Answer:
I believe its an alphabet
D. Backbones.
All of the animals listed are vertebrae, a classification of animals that means they have backbones. Fish do not have legs, lungs, or hair, and birds lack hair as well.
Generally speaking, it was the "slum dwellers" and the "immigrants" who did not share in the prosperity of the late 1800s in the United States, since they were the lower class workers.
