What characters do whales posses that make them mammals rather than fish? Why are whales thought to have evolved from land dwelling animals? What is the evidence?

What characters do whales posses that make them mammals rather than fish? Why are whales thought to have evolved from land dwelling animals? What is the evidence?.

What characters do whales posses that make them mammals rather than fish? Why are whales thought to have evolved from land dwelling animals? What is the evidence?