THE NAUTILUS. / 



There are no inherent tendencies in a species to depart from it- 

 self, but when estrangements do occur, they are effected by ulterior 

 causes, natural or artificial. Where color markings are bleached 

 out in mature shells in clear streams, the} 1 are retained in the same 

 species in muddy waters. Shells thin and fragile in cold, limeless, 

 pure water, become thicker and coarser in dirty streams. Spinose 

 and verrucose shells are found in rapid waters, with a maximum de- 

 velopment of spines and warts, while the same species in sluggish 

 waters have these characters minimized or even absent. The thin 

 edentate Anodontas of ponds or lakes, need no teeth to keep their 

 valves in place, and hence have none. Nature's argument for an 

 organ or an accessory is the need of it, which is furnished by a pro- 

 cess of slow development the heavier species showing rudimentary 

 teeth. 



Mr. Darwin, in " The Origin of Species," shows that in a genus 

 having many species, if it has not reached a maximum de- 

 velopment, many other species are *till fm-mhiy in it. This is con- 

 firmed in Unio and Auodonta, and we may reasonably expect new 

 species will be discovered in them. 



Departures of a Unio from its parent stock, when seen as features 

 of whole colonies, entitle it to specific distinction without hesitation, 

 provided the habitats differ, and to varietal distinction where found 

 in company with or near its next in affinity. On such a basis spe- 

 cific recognition is accorded in other branches of zoology, and also 

 in botany. Plenty of land and marine shells, are specifically sep- 

 arated, only by the most minute or microscopic differences of the 

 shells. We would not advocate such peering minuteness in the 

 UuionidiJe where the tendency to variation is much greater, and 

 where expert comparative anatomists are unable to find distinguish- 

 ing generic or specific differences in the soft parts. 



In the Unionidae, the constants of nature are few and the differ- 

 entials many. How then shall a rule be formulated by which we 

 can confidently say a given Unio is distinct from another? The 

 hiatus necessary for the founding of a new species must be such an 

 aggregation of differences of character, such an estrangement from 

 its next in affinity, that the gap will be large enough to justify a 

 specific separation of the two. A substantial agreement in the out- 

 line of two Unios may be a fact, and yet other distinctive characters 

 easily and unmistakably separate them. A process of differentia- 

 tion must be applied in uuiology but with extreme care. But just 



