xlvi FAUNA HAWAIIENSIS 



that all these may have been variable on their arrival in the islands, or are the 

 descendants of variable immigrant species, but the variability in this class (i.e. immi- 

 grant species or comparatively recent developments from immigrants) is so very 

 general, that this seems improbable, comparing them with the species accidentally 

 introduced by man. 



The endemic fauna of the islands consists for the most part of genera containing 

 an inordinate number of species, evidendy allied to one another, e.g. Scotoiytkra and 

 Hyposmochoma (Lepidoptera), Proterhinus (Coleoptera), Nesoprosopis (Hymenoptera), 

 Anomalochrysa (Neuroptera), Paratrigonidium (Orthoptera), Drosophila (Diptera), 

 Achatinella (Mollusca), and many others. If the genera are weak in species they are 

 generally, if isolated in the fauna, highly peculiar in structure, or they are evidently 

 merely offshoots from some of the larger genera. In the Passerine birds the genera 

 are numerous, but they are mostly weak in species. The large genera of insects and 

 shells often consist of many variable species, though in these large genera some of the 

 species are generally stable. More rarely there are genera, containing many species, 

 few or none of which are conspicuously variable, at any rate not sufficiently so, as to 

 cause any serious difficulty in defining the species. Probably these genera of stable 

 species have reached the furthest point of development possible under the existing 

 conditions in the islands. If we examine the series of allied forms of the Passerine 

 birds, as they exist to-day, we, see that a large number of them have reached such a 

 stage of specialization for a certain mode of life, that we can hardly conceive that they 

 could under any circumstances give rise to new forms. These could at most arise from 

 some of the more generalized. Possibly the same is the case in the large genera of 

 insects, of which the species are in a stable condition, though we are unable to appre- 

 ciate the relationship of structure to habit as easily as we can in the birds. 



Distribution of animals by natural agencies. 



All the islands being volcanic and having been built up from a great depth of 

 ocean at various periods, their entire fauna naturally originated from immigrants derived 

 from other lands. These immigrants must have arrived either by flight, like the birds, 

 or in drift like the flightless insects and probably the land Mollusca. Some of the 

 flightless insects, however, that we now find in the islands, have no doubt lost their 

 powers of flight after their arrival, or more properly speaking are the descendants of 

 well-winged ancestors, and in this respect are similar to the little flightless rail iyPcnnula 

 ecaudala) now extinct, but others, for instance some of the genera of weevils were, 

 equally certainly, flightless when they arrived. These can only possibly have reached 

 the islands in drift. That insects can pass over enormous distances in floating logs of 



