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a 



ON THE MARINE ZOOLOGY OF THE IRISH SEA. 463 



A point which comes out in making complete lists, such as the above, 

 of the contents of the net on one haul is the relatively large number of 

 genera represented by the species.' In the haul, quoted above, from the 

 expedition of June 23 the 112 species were referred to 103 genera ; in 

 the haul from the Fisheries steamer on July 23 tlie 39 species obtained 

 belong to 34 genera ; and in the haul on August 3 there were 78 species 

 and 67 genera. Taking a few instances of particular groups — on August 25, 

 1894, the 12 species of Tunicata taken in one haul represented 10 

 genera ; and Mr. Walker reports the following numbers of species and 

 genera in hauls of the higher Crustacea : — March, 1893, off Rhos, shallow, 

 19 species in 18 genera ; May 1893, off E,hos, 2 fathoms, 24 species in 21 

 genera : July 1893, off Little Orme, 5 to 10 fathoms, 31 species in 28 

 genera ; October 1893, off Little Orme, 4 to 10 fathoms, 41 species in 36 

 genera ; September 1894, off Little Orme, shallow, 39 species in 3.5 genera ; 

 and April 1895, off Port Erin, 34 fathoms, 40 species in 35 genera. 



These figures are particularly interesting in their bearing on the 

 Darwinian principle that an animal's most potent enemies are its own 

 close allies. Is it theii 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 lamellibranchs, amphipods, or annelids they will 

 probably belong to as many genera, and if these genera contain other 

 British species these Avill 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 par- 

 ticular hauls. That has been done to some extent with the ' Fauna ' of 

 Liverpool Bay, and the following instances may be taken as samples. 

 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 the 

 collections quoted from Mr. Walker above 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 (August 25, 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 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. We 

 know of some cases, however, of allied species occurring together, but these 

 do not necessarily affect the general argument. It is possible also that 

 sessile animals, such as hydroids and polyzoa, may form a partial excep- 

 tion, and may differ from wandering forms in their method of competition. 

 We are accumulating further statistics on these points. 



THE SUBMARINE DEPOSITS. 



In last year's report the nature of the deposits forming on the floor 

 of the Irish Sea was discussed in a preliminary manner. During this 

 season's work the bottom brought up on each occasion has been carefully 

 noted and a sample kept for future study in the Jermyn Street Museum. 

 One point which this collection of deposits from comparatively shallow 



' Dr. Murray, in the ' Challenger ' Summary, notes this fact in the case of deep- 

 sea hauls, but does not seem to recognise its application to shallower waters. 



