It would be if there was a road connecting Central America (Panama) and South America (Colombia), but there is no road, there is a small no mans land of bandits, narco traffickers, rebels, military and indigenous hostile locals that will shoot first and not ask questions after.
No fear, you can circumvent the Darien Gap, with two options:
1- Time to kill, go across to Colombia via a sailboat, which is arranged in PC and takes a 4 days to cross to Turbo Colombia via San Blas Islands, choosing a good boat, a safe captain is half the trouble, the other is about $400. Allow 7-10 days when all said and done.
2- Fly from Panama City (PTY) to capitols or major cities in South America O/W, from $200 normally.
Rainy season in CA is at its peak from Sept to Nov, Costa Rica/Panama best avoided in Oct.
Most backpackers allow 3-4 months for CA region not including Mexico. For 8 countries, thats 2 weeks per country, depending on your interest, budget and route.
VIP top notch bus services connect all the capitols. [http://www.ticabus.com/ingles.html]