1232 BULLETIN OF THE BUREAU OF FISHERIES. 



months. It is likely that all the waters of the region approach very nearly to 

 the freezing point of salt water for a longer or shorter period nearly every winter. 

 It may be that the rule which has been formulated by Verrill" and Allen for 

 birds, and by Merriam'' for terrestial animals and plants in general, applies 

 here — namely, that the limits of distribution are determined by the temperature 

 at the breeding season only. If this be true it would follow that adult animals 

 and plants might survive temperatures in which propagation could not occur. 



It is certain, however, that an actual destruction of adult organisms may 

 occur as the result of a too high or too low temperature. The case of the com- 

 mon sea urchin {Arbacia punctulata), which, as our records show, was almost 

 exterminated in Vineyard Sound during the particularly cold winter of 1903-4'" 

 is a good illustration of this point; and it is a matter of common observation 

 among fishermen that great numbers of dead fishes of certain species are fre- 

 quently found during the thaw following a particularly hard spell of cold weather. '^ 

 Conversely, certain members of our local fauna (e. g. the noncolonial hydroid 

 Tubularia couthouyi) are known to be able to grow and maintain an active exis- 

 tence only at a low temperature. It is manifestly impossible, therefore, to make 

 any single, all inclusive statement as to mode of operation of temperature in 

 restricting the distribution of species in the Woods Hole region. 



As regards the depth factor, we can find little evidence of actual bathy met- 

 ric distribution in the waters under consideration. It is true that certain species, 

 according to our dredging records, seem to be restricted to the sublittoral zone 

 (e. g. Crepidula convexa, fig. 23), but these are probably also littoral in their 

 habitat, and it is possible that proximity to shore rather than depth proper 

 may be the determining factor in such cases. 



Salinity, likewise, though undoubtedly a potent factor in determining the dis- 

 tribution of species in or near the mouths of streams, seems to play a negligible 

 part in the explanation of our dredging records. The only point in the region 

 covered where the dilution of the sea water is at all considerable is near the 

 head of Buzzards Bay (fig. 4) ; but we have recorded hardly a single species 

 which was dredged here exclusively or even predominantly. 



One question which will naturally present itself to the student of geographical 

 distribution is this : What is the position of the Woods Hole fauna in the fauna 

 of our American coast? To which of the larger zoogeographical regions does 

 it belong? And is it situated in the middle of that region or close to one of its 



a American Journal of Science, March, 1866, p. 249. 



6 North American Fauna, no. 3, p. 26; Proceedings of the Biological Society of Washington, April, 



1892. P- 45- 



c The mean water temperature at the Woods Hole station for January and February, 1904, was 

 29.3° F., that for the same months during the other four years of the period igoj to 1906 being 32.3°. 



<* Gould cites a case of the wholesale destruction of oysters by " ground frost" (see Boston Journal 

 of Science, 1840, p. 492). 



