On Darwin's voyage, he observed ostriches and rheas living on grasslands on separate continents. These two types of birds are similar, but not identical. How did Darwin apply these observations?

A. He tested the birds to see if each would survive in the other's environment.
B. He developed hypotheses to explain the distribution and differences of the birds.
C. He concluded that one of the birds must have evolved from the other bird.
D. He developed hypotheses to explain how each bird produced more of its own kind.