In fact, the chosen name of our class of animals - Mammalia - originates in Linnaeus's campaign for women to nurse their children at their own breasts, at a time when most of his own circle did not do so.
Like most arthropods, they date from the Jurassic period, biologically so much older than mammalia that they might as well be from another planet.
At first glance at these astonishingly diverse creatures, which include koala bears, armadillos, platypuses, beavers, whales and humans, it might seem that mammalia as a category is also arbitrary: is there any significant connection between the lumbering Indricotherium and the gnatty Batodonoides?
Spencer enlists evolutionary theory to claim that different races of human beings exist and form a clear hierarchy: "The civilized man departs more widely from the general type of the placental mammalia than do the lower human races" (1857, 18).
And this class will no doubt be a subclass of many sub-categories: the genus canis, the class (in the biological sense) mammalia, …, and so through a chain of sub-categories that eventually reaches some very general sub-category like "substance" or "material object".
The class Mammalia is worldwide in distribution.
Some authorities classify the elasmobranchs into one class (Selachii) and classify the chimaeras into another (Holocephali); however, assigning the two groups class rank implies a degree of distinctness equal to that of the amphibians (Amphibia), reptiles (Reptilia), birds (Aves), and mammals (Mammalia).
The greatest present-day diversity is seen in continental tropical regions, although members of the class Mammalia live on (or in seas adjacent to) all major landmasses.
Tetrapod (superclass Tetrapoda), a superclass of animals that includes all limbed vertebrates (backboned animals) constituting the classes Amphibia (amphibians), Reptilia (reptiles), Aves (birds), Mammalia (mammals), and their direct ancestors that emerged roughly 397 million years ago during the Devonian Period.
To simplify definitions and to allow the strict delimitation of the Mammalia, some authors have suggested basing the boundary on a single characteristic, the articulation of the jaw between the dentary and squamosal bones and the attendant movement of accessory jawbones to the middle ear as auditory ossicles.
