In looking at the life on the grasslands of Africa and Australia around the time of man and comparing it with the life that existed on the plains of South America some time earlier, during the middle Tertiary, we can see that animals with similar life styles appear to evolve similar shapes and sizes in corresponding environments. It makes no difference whether these environments are separated by time, space or both, they are by far the most important single evolutionary factor governing the shape and form of living creatures. Large herbivorous animals, very similar in appearance to the rhinoceros, and long-legged, swift-running grazing animals appeared in all three environments. Carnivores, insectivores and omnivores all superficially similar to one another evolved. The most strikingly similar groups were the burrowing insect-eaters and the flightless birds, which because of their highly specialized modes of life developed along broadly the same lines.