Chapter V. THEORETICAL CONSIDERATIONS. 

 1. FACTORS DETERMINING DISTRIBUTION. 



As a result of our labors, the distribution patterns of a large number of species 

 have been portrayed graphically. Even if these were offered merely as empirical 

 facts, without any attempt at an explanation, we feel that their publication would be 

 fully justified. But many of these distribution patterns are not purely empirical. On 

 the contrary they stand in evident relation to certain physical factors in the environ- 

 ment. The nature of these factors has been already discussed rather fully in chapter 

 II, and concrete examples of their influence upon distribution have been instanced 

 repeatedly in chapter iv. The factors which we believe to be most effective directly 

 and indirectly in determining the distribution of the bottom-dwelling species through- 

 out these waters are (i) the character of the bottom, considered chiefly in relation 

 to its physical texture; and (2) the temperature of the water. To these we may add 

 another factor of far less extended application, so far as concerns our dredging results. 

 This is (3) depth of water, or, perhaps, more strictly, proximity to shore, though this 

 statement demands considerable explanation. 



We are quite aware that several other factors are generally recognized" as being 

 of importance in determining the distribution of marine organisms ; and we do not 

 wish to be understood as limiting these agencies to the ones here enumerated. But we 

 are concerned at present only in explaining the phenomena encountered by us during 

 our dredging operations in the vicinity of Woods Hole. The factor of salinity is doubt- 

 less of the highest importance in determining the fauna of salt marshes and estuaries, 

 and even that of the open sea near the mouths of rivers.* But there are, within the 

 limits of our region, no streams of sufficient magnitude to seriously affect the fauna at 

 any considerable distance from shore. Among all the species taken in the course of our 

 dredging, we have encountered not more than two or three which seemed to be restricted 

 to the upper portions of Buzzards Bay, where alone the water was found to be diluted 

 in any considerable degree (cf. charts 215-218). The most striking case of this sort was 

 that of the sponge Tethya gravida, which, so far as we know, has only been taken near 

 the head of Buzzards Bay. 



The presence or absence of other organisms, which may serve as the food of a given 

 species or which may furnish it with a necessary basis for attachment, is surely to be 

 ranked as an important factor in influencing distribution. But these other organisms 

 are, in turn, dependent upon inorganic factors, such as those which we have mentioned, 

 and thus the latter may be regarded as ultimately responsible for the distribution in all 

 cases. 



o For admirable general discussions of this subject the reader is referred to C. G. J. Petersen (1893). Herdman et al. (1894), 

 Walther (1S94). and Allen (1899). 



6 Certain marine fishes have been held to be sensitive, directly or indirectly, to comparatively slight differences in the density 

 of their surrounding medium. Petterssen (1894) has shown that the appearance of herring upon the Norwegian coast is corre- 

 lated with periodic changes of water salinity; but since the latter changes are simultaneous with changes in the temperature 

 and in the food supply, it would seem difficult to exclude the influence of these latter factors. On the contrary, it is well known 

 (vide Sumner. 1906. p. 68) that many marine fishes are capable of Uving equally weU in waters of widely different degrees of 

 saUnity. 



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