What is the meaning of this line from As You Like It: "but as all is mortal in nature, so is all nature in love mortal in folly?"Said by Touchstone...

This is the ending line from Touchstone's speech in which
he makes fun of lovers and the things they say.  Here, Touchstone is saying that all
people who are in love act in stupid ways.  He is also implying, though, that the
foolishness does not last.

First, Touchstone says that all
in nature is mortal, meaning that everything that is natural will die sometime.  He then
says that the nature of love is to be foolish.  This is what he has been trying to get
across by all his silliness about kissing cows' udders and such.  Finally, he is saying
that the folly of love is mortal just like everything in the natural
world.

Overall, then, he is saying something along the
lines of "Everything in nature dies sometimes and so does the foolishness of people in
love."