The mammals

 With the mammals the evolutionary line of the vertebrates is completed at the moment. They are the dominant tetrápodos in the terrestrial way, with a great diversity of forms and especially with a great capacity of adjustment to the diverse means.They represent the maximum degree of adjustment to the terrestrial life obtained till now by the animal kingdom. Nevertheless, some groups have returned to the sea, as the cetacean ones, whereas others, the quirópteros, have conquered the air way. The great diversity and, especially, the enormous capacity of adjustment have allowed to the mammals to expand all over the world and to take advantage of the diverse mas available resources. Departing from a basic structure, they have developed forms as diverse as a dolphin, a shrew, an elephant and the own human being.

The stretching of the neck of the giraffes they have not supposed increase of the number of the vertebrae but an increase of his length.

Some mammals have adapted to the life in the water, as the case of the dolphins, acquiring his body a fish-shaped aspect, reflection of the adjustment to the way.