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Have you ever thought about how different animals are from each other? For example, think of a jellyfish and an elephant. Apart from the differences in size, the jellyfish does not have a skeleton. It lives in water and its shape can change. Why are animals so different? Because if they are to survive as a species, they must be able to change slowly to live in their surroundings. So animals living in different places have to develop different ways of fitting in with their surroundings. Over thousands of years, climate changes, and so does vegetation. Animals have to change as well, or die out. Today we find the fossils of many types of animals which did not change in the right way, and so died out millions of years ago. One of these was the dinosaur. Dinosaurs were very large animals, some of them 50 feet long and 20 feet high. Think of the size of an elephant, and you will see how very big dinosaurs were. What does an animal need if the species is to survive? Several things. It must certainly be able to get enough food. It must be able to go on living without being killed off. And it must produce enough young ones.