Some surnames become more concentrated over time, multiplying in certain districts. Others remain few in number, quietly plodding their way down through the centuries. Here are a couple of Cornish examples, one from the west and the other from the east.
The name Berryman or Berriman is particularly associated with West Penwith. In 1861, 64 per cent of the Berrymans in Cornwall lived there. Yet in earlier times the name Beryman or Beriman, always spelt with just one <r>, was more scattered, found as far east as Fowey.
Nevertheless, a concentration of Berrymans in the far west had occurred by the mid-1700s as the following map, based on the former district council areas, shows.
Many miles to the east, Pengelly is a surname that has become associated with Looe. Yet, with at least 13 examples of the placename Pengelly or Pengilly in Cornwall, it was much more evenly spread in earlier times. Its expansion at Looe does not seem to have taken place until the nineteenth century, a period when fishing ports experienced little in-migration.
The causes for the ramification and growth of some surnames but not others must lie in the detailed demographic and economic history of the local area. Each surname has its own subtly differing story to tell.
For more examples see The Surnames of Cornwall.
Cornish studies resources 
There is a town of Pengilly on Minnesota’s Iron Range. It is said to be named for a Doctor Pengilly who got off the train at that location to care for the local population. My great-grand uncle the Rev. William Downing served the Methodist church there. He came to the U.S. from Cornwall in 1911.
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