As I recall, Florida and Texas are JUST north of the tropic region officially. But I agree there should be a variation. I seem to recall reading somewhere that there have been some studies done, and there are variations about as one would expect currently (however details escape me)

I'm more interested in how Wiley & Co. explain the variation if we supposedly evolved where it wouldn't have been as much of a factor. My science classes were a long time back, but I seem to recall there isn't supposed to be a significant variation in the day / night length in the tropics. There may have been a rainy season or some such that had similar effects, but presumably that would have had a different trigger.

Guess I'll need to get the book to find out

Gooserider