who was adam and what is reffered to as the penaltyof adam

The Duke says "Are not these woods more free from peril than the envious court? Here feel we but the penalty of Adam, the seasons' difference." The seasons' difference is the change in weather from season to season.

The penalty of Adam is a direct reference to the fall of man in the Bible, and his expulsion from Eden. Normally Adam's penalty would be thought of as death, but here the thought seems to be more that in Eden there was no change from season to season, but once expelled, Adam had to endure the change of seasons. Thus the penalty of Adam is Change, or if you like, Time.