1 88 BULLETIN OF THE BUREAU OF FISHERIES. 



correlated, with marked physiological differences sufficient to adapt the two to differing 

 habitats. Thus the assertion so often made that the slight structural differences by 

 which we distinguish one species from another are commonly of no conceivable utility, 

 and therefore can never have arisen through the action of natural selection, loses much 

 of its force. While it may be true that these slight structural differences in themselves 

 can play no significant role in the life of the organisms concerned, it is likewise evident 

 that there are certain correlative physiological changes sufficient to adapt the organisms 

 to somewhat different modes of existence. That natural selection has been the con- 

 trolling factor in the origination and perpetuation of such specific differences, whether 

 morphological or physiological, is far from certain. But that the characters con- 

 cerned are in most cases too insignificant to be of selective value is also far from certain. 



Where we have to do merely with the adoption of a more restricted habitat by 

 one species than by another, it is quite possible that the physiological difference in 

 question relates merely to general constitutional vigor; i. e., the less hardy species 

 may restrict itself to the more favorable portion of the habitat. Where, however, the 

 ranges of the two species are more or less complementary to one another, particularly 

 if they do not coincide throughout any portion of their extent, such an explanation is 

 of course out of question, and we are obliged to fall back upon the assumption that each 

 is more or less specifically adapted to its respective habitat. 



In order to throw light upon the second of the above questions (i. e., Are members 

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

 closely related?), we have adopted a method employed by Herdman (1895). This author, 

 after noting the relatively large number of genera represented by the species taken 

 in a single dredge haul, 6 writes: "These figures are particularly interesting in their 

 bearing on the Darwinian principle that an aminal's most potent enemies are its own 

 close allies. Is it then the case, as the above cited instances suggest, that the species 

 of a genus rarely live together; that if in a haul you get half a dozen species of lamel- 

 libranchs, amphipods, or annelids they will probably belong to as rnany genera, and 

 if these genera contain other British species these will probably occur in some other 

 locality, perhaps on a different bottom, or at another depth? It is obviously necessary 

 to count the total number of genera and species of the groups in the local fauna, as 

 known, and compare these with the numbers obtained in particular hauls." In Liver- 

 pool Bay, for example, "the known number of species of higher Crustacea is 90, and 

 these fall into 60 genera. So the genera are to the species as 2 to 3," whereas in certain 

 dredging collections cited "the genera are to the species on the average about as 28 to 

 31, or nearly 7 to 8. Again, the total number of species of Tunicata is 46, and these 

 are referred to 20 genera ; while in the case given above * * * the 1 2 species taken 

 on one spot represented 10 genera, or a little over a quarter of the species represented 

 half the genera. These, and many other cases which we might quote, seem to show 

 that a disproportionately large number of genera is represented by the assemblage of 

 species at one spot, which means that closely related species are, as a rule, not found 

 together" (p. 463). 



a This suggestion has been made to us by Prof. Herdman. 



l> This fact was pointed out by Sir John Murray (Challenger "Summary," p. 1435). who, however, restricts its application 

 to great depths, concluding "in the deepest zone, therefore, the species stand to the genera in the ratio of 5 to 4, and in the 

 shallowest zone nearly as 3 to i.'.i 



