• MARINE BIOLOGICAL STATION AT PORT ERIN. 59 



of Amphipoda so far recorded in the L. M. B. C. District 

 (about 5000 sq. miles) is 124 species in 78 genera, or 

 in the proportion of 159:100; while G. O. Sars dealing 

 with the Amphipoda of Norway — a very much more 

 extended area — gives 365 species in 157 genera, or in the 

 proportion of 232 : 100. 



To sum up, the proportions of species and genera, in 

 these Amphipoda are : — 



Ehos, &c, 1 to 10 fms., 115 sp. in 100 gen., or 1J : 1. 



L.M.B.C. dist., to 70 fms., 159 sp. in 100 gen., over lj:l. 



Norway, to 1215 fms., 232 sp. in 100 gen., or 2| : 1. 



So, it is clear that as one increases the area and depth 

 investigated the proportion of species to genera in the 

 fauna increases, until e.g., on the coasts of Norway it has 

 become more than twice what it is on the north coast of 

 Wales. 



Again, the total number of recorded species of L.M.B.C. 

 Tunicata is 46, and these are referred to 20 genera ; while 

 in the case given above (August 25th, 1894) the 12 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 series of statistics in regard to 

 other groups 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. 



We know of some individual cases, of course, of allied 

 species occurring together, but these do not necessarily 

 affect the general argument. Exceptional cases may be 

 due to some special habit which, although the species are 

 allied forms, prevents them from being severe competitors. 

 It is possible also that sessile animals, such as hydroids 

 and polyzoa, may form a partial exception, and may differ 



