i8 THE SCOTTISH NATURALIST 



Coll/ in both cases being associated with //. lituratus. Now 

 I examined many such pools in the north in my search for 

 OctJiebius lejolisii, but whereas the habitat at Tarbert and in 

 Coll was in a very sheltered part, I was working the exposed 

 Atlantic face of the island in the north, and this may well 

 account for the absence of H. planus. These species might 

 have been expected to occur in the pools in the sea-turf behind 

 the Melbost sands, near Stornoway, as I have found them 

 in sea-turf pools in several places, but these pools and similar 

 ones behind the Luskentyre sands in South Harris were 

 very barren. Gyriniis opaais only occurred high up in the 

 mountains in Harris, so that its absence from the northern 

 district is quite easily explained. 



There are, however, one or two species whose localisation 

 is difficult to account for. Thus, H. ruficollis occurred, not in 

 abundance, but in 9-5 per cent, of the northern collections, 

 and usually it occurred in ballast-holes. Why should this 

 species fail to occur in similar holes in the south? Again, 

 A. chalconoUis occurred in 27 per cent, of the northern 

 collections, and always in pools, and yet in the south where 

 pools were more abundant the species was entirely absent. 

 Possibly in both cases the absence from the south is in some 

 way associated with the physiography of the land, and 

 narrow valleys between mountains may be unsuitable, but 

 such instances only indicate what factors there may be in 

 determining the presence or absence of a species in an 

 apparently easily accessible district. 



In any collecting of natural objects there is obviously 

 a very large personal element concerned in getting results. 

 There are not only good and bad collectors, but there are 

 many different types of collectors, and two men working 

 the same ground ma}-, and probably will, each discover 

 some species not found by the other ; since it is obviously 

 impossible in any extensive area to examine every piece of 

 ground suitable for collecting, the results depend upon what 

 ground each collector misses just as much as they do upon 

 the capacity of each to discover different species in the same 



' "The Aquatic Coleoptera of the Mid-Ebudes," Aim. Scot. Nat. 

 Hist.., April 1910, pp. 79 and 84. 



