136 FAUNA HAWAIIENSIS 



appreciate slight visible differences, how are these differences preserved from loss ? It 

 seems difficult to imagine that the numerous white and black species of the jiicunda 

 group of Scoparia, which are no doubt well concealed by their colouring amongst the 

 lichens, derive any advantage in this respect from the slight characters of marking in 

 which they differ from one another ; and if there were not some influence tending to 

 the maintenance of these characters, variation in that particular would be unrestricted 

 and they would disappear. In an ordinary fauna this difficulty does not usually present 

 itself 



(r) Tendency to variability. 



An unusual proportion of species show strong variability, which in perhaps 1 5 or 

 more species becomes excessive. It is impossible to give precise figures on this point, 

 that would be of any real value, as many of the species only exist in one or two speci- 

 mens. The variation of such insects as Hypenodes altivolans and Eticymatoge nionti- 

 colans is quite phenomenal, and the proportion of highly variable forms is certainly 

 much larger than in an ordinary fauna. In New Zealand, however, highly variable 

 species of the same character are found in perhaps much the same proportion. Such 

 species seem to resemble nebulae, which time, or a modification of circumstances, will 

 break up and condense into groups of stars. I conceive that species in this state of 

 potential multiplication are more plentiful in thinly stocked regions, such as the 

 Hawaiian Islands and New Zealand, because the number of unoccupied stations for 

 species acts as an apparent incentive by removing a real check. In other words, 

 species are being formed relatively much more frequently in such regions, and therefore 

 these afford the most favourable opportunity for studying the process. In confirmation 

 of this view, it may be noted that the 57 species of Scoparia are remarkably constant, 

 not one being even moderately variable, on the available evidence ; in this genus 

 specific development has already proceeded so far that suitable stations (assuming 

 restriction to a lichen-feeding habit) are probably almost all occupied. 



{d) Tendency to dull colouring. 



It might perhaps be expected that a group of tropical islands, enjoying abundant 

 sunshine and possessed of a luxuriant vegetation, would be remarkable for the bright 

 colouring of its Lepidoptera, but the case is quite the reverse. Considered as a whole, 

 the general effect is exceptionally sombre. This is in some part attributable to the 

 accidental constitution of the limited fauna, of which the principal genera are such as 

 are usually inconspicuously coloured in all regions ; thus Agrotis habitually assumes the 

 obscure tints of the dead leaves and rubbish amongst which the species conceal them- 



