Diversity of structure

The varying modes of life which mammals have adopted are associated with the great diversity in structure. For there is always an intimate relationship between the structure of an animal, its mode and habits of life, and the conditions under which it lives. Adaptation to the particular kind of life which various groups of mammals chose to follow led to the evolution of that bewildering diversity in form and structure seen among mammals today, a diversity which fits them for the most varied modes of existence. Arboreal mammals exhibit grasping limbs rendering them fit to live and secure their food in trees. With parachute-like folds of skin stretched between their extended limbs, flying squirrels can glide and sail from tree to tree. The thicket cylindrical body of a mole, its enormous forefeet modified for digging, fit it especially for a life underground. Many mammals sought their food and took to a life in rivers and lakes, and some of the aquatic forms invaded the sea; among them are the whales, porpoises, and dolphins, whose fishlike bodies fit them for a marine existence almost as perfectly as fishes.