Contrary to what many believe, evolution is not a "theory" in crisis. The evidence for evolution is ubiquitous. Evolution is the branch of biology that binds our knowledge of life together.
For example, Mammals (animals that are warm blooded, possess hair and mammary glands and normally give birth to live young) evolved from aquatic creatures that took to land approximately 200 million years ago. Whales of course are mammals as well, so this brings forth an interesting question. That question being, "How and why did whales end up back living in the water if the ancestors of mammals evolved on land?
In 1978 a skull was found in Pakistan that looked like it belonged to an early wolf like creature. But upon closer examination by a Paleontologist from the University of Michigan, it was discovered that this skull contained the "ear" of a whale! This discovery provided the world with concrete evidence that supported one of Darwin's most controversial theories, that being that whales descended from land dwelling mammals!
Whales are a great example of evolution on our planet! They are so different anatomically from any other mammal that they are regarded as a separate branch of mammalian evolution. The skull found in Pakistan in 1978 represents concrete evidence of the beginning of that "branch". That particular skull was very important because it was discovered among land mammal fossils, not in sea rock! The whale skull and thus the creature was named "Pakicetus" or "whale from pakistan". This creature was the land mammal whose descendants bacame what we know of today as whales!
Years would pass and eventually scientists went back on the search, but this time for the creatures legs. They did not find them, but instead found skeletons in Egypt of another whale ancestor. This find was Basilosaurus, and was a fully aquatic whale ancestor (as opposed to Pakicetus who was a land creature). A few days later came an even greater discovery that was concrete evidence of evolution. That discovery was that Basilosauras had "legs" !
What would a fully aquatic creature need with legs?
Basilosaurus' legs were "left overs" from the distant past of Pakicetus ! This was extremely fascinating and revolutionary because 10 million years seperated Pakicetus and Basilosaurus, yet, Basilosaurus still had visible vestigial features of legs and feet
Scientists have since discovered a series of transitional fossils (fossils of creatures that are intermediates between two types)! Including a 55 million year old land dwelling "walking whale" called Ambulocetus. Ambulocetus could walk on land but yet also swim. Another find was Rhodhocetus, a mostly aquatic mammal that could also walk on land a bit. NOTE It is very unlikely and uncommon for all of the transitional creatures to be found simply because only a small percentage of creatures ever become fossils!
Other Types of Evidence
Anatomy: Evolution is not only backed up by fossil evidence. Evolution is also "backed up" by anatomical evidence that determines relationships. For example, a human arm and hand contain the same analogous structures seen in a whales fin! This is called an anatomical homology, and is proof of evolutionary relationships between us and whales (human and whales are both mammals and share a common ancestor!)
Molecular Evidence: Molecular evidence contributes just as much (and possibly more than anatomy and fossils) to our understanding of evolution. DNA base sequences and amino acid sequences can be compared from different creatures in order to determine how closely related they are! Creatures that are not closely related will have more differences in their DNA and amino acid sequences. The more closely related 2 creatures or species are, the fewer DNA differences they will have!
Current DNA studies show that the whale is most closely related to the Hippopotamus than any other mammal! Showing us that whales and Hippos possess a recent common ancestor!