THE NAUTILUS. 
There are no inherent tendencies in a species to depart from it- 
self, but when estrangements do oceur, they are effected by ulterior 
causes, natural or artificial. Where color markings are bleached 
out in mature shells in clear streams, they 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 still forming in it. This is con- 
firmed in Unio and Anodonta, and we may reasonably Eaeoe 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 
Unionidz 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 uniology but with extreme care. But just 
