186 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 
“‘seographical 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? 
(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.© 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). Pecten, 2 species (charts 125, 126). 
Tubularia, 2 species (charts 18, 19). Arca, 3 species (charts 131-133). 
Asterias, 2 species (charts 48, 49). Astarte, 2 species (charts 138, 139). 
Nephthys, 2 species (charts 57, 58). Busycon, 2 species ( charts 164, 165). 
Ampelisca, 2 species (charts 87, 88). Crepidula, 3 species (charts 183-185). 
Pagurus, 4 species (charts 109-112). Polynices, 3 species (charts 186-188). 
Cancer, 2 species (charts 115, 116). Amaroucium, 3 species 4 (charts 195-197). 
Anomia, 2 species (charts 123, 124). 
a In any case the relationship will be conceded as being very close. 
b 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. 
