MARINE BIOLOGICAL STATION AT PORT ERIN. 57 



in the two hauls of October 27th there were 93 species in 

 81 genera and 111 species in 93 genera. Taking a few 

 instances of particular groups — on August 25th, 1894, the 

 15 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 Khos, shallow, 19 species in 18 genera ; 

 May, 1893, off Khos, two 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 35 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 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 lamellibranchs, amphipods, or annelids 

 they will probably belong to as many genera, and if these 

 genera contain other British species these will probably 

 occur in some other locality, perhaps on a different bottom, 

 or at a greater 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. That has been 



* These numbers refer to the Higher Crustacea only. There were many 

 other animals in the hauls. 



t " As the species of the same genus usually have, though by no means 

 invariably, much similarity in habits and constitution, and always in struc- 

 ture, the struggle will generally be more severe between them, if they come 

 into competition with each other, than between the species of distinct 

 genera." Darioiu, The Origin of Species, sixth edition, p. 59 ; see also 

 Wallace, Darwinism, second edition, p. 33. 



