Anculosae of TtiE Aeabama River Drainage 5 



particle (Hannibal, 1912). As Mr. Smith saw it, and as I see it n^yself now, 

 the undesirable conclusion is forced on one that there are far more exist- 

 ing species than have been described, that it may be a very long time before 

 the last one has been found and the books closed. 



The reason lies in the manner of life of the Pleuroceridae for one thing 

 and for another in the apparent fact that the family is in the active fer- 

 ment of evolution. The greater number of the species, in other genera as 

 well as the Anculosae, inhabit rocks and gravel bars in swift-moving 

 streams. The migrating impulse is absent. Observation leads to the con- 

 viction that in the case of a species of the Anculosa, as already mentioned, 

 every moment of living may be spent upon a single spot of a single stone. 

 Not only do the ordinarily recognized barriers restrict the spread of the 

 animals, but the deep water of a river turns back creek forms, the deep 

 water between bars in the same stream interrupts dispersal, in instances 

 quite narrow rifts on a single group of shoals serve as effectual barriers. 



The influences of isolation working from without thus exercise their 

 greatest powers. Working from within the forces of evolution carry on 

 differentiation still farther. 



Speaking of one group of this family, Dr. Lewis (1873)- made the des- 

 pairing remark : "One cannot tell where to assign limits. Limits are appar- 

 ently obliterated and species have no existence. They are a confused mass 

 and must be referred to one type." I believe it is true that species in this 

 family, except occasionally, do not exist as Dr. Lewis and his contempor- 

 aries wished to define the word species. One is lost who tries to think of 

 these animals as having any such fixity of characters as occur in other 

 families and orders. We have rather to think of the characters as overlap- 

 ping from one race to another, even from genus to genus. Thar collection 

 of individuals in the Pleuroceridae may be called a species whose predomin- 

 ant characters are not the predominant characters of another collection of 

 individuals. If we see only a few specimens of a single species its own 

 peculiar characters may often seem to be submerged by characters linking 

 it with another species. But in a long series the individual characters stand 

 out, and we are compelled then to recognize the existence of definable dif- 

 ferences and to proceed to describe them and provide the label of a name. 

 If we adopt the policy^ — ^the tempting course — of referring all these many 

 collections to one or several types we surrender whatever value there is in 

 the defining of local races and lose with it the means of tracing geographical 

 distribution. Dr. Lewis' "confused mass" would become more confused 

 than ever. All the tribes of American Indians — to go far afield for an 

 analogy — are alike in certain regards, tribal characters overlap tribal char- 

 acters, yet it is possible to differentiate tribe from tribe, and the right and 

 necessity of the scientist and historian to speak of these collections of in- 

 dividuals as separate, distinct, diff'erentiated, are not to be questioned. 



With the method of evolution in this family, the writer is incompetent 

 to deal. There has been so far no intensive study of the anatomy, no broad 

 inquiry into the rules or rhythm of variation if any such things exist, no 

 breeding and interbreeding to discover whether known rules of heredity 

 apply here. It is a field still fallow for the experimenter. 



