ISOLATION 135 



proportion of endemic species is very high, and often what was 

 probably a single immigrant species is at the present day 

 represented by a large genus (cf. Perkins, 191 2). 



The effects of isolation in these extreme cases appear 

 sufficiently striking, but there is a danger of overestimating 

 the part that geographical isolation has played in the evolution 

 observed. The enormous area of continuous tropical forest 

 covering the larger part of northern South America is probably 

 proportionately quite as rich in endemics. The distribution 

 of the fauna of South America is still very imperfectly known, 

 but it appears likely that an enormous number of species have 

 developed under the relatively constant rain-forest conditions 

 without the intervention of any very definite barriers. Some 

 species would appear to occur over the whole area, while others 

 are apparently definitelylocalised ; but much more information 

 is needed on this point. Again, in the Hawaiian Islands with 

 their singularly stable and relatively uniform environment 

 (especially before the arrival of Man), numerous allied species 

 have often been evolved on one island. Further, while islands 

 as a whole are characterised by the endemism of their fauna, 

 there are a good many exceptions. We may instance the 

 following : 



Crustacea. Lowndes (1930) records that, in a collection 

 of Copepods from the New Hebrides, practically none 

 of the species are endemic. Many are identical with 

 British species, though in this group dispersal powers 

 would not be expected to be very effective. The 

 Ostracods, on the other hand, are nearly all endemic, 

 though special dispersal mechanisms (resting eggs, etc.) 

 are developed. 



Spiders. No peculiar forms occur on the Scilly Isles, Lundy 

 Island or Channel Islands (Bristowe, 1929a, 1929^, 

 1929c). On the whole, dispersal power (by gossamer) is 

 good, but the incidence of this power throughout the 

 order requires investigation (cf. Bristowe, 1929c). 



Hydracarina. Lundblad (1930, p. 24) records only one 

 endemic variety on the Faroes. 



Myriapoda. No endemics * on the Faroes (Hammer and 

 Henriksen, 1930). 



1 i.e. definite subspecies. 



