Darwin Noted the Separation of Different Marine Species by the Isthmus

Multiple Choice

Darwin noted the separation of different marine species by the Isthmus of Panama and concluded that this separation of species meant that

A) marine animals were not good at crossing dry land.
B) there was no good way to explain this oddity.
C) an external agent scattered species across the planet.
D) the species on each side came into being where they lived.