Answer:
Vestigial organs
Explanation:
Vestigial organs/ structures are those parts of the body that have become redundant, weakened, or useless to the body as a result of prolonged inactiveness.
According to Charles Darwin's theory of evolution, he explains that parts of the body that are not used quite often do not develop any further, and as generations of offspring come and go, such parts become useless to the organism. This is the law of use/misuse of parts.
This is what happened to the whales. Over the years, the whales must have discovered that the legs were useless to them as they spent most of their time in the water. This led to the degeneration of such body part over thousands of generations, leaving them with just a faint trace of the hind limbs which were observed
