1 86 BULLETIN OF THE BUREAU OF FISHERIES. 



Jordan, who is one of the foremost recent advocates of the theory of evolution by 

 isolation, tells us (1905) that "it is extremely rare to find two subspecies inhabiting or 

 breeding in exactly the same region." Again: "Given any species in any region, the 

 nearest related species is not likely to be found in the same region nor in a remote region, 

 but in a neighboring district separated from the first by a barrier of some sort" (p. 547). 

 This, he says, "may be raised to the dignity of a general law of distribution." 



Few groups of marine animals have been worked as intensively as have the birds 

 and fresh-water fishes upon which Jordan chiefly relies for evidence in favor of his 

 theory. It is largely due to this fact, probably, that "subspecies," "varieties," and 

 "geographical races" play a relatively minor part in the taxonomy of marine animals. 

 We thus have practically no data at our present disposal to test the first of Jordan's 

 assertions quoted above. A case which perhaps deserves mention at this point, though 

 its relevance may well be questioned, is that of the mollusk Polynices heros, and its 

 supposed variety, triseriata. We must make the reservation at once that these are 

 regarded by some conchologists, e. g., Dall, as distinct species, and in fact we have our- 

 selves followed Dall in so listing them in our catalogue. A glance at the charts 6 

 (187,188) reveals the fact that while the two forms coexist throughout much of 

 their range, they nevertheless do not present the same distribution patterns, but appear 

 to show distinct preferences as to habitat. There is, however, no real geographical 

 isolation, for the two forms occur on closely adjacent parts of the sea floor, being 

 taken together, not infrequently, in a single dredge haul. c Whether or not these two 

 species (or varieties ?) cross freely, and with what results, we have no means of know- 

 ing at present. 



It is impossible, likewise, for us to state whether or not the species nearest related 

 to any given one among our local fauna occurs in this region, or in a "neighboring 

 district. ' ' Such a question could be answered only after an exhaustive research into 

 the fauna of neighboring parts of our coast. We have a considerable collection of 

 data, however, with which to answer the kindred questions: (1) To what extent do 

 members of the same genus tend to differ in habitat? and (2) Are different members 

 of the same genus less likely to be associated together than species not so closely 

 related ? 



As bearing upon the first of these questions, the comparative distributions of dif- 

 ferent species of the same genus have been presented by us in a large number of cases. 

 The reader is especially referred to the following examples : 



Eudendrium, 2 species (charts 16, 17). 

 Tubularia, 2 species (charts 18, 19). 

 Asterias, 2 species (charts 48, 49). 

 Nephthys, 2 species (charts 57, 58). 

 Ampelisca, 2 species (charts 87, 88). 

 Pagurus, 4 species (charts 109-112). 

 Cancer, 2 species (charts 115, 116). 

 Anomia, 2 species (charts 123, 124). 



a In any case the relationship will be conceded as being very close. 



6 Only the occurrence of living specimens (designated by circles) can be taken into account here, since the dead shells are 

 probably transported considerable distances by hermit crabs. 



« As regards geographical range, that of Polynices heros is stated by Dall as extending from Labrador to Virginia; that of 

 triseriata being practically identical, i. e., from Labrador to Cape Hatteras. 



d In our catalogue we have followed Dr. Van Name in not regarding one of these as a distinct species. 



Pecten, 2 species (charts 125, 126). 

 Area, 3 species (charts 131-133). 

 Astarte, 2 species (charts 138, 139). 

 Busycon, 2 species ( charts 164, 165). 

 Crepidula, 3 species (charts 183-185). 

 Polynices, 3 species (charts 186-188). 

 Amaroucium, 3 species'* (charts 195-197). 



