evertheless, we may ask, with Farlow : "... is our belief 

 )n evolution merely dogmatic, like some of the theological doctrines 

 wdiich we believe thoroughly but which we do not allow to inter- 

 fere with our daily life, or, . . . has our belief modified the 

 manner in which we treat what we call species?" (10) When we 

 note how unwilling systematists are to-day to recognize more than 

 one species in a series whose end forms dififer widely, when a suf- 

 fxient number of members are known to bridge over all the more 

 striking gaps, we are forcibly impressed with the fact that, uncon- 

 sciously though it may be, the majority of systematists are still 



