the significance of the migrations during the *high* middle ages in europe in terms of numbers of migrants and long-term effects overlooked by most. from The Making of Europe: Conquest, Colonization and Cultural Change 950-1350 (books.google.com/books?id=9R0fm…): 
the general pattern was expansion outta nw "core" europe regions to more peripheral areas. (also northern spain southwards.)
there's some thinking that the Great Divergence (en.wikipedia.org/wiki/Great_Div…) began in the north sea region as early as 1000 (see @Scholars_Stage's blogpost here: scholars-stage.blogspot.com/2013/11/anothe…). one consequence seems to have been a pop increase resulting in a subsequent heavy emigration...
...d*mn flemings got everywhere! =P (disclaimer: very likely i have some flemish ancestry.) @edwest
just going to pause here for a second to remind everybody of the four forces (mechanisms of change) of evolution: mutation, MIGRATION (gene flow), genetic drift, and natural selection. evolution.berkeley.edu/evolibrary/art…
