Answer:
Living birds lack teeth.
Most mammals lack shelled eggs.
Whales and snakes lack legs.
Step-by-step explanation:
Living birds lack teeth, Most mammals lack shelled eggs and Whales and snakes lack legs are the characteristics that are present in modern animals as compared to their ancestors. These changes occur in the organism due to evolution that occurs with the passage of time and the main reason for that is change occurs in the environmental conditions for organism which force them to make changes which is best for the environment.