When there are no more animals of a particular species left alive, that species is said to be extinct.
In the distant past, several species of animals became extinct through natural causes. There might have been a change in the climate of a particular region, and the plants or animals on which the species lived could have died out because of the change in the weather. Or one animal might have been destroyed by another that was stronger or faster.
But ever since human beings have been on Earth, and especially in the last hundred years, animal species have become extinct because of hunting, pollution, or the destruction of the places where they live.
So many animals are now in danger of extinction that a list is kept in a Red Book. unfortunately, the list gets longer every year.