Read the sentence below and select which is a singular pronoun and which is a plural pronoun from the table.
I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when I first kept pigeons and watched the several kinds, knowing well how true they bred, I felt fully as much difficulty in believing that they could ever have descended from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other large groups of birds, in nature.
Darwin, Charles Robert. The Origin of Species. New York: P.F. Collier & Son, 1909–14,