Consider the following statements regarding the Heartland Theory proposed by Halford J. Mackinder:

1. The Heartland refers to the central part of the Eurasian continent.
2. 
   
3. The theory emphasized the strategic importance of sea power over land power.
4. 
   
5. Control over the Heartland could potentially lead to dominance over the “World Island.”
6. 
   
7. The Heartland was considered difficult to invade due to natural barriers and continental position.
8. 
   

Which of the statements given above are correct?