154 



where*. But, after making due allowance for these 

 powerful means of dissemination (which, in the common 

 order of things, must necessarily obtain in mountain 

 islands, as it were, par excellence), the fact still remains, 

 that in the Madeiran Group the acquired areas, even up 

 to the present date, of a vast proportion of the insect 

 inhabitants, are wonderfully circumscribed. The real 

 state of the case, however, would appear to be simply 

 this : that the floods, although they may have tended to 

 diffuse the members of a comparatively uniform alpine 

 fauna in the various clefts or gorges beneath, can have 

 had no power to combine the aborigines of the several 

 gorges themselves ; and, since a large proportion of the 

 endemic species of those islands are (as I have previously 

 stated) apterous, the perpendicular edges of the ravines, 

 which in many instances rise to an elevation of 2000 

 feet, have acted (and ever will act) as impassable barriers 

 to vast numbers of the insect tribes. 



With this single example (by way of illustration), 

 which the Madeiras have supplied, I will take my leave 

 of the question of natural barriers, as tending to regulate 

 the topographical diffusion of the Annulosa, feeling that 

 I have already devoted too much time and space to this 

 portion of the subject (if such indeed it be) which I had 

 proposed in the present treatise to discuss. Other 

 barriers might have been adverted to, such as large 

 rivers, extensive deserts, and thickly set forests (espe- 

 cially of pine-trees, which frequently offer a very decided 

 * Insecta Maderensia, p. 81. 



