Answer:
No need for that part in the body.
Step-by-step explanation:
The loss of various parts by the organisms occurs due to natural selection because there is no need for that organ for the organism which allows them to remove and survive without that organ. Whales have lost their hind limbs because there is no need for it in swimming so it becomes useless and with the passage of million of years these legs are disappeared from whale's body so we can conclude that natural selection is the main cause for these losses in organisms.