xlvii 



but that each area being kept well stocked "with allied species 

 already adjusted to its minute conditions, such migration rarely 

 occurs. Thus a limit is put to the spread of species by species 

 themselves, which produces similar results on the actual distribu- 

 tion of forms throughout the world, to those produced by mighty 

 physical barriers such as the Andes. 



There is yet one other consideration remaining. If these bar- 

 riers are not required to explain the limitation of faunas, it does not 

 follow that they do not act as barriers all the same ; but it is, I think, 

 difficult to prove it. If 1200 miles of sea do not form a sufficient 

 barrier against the stocking of the Azores with insects from "Western 

 Europe, I do not think sixty miles of mountain should be assumed 

 to prevent for tens of thousands of years the transport of species, 

 in the egg state, by birds or currents of air, from one side to 

 the other. I may add, in conclusion, that if the efficacy of barriers 

 of this nature has been overrated, some important conclusions 

 regarding changes on the earth's surface will have to be re- 

 considered ; such, for instance, as that of the extension of a glacial 

 epoch over nearly the whole earth — a hypothesis conceived by 

 Darwin to explain the existence of the same genera and sometimes 

 the same species in high latitudes, both in the northern and southern 

 hemisphere, whilst absent from the intervening zones. I believe that, 

 with some very obvious exceptions, such as Mammals and Batra- 

 chians, there can be no limit placed to the dissemination of a 

 species, provided there are unoccupied areas suitable to it, in any 

 part of the earth, and provided also time sufficient be allowed for the 

 process. 



The discussion of points connected with the labours of British 

 Entomologists has extended to so great a length that but little space 

 is left me for noticing the progress of Entomology abroad. I am not 

 aware, however, of the appearance of many new works of striking 

 importance on the continent or elsewhere during the yeav. The 

 ' Genera des Coleoi^teres,' by Professor Lacordaire, has progressed 

 to the extent of another volume since our last Anniversary, and the 

 author's marvellous power of making a difficult subject compara- 

 tively easy shows no sign of decay. The great ' Catalogus Cole- 

 opterorum,' by Gemminger and Harold, is also advancing rapidly, 

 the sixth volume having appeared within the last few days. The 

 students of special groups in the Order find much to criticise and 



