Natural selection lays down rules about precisely which form of life is most suitable for colonizing a particular environment. This evolutionary feature can give rise to a large number of different animals with the same superficial appearance. When the animals concerned have evolved from the same ancestor and have developed independently along similar evolutionary lines, they are said to have evolved in parallel. When the ancestors are different and the animals have evolved along quite different lines to produce the same final shape, their evolution is called convergent. An example of parallel evolution can be seen in the development of Equus, the horse, which appeared at the end of the Tertiary in North America, and Thoatherium, a remarkably similar ungulate which evolved at the same time in the then isolated continent of South America. The two forms developed independently along similar lines from similar ungulate ancestors in response to the same set of environmental conditions. An example of convergent evolution is found in the development of the shark, Carcharodon, the fish-lizard, Ichthyosaurus, and the dolphin, Delphinus � three animals from totally different classes but having adopted the same streamlined shape, swimming fins and tail in order to exploit the same niche in the same environment, that of active fish-eaters in the sea.