For a day in the life := this day to a far distant future
. perform some meaningful calculation on the date
So it became fashionable to use the underscore in place of a space:
For a_day_in_the_life := this_day to a_far_distant_future
So the underscore improved the readability somewhat by making it easier to see where one program element began and the other left off. Underscores are still somewhat popular today, but they also have a problem. It is difficult to type them since it requires a shift key. Also, for a time, there were actually keyboards that didn't include the underscore, which limited its popularity somewhat.
Enter "camel case" (or CamelCase) as the most recent solution to the problem of creating readable multiword identifiers. Our example becomes :
For aDayInTheLife := thisDay to aFarDistantFuture
Surprisingly, it doesn't seem to have lost any readability once you get used to it. The brain is a pretty adaptable thing when it comes to reading. After all, ancient Latin was often written without spaces between the words, and in all upper case. It works for literature too, although it probably won't catch on.