"We can judge the heart of a man by his treatment of animals"
-- Immanuel Kant
"Until he extends the circle of his compassion to all living things,
man will not himself find peace."
-- Albert Einstein
Then, of course, there's the wonderful English hymn written by Cecil Alexander and used by James Herriot for his series of books about being a vet in Yorkshire in the 1930s:
All things bright and beautiful,
All creatures great and small,
All things wise and wonderful,
The Lord God made them all.