As one of the last Whigs, and long after his academic career, the 1926 date of publication is misleading. The thinking is more than a century old, reflecting both the conscious biases he was proud of, and the unconscious biases of his place and time.
The English of England have always been singular for caring little about their cousins and ignoring their distant relatives: the very different practice of the Scot is partly due to the fact that he carries more Celtic blood in his veins.HBDchick's theory is that the Catholic Church forbidding cousin marriage - and NW Europeans going along with it more than other peoples - became the basis for some genetic selection for those who could venture out from clannishness. Trevelyan would push this characteristic even farther back than she does. He points to the preference Anglo-Saxons and other nearby tribes had for swearing loyalty to a leader, even if from an other tribe, e.g. Beowulf the Geat.
And from The Revenge of Geography, by Robert D. Kaplan.
...elite molders of public opinion dash across oceans and continents in hours, something which allows them to talk glibly about what the distinguished New York Times columnist Thomas L. Friedman has labeled a flat world. Instead, I will introduce readers to a group of decidedly unfashionable thinkers, who push up hard against the idea that geography no longer matters.