no THE SCOTTISH NATURALIST 



it to establish itself. The inter-relationships of species in a 

 community are exceedingly intricate, and because we find 

 two kinds living side by side on the mainland we must not 

 assume that they will always succeed in living side by side, 

 even in places with identical environmental conditions. 

 Among other things, the composition of a community prob- 

 ably depends, to some extent, upon its method of origin and 

 its process of development, and, probably to a large extent, 

 the success of a species depends upon the time of its arrival 

 in relation to other species. It is quite easy to see how 

 this may be true. Every species makes certain demands 

 upon its environment, and the environment can maintain 

 a certain number of individuals. If two species make similar 

 demands, and if one has the maximum number of individuals 

 that the environment can maintain before the other puts in 

 an appearance, the chances of the latter getting in at all 

 are very poor ; on the other hand, if the former species is 

 not fully occupying the position, the latter may come in 

 quite easily and share. 



Previous occupation by some other species, probably 

 H. gyllenlialii, may quite well account for the case of 

 H. niorio^ and, if the earlier record of the occurrence of this 

 species at Tarbert is correct, we have evidence that at least 

 one individual of the species has succeeded in reaching 

 the island. The rarity of H. tristis is also possibly due to 

 the fact that some other species was there before it which 

 has prevented it from obtaining more than a precarious 

 foothold up to the present time, while RJi. bistriatus^ on the 

 other hand, probably arrived earlier, and has, therefore, 

 obtained for itself a much better position than it has in the 

 other islands where I have taken it. 



So far as water-beetles are concerned, therefore, I consider 

 that transportation across the sea is ample to account for 

 the faunas of these western islands. I do not propose to 

 argue the question for other groups on the very incomplete 

 material at my disposal. It is for the specialists in each 

 group to say whether they regard land-bridges as essential, 

 and, of course, if any group of animals has, on these islands, 

 species which are to be regarded as quite untransportable 



