common to North America and Europe. 483 



mal ; one with a columellar fold, a thickened outer Hp, armed 

 with teeth, and a wide umbilical depression ; the other with- 

 out a thickened lip, teeth, fold, or depression. In converting 

 these dissimilar shells into Ancylus, they must produce but 

 one species, notwithstanding the extent of their disagreement, 

 because the latter genus never has a thickened lip nor teeth, 

 and cannot possibly have the remaining characters which 

 Helix presents. The same reasoning has a bearing upon 

 higher divisions, and prevents us from setting aside a genus 

 like Helicina, because there cannot be a corresponding genus 

 allied to Patella; and it is fatal to the quinary, or any other 

 numerical arrangement, because, in proportion to the simplici- 

 ty of the form, and the reduction of the organs, must the 

 number of groups and aggregates diminish, of whatever value 

 or denomination they may be.* It appears needless to assert 

 that the absence of characters upon which generic and spe- 

 cific distinction would depend, renders two forms identical ; 

 as, in natural history, we cannot admit ''a distinction without 

 a difference." 



I am aware that species from distant localities, long con- 

 founded, have eventually been found to present distinctive 

 characters ; but, whilst this ought to teach us caution, it 

 should not lead us to pronounce all such objects distinct, until 

 similar diagnostic characters be detected, or we adopt a rule 

 in the one case which we reject in the other ; namely, that 

 objects are distinct which appear identical, under the operation 

 of a peculiar law% only when remote localities are concerned, 

 which exerts no influence upon animals of the same region. 

 Thus, if Cicada septendecim were to occur in Australia, it 

 would be considered a distinct species, whilst the seventeen 

 broods of our own coimtry constitute but one ; although it 

 admits of a doubt whether all are the descendants of a sini^le 

 pair. The same remark will probably apply to all the species 

 of this genus, the period required to bring them to maturity 

 being unknown. Moreover, the very fact of a species living 



* This idea is borne out by the fact that an ornithologist is the prbposer of the 

 quinarj-, and an cnlomologist of the septenary arrangement. 



