dot instead of a line. Compounds often start out as separate words, gradually growing closer through hyphenation, and finally merging into a single unit. (Although there is no evidence that doughnut was ever actually two discrete words.) The shortening (no pun intended) of doughnut to donut occurred at some point as well, at least in American English (the Brits still prefer to make the "dough" plain). A related example of how words and their meanings can change over time is the saying "I'll bet [or give] you dollars to doughnuts..." The idea was that dollars were worth so much more than doughnuts back then that this would perforce be a very good wager. Of course, given the rate of inflation, a doughnut (or donut) now often costs more than a dollar, making the expression nonsensical, or in any case, backwards. The phrase first arose around 1870, but was originally given as "dollars to buttons" or "dollars to cobwebs." It wasn't until the turn of the century that the alliterative appeal of "dollars to doughnuts" proved stronger. We found two cases of Inflaton* (for inflation*, etc.) in OhioLINK today, and 370 in WorldCat.
(Photo of store advertising donuts and brains, found on the Web.)