2 I 2 
Willis. —Age and Area. 
entered from the north-east. Let us, for the sake of prediction, suppose 
that this invasion centred at Duncansby Head, and draw a circle round that 
to include Dublin county. This circle crosses England and Wales from 
Merioneth to the Wash, and one will expect the Dublin flora to contain 
those species that reach to this circle in Britain. There may have been 
other invasions, and the directions may not have been exactly as here 
