Internal fertilization takes place inside the body of the female. It is necessary for animals which live on land (terrestrial animals) and is retained by aquatic animals descended form terrestrial ancestors eg whales and dolphins.External fertilization takes place outside the body of the female. It usually takes place in water, usually the environment in which the animal lives eg fish and amphibians. This is because eggs and sperm would dry out and die if they are not kept wet. Internal fertilization is more certain than external fertilization so fewer eggs need to be produced. After internal fertilization eggs can be protected and fed while developing inside the mother's body. After external fertilization the eggs are potentially exposed to predators and so must external fertilization the eggs are potentially exposed to predators and so must either be produced in very large numbers or protected by the parents or both.
