152 The Scottish Naturalist. 



struck by the fact that none of the mountain Lepidoptera had 

 been caught in this manner. Of course it is quite, and perhaps 

 extremely, probable that this habit of avoiding wind-currents 

 may have been acquired since these insects colonised their pres- 

 ent mountain homes, and that they were in former times as in- 

 cautious as the lowland species. Moreover, even supposing that 

 they were cautious, a single female of each of the species may 

 have been carried by the wind and founded a colony. 



On the whole, however, I think the probabilities are in favour 

 of the passage by land, or rather by land only slightly interrupted 

 here and there by very narrow water-barriers ; and if we can 

 judge from the present distribution of the species, we may hazard 

 a guess at the sequence in which they came. I say the " present 

 distribution," for we cannot tell of course what changes may have 

 taken place — such as the extinction of species in localities once 

 frequented by them — during the many thousand years that have 

 elapsed since their first arrival. 



But, first of all, what kind of a country did the dry sea-bed make, 

 and what were its physical features ? It might be thought that 

 some idea of these might be obtained by studying the Admiralty 

 charts of the various depths at present existing round our shores 

 and in the German Ocean ; but geologists think that many of 

 the great banks which occur in the southern reaches of the Ger- 

 man Ocean probably consist, in large measure, of glacial deposits, 

 and their presence tends to obscure the physical features that 

 obtained in pre-glacial and inter-glacial times. Were one to 

 judge from the present depths, the first dry land lay between 

 Holland and North Lincoln or South Yorkshire, and the Rhine, 

 Thames, and other rivers ran west through the English Channel ; 

 the Elbe and other rivers north of the Rhine running, on the 

 other hand, northwards. But geologists incline to believe that 

 the Rhine flowed northwards, and was joined by the Elbe, 

 Thames, and other rivers. However that may be, it is pretty 

 certain that great plains, intersected by rivers of varying breadth, 

 occupied the present bed of the German Ocean, and that all 

 parts of that sea of less depth than one hundred fathoms were 

 then dry land. The hundred-fathom line (beyond which the 

 depths increase suddenly) reaches from Denmark to the north 

 of the Zetland Islands, but does not touch Norway, which is cut 

 off by a deep channel. From beyond the Zetlands it passes 

 outside the Hebrides, Ireland, and the south-west of England, 

 and gradually approaches the coast of France in the extreme 



