A frog’s internal organs. Brehms Tierleben. Vol. 4. 1912,
The grasshopper and the frog. Nature and Science Readers: Outdoor Visits - Book Two. 1935.
The head of a frog morphs into the head of a person. Colored etchings by Christian von Mechel after Lavater, 1797.
“The appendages of the vertebrates are homologous, as shown by comparing the skeletons of these three. It would be difficult to explain these similarities on any other basis than organic evolution.”
Zoology. 1952. Internet Archive.