That's a good idea Pooh. But there would be rivers to sail on too! there are ways to make it so any tech could be learned without what is shown as required prediscoveries, ie horses for horseback riding. It can be we chose to ride cows, eh? Therefore, there should just be MAJOR detriments, not complete cutoffs, for not having horses for horseback riding- for we would have ridden stuff, just not horses. And if you see another civ with the tech/horses/whatever, then that detriment is taken away. To go with my example of horsebackriding: Nation has no horses. So Horseback riding costs 500 beakers. THen See horses. lowers to 200. (or whatever the amounts would be).