Im not sure but i need the answer as well.
I believe the answer is: To control trade along the Mississippi River.
By controlling the Mississippi river, Thomas Jefferson believe that they could eliminate british influence in products distribution to different states. He counted that the amount of products transported from this river could accounted to around 500 million tons of shipped goods per year.
Answer: In 1881, in the general american election.
<em>On the Origin of Species</em> was a book written by Charles Darwin regarding the observations he made on his trip to the Galapagos Islands.
Hope that helped =)