29 12 Proceedings of the Royal Trish Aademy. 



Coll, which is larger bub farther north, the same number of species occurred 

 while on Eigg, still farther north, and with very poor accommodation for 

 helophiles, there were only thirty-seven species. I doubt very much, 

 however, whether there is much relationship between the size of an island 

 and the number of species it possesses— which seems to me to depend 

 entirely, or almost so, upon the variety of habitats. I have spoken in a 

 general way of fresh-water-marsh, and peat-moss, but each of these names 

 really covers a number of different habitats which grade into one another in 

 such a way that only a very critical examination would reveal the differences. 

 For instance, the heating capacity of a pond depends upon its depth and the 

 nature of the water-supply and of the soil in which it lies. This will affect 

 the vegetation and the fauna and assuming that heating capacity alone were 

 what determined these — and I believe it is of some considerable importance — 

 we might find two or three apparently different kinds of ponds possessing the 

 same general fauna and flora. Then again Birge has shown in his " Plankton 

 Studies on Lake Mendota '" that of two species, one was abundant and the 

 other scarce according as to which got the start at the beginning of the 

 season and, from what is known of the oecological relations between species j 

 the result of such a struggle might quite well affect the whole fauna and flora 

 of the habitat, so that two ponds closely similar in all their physical characters 

 might contain very different species and yet be classed as the same habitat. 

 Further, the nature of the habitat is constantly and often rapidly changing 

 owing to the growth and decay of the fauna and flora. A small, shallow 

 pool with no surface vegetation heats up rapidly and cools rapidly while a 

 pool filled with vegetation has a much more equable temperature ; in a pool 

 partly covered with vegetation the temperature differs in the bare and pro- 

 tected parts as much as 8 or 10 degrees at the hottest time of a summer day. 

 Thus a pool which one year is a suitable habitat for species which endure 

 extremes of heat and cold, may next year be suitable for those which only 

 endure small variations. 



There seems, however, to be much less variation in the " peat-moss " pool 

 habitat than in the " fresh-water marsh " pool habitat and hence the similarity 

 between the faunas of oxylophytic areas such as Clare Island, Coll, Eigg, and 

 Skye. I would therefore account for the poverty of Clare Island fauna in 

 comparison with that of the adjoining mainland merely through lack of 

 variety of habitat. There seems to be geological evidence that the island 

 has become separated from the mainland only in comparatively modern times, 

 so that no question arises as to how the fauna may have reached it. 



Trans. Wisconsin Acad. Sui., ic, x. &c xi. 



