Which statement best describes the purpose of this excerpt

The statement that best describes the purpose of the excerpt is the third one: to explain how balance came to exist in the natural world.

Explanation:

In this excerpt, the speaker is explaining how natural balance works, how nature works things out in order to create balance between things. In this case, the speaker talks about a pair of twins -one right handed and the other left handed- one creates the plant eating animals, and the other creates the meat eating animals. This keeps the world balanced, not every animal eats meat, not every animal eats plants, so there will always be plants and meat because not every animal eats the same and this makes the world function well.  

the counterarguments for Roosevelt's position regarding America's duties is found in the paragraph outlined below

Other good people seek to deter us from threading the path of the hard but lofty duty by bidding us remember that all nation that has achieved greatness that has expanded and played their part as world power has in the end passed away.

Explanation:

This paragraph points out the fact about previous world powers in times past and the inevitable truth of their eventual crumble and the constant shift in world power throughout history.