180 TRANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



and others an equal but limited homogeneity, which seems 

 to imply a recent arrival. 



I think we must admit from their generally southern 

 range that they are all post-glacial, and it might also be a 

 plausible conjecture that the Iberian element, that is, the 

 Irish or Irish and West Anglian species were earlier than 

 the South Anglian. It seems also probable that while it 

 is certain Ireland must have been joined with an extreme 

 western extension of Europe during the advent of the 

 former, that country may have been quite insulated while 

 the advent of the latter took place across what is now the 

 eastern part of the English Channel. Moreover, there 

 seem to be grounds for assuming that at a time when 

 Cornwall, Kerry, France, and Portugal were all united, 

 some obstacle, such as an immense lake, existed where 

 the Irish Sea and St. George's Channel are now. This 

 would explain the phenomenon of bifurcation in the lines 

 of distribution of such a species as Getonia aurata, which 

 does not extend much beyond the midlands in England, 

 while it goes right up to Donegal in Ireland, and also 

 occurs in Man; or of Pyropterus affinis, which, as I 

 have mentioned before, occurs in Kerry in Ireland and 

 Nottinghamshire in England. 



Further than such problematical conjectures, or, at 

 best, working hypotheses, I doubt if we are justified in 

 going for the subject is a very difficult one, and in 

 my own opinion hardly at present ripe for discussion. 

 "We have after all not a very complete basis of facts to go 

 upon, there may have been many streams, and many 

 periods of migration from the same region, and I doubt 

 whether it is possible, perhaps ever will be possible, to 

 quite disentangle these threads, or to prove the exact 

 limit as regards species of northern, eastern, or southern 

 derivation. 



