BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 185 



Of the entire 202 species, 101, or exactly 50 per cent, are believed to have a range 

 upon our coast which is predominantly southward; 46 species (23 per cent) have a 

 range which is predominantly northward; while 31 of them (15 percent) have a range 

 of approximately equal extent, so far as known, in both directions. The remaining 24 

 species have been relegated to the doubtful column. The fact to be emphasized is that 

 the ratio of southward-ranging species (as thus defined) to northward-ranging species is 

 greater than two to one, while about 15 per cent of them do not seem to be thus 

 restricted in latitude." 



Viewing these 202 species in another way, it is to be noted that 130, or about 64 per 

 cent of them, are known to have a range extending north of Cape Cod, leaving 72 of 

 them (36 per cent) which, so far as reported, have not transcended this barrier. Doubt- 

 less more complete information will reduce the latter figure. As has already been 

 pointed out, any locality where extensive collecting has been done is sure to figure as the 

 reputed limit of distribution, whether northern or southern, for many species. It is 

 significant, therefore, that only 37 of the species under consideration (18 per cent) have 

 not yet been recorded from points south of Woods Hole. 6 Comparing this figure with 

 the 36 per cent which are not known to occur north of Cape Cod, it may be that we 

 have some measure of the real effectiveness of the last as a barrier to distribution. 



Crude, in the extreme, as any such computations must be, the conclusions seem to be 

 fairly well grounded (1) that Cape Cod does have an appreciable influence as a barrier to 

 distribution, and (2) that the southern types preponderate considerably over the north- 

 ern ones in our Woods Hole fauna, or at least in that part of it which is accessible to the 

 dredge. These generalizations may not be true of each individual group (e. g., coel- 

 enterates and amphipods) ; and in general it must be remembered that a considerable 

 minority of northern forms are included in our local fauna, while about 64 per cent of 

 our species are known to occur north of Cape Cod. On the other hand, it is well to 

 state that our local fish fauna, which is but sparingly represented in our dredging records, 

 and consequently plays little part in the foregoing tabulation, is overwhelmingly south- 

 ern, 75 per cent being southward-ranging in the foregoing sense of the term, while nearly 

 50 per cent of the total number of recorded species are such as are reputed to find in 

 Cape Cod their northern limit of distribution. And, lastly, we must bear in mind that we 

 are here dealing only with the benthos of the region, the plankton, as well as the littoral 

 fauna, being left out of consideration. 



6. COMPARATIVE DISTRIBUTIONS OF CLOSELY RELATED SPECIES. 



Turning to another phase of our subject, it would be unreasonable to look to the 

 results of such a survey as the present one for any considerable light upon the origin of 

 species. Those who insist upon the importance of isolation as a factor in species differ- 

 entiation are wont to maintain that different subspecies do not coincide in their ranges, 

 but that these supposedly incipient species are practically always separated from one 

 another, geographically or otherwise. After the complete splitting of a species, i. e., its 

 replacement by several specifically distinct forms, these latter may by migration, it is 

 said, come to occupy the same territory. 



° The similar treatment by Hoyle of the deep-water fauna of the Clyde sea-area was unknown to the present writers at the 

 time when the foregoing discussion was written. (See Hoyle, 1890, p. 463 et seq.). 



'< More strictly, south of Vineyard Sound and Buzzards Bay. Block Island and Long Island Sound have not been regarded as 

 farther south. 



