All morphologists arrive at the firm conviction that all vertebrata, from the Amphioxus upwards to man himself, all fishes, amphibia, reptiles, birds, and mammals, descend originally from a single vertebrate ancestor, for we cannot imagine that all the different and highly complicated conditions of life which, through a long series of processes or stages of development, led to the typical formation of a vertebrate, have accidentally happened together more than once in the course of the earth's history.
"The Old Riddle and the Newest Answer"
Radiated animals ascend towards the mollusks, the mollusks towards the articulata, the articulata towards the vertebrata.
"Orthodoxy: Its Truths And Errors"
Among the superior animals however, and especially among the vertebrata, pain exists.
"The Pros and Cons of Vivisection"