i9i7.: 95 



the other ; for example, the range of the Staphylinid beetle Quedws 

 othiniennis Joh. (falparum Dev.), whose habitat is apparently restricted 

 to moles' nests, cannot be wider, and may be narrower than the ran^e 

 of the animal with which it is associated, and thus the rarity of the one 

 species must of necessity imply the corresponding rarity of its 

 dependent. 



6.— Past GEoroGicAL History. 



Then again, the geological or climatic changes which have taken 

 place in past ages may have been in some cases only local, so that we 

 may find in scattered localities species which are either identical or 

 closely allied. If the separation has been of comparatively brief dura- 

 tion, probably the former condition of things will hold ; but if of 

 longer duration, probably the second — and this more especially if (i) 

 the habitats ai'e separated by barriers difficult of passage, such as wide 

 stretches of sea, deserts, mountains, or land between now unconnected 

 seas, e.g., the Black Sea and the Caspian ; (ii) the two habitats possess 

 even slightly differing conditions of physiography, geology, and 

 climate, with their correlated differences of fauna and flora ; (iii) the 

 genera are of a plastic nature, such as Salix, Rosa, and Rubiis among 

 plants, Helix among the Mollusca, and Longitarsus among the Coleo- 

 ptera. Hence we find that isolated mountains, islands, areas of inland 

 drainage, etc., frequently possess peculiar forms of extremely restricted 

 distribution, such as, for example, the numerous forms of Helix in 

 the Philippines, where almost every island possesses one or more 

 peculiar species. It is from considerations such as these too, that we 

 can explain the discontinuous distribution of Alpine and Arctic plants 

 and animals in Britain, though it would be well, within the limits of 

 this short paper, to avoid the still vexed question of the survival or 

 destruction of all terrestrial life, in northern Britain at least, during 

 the glacial period. 



7. — The Past Geographical History of the Species. 



In some cases of discontinuous distribution, however, the ob- 

 served phenomena cannot always i)e satisfactorily explained, either 

 on the theory of past geographical continuity of range, or on the 

 theory of divergent descent from such a geographically continuous 

 species. As pointed out in Section 1, a species or genus extending its 

 range makes its way outwards in various directions. It is quite con- 

 ceivable that a species might reach a given country by two or more 



