I am quite conscious that my speculations run quite beyond the bounds of true science. … I inferred that genera & Families with very few species (i. e. from Extinction) would be apt (not necessarily always) to have narrow ranges & disjoined ranges. You will not perceive, perhaps, what I am driving at & it is not worth enlarging on, — but I look at Extinction as common cause of small genera & disjoined ranges & therefore they ought, if they behaved properly & as nature does not lie to go together!

Letter to Asa Gray, 18 June 1857