Cambridge Natural History Mammalia/Chapter I
The Mammalia form a group of vertebrated animals which roughly correspond with what are termed in popular language "quadrupeds," or with the still more vernacular terms of "beasts" or "animals." The name "Mammal" is derived from the most salient characteristic of the group, i.e. the possession of teats; but if the term were used in an absolutely strict etymological sense, it could not include the Monotremes, which, though they have mammary glands, have not fully-differentiated teats (see p. 16). There are, however, as will be seen shortly, other characters which necessitate the inclusion of these egg-laying quadrupeds within the class Mammalia.
The Mammalia are unquestionably the highest of the Vertebrata. This statement, however, though generally acceptable, needs some explanation and justification. "Highest" implies perfection, or, at any rate, relative perfection. It might be said with perfect truth that a serpent is in its way an example of perfection of structure: not incommoded with limbs it can slip rapidly through the grass, swim like a fish, climb like a monkey, and dart upon its prey with rapidity and accuracy. It is an example of an extremely specialised reptile, the loss of the limbs being the most obvious way in which it is specialised from more generalised reptilian types. Specialisation in fact is often synonymous with degradation, and, this being the case, implies a restricted life. On the other hand, simplification is not always to be read as degeneration. The lower jaw, for instance, of mammals has fewer bones in it than that of reptiles, and is more concisely articulated to the skull; this implies greater efficiency as a biting organ. The term highest, however, includes increased complexity as well as simplification, the two series of modifications being interwoven to form a more efficient organism. It cannot be doubted that the increased complexity of the brain of mammals raises them in the scale, as does also the complex and delicately adjusted series of bonelets which form the organ for the transmission of sound to the internal ear. The separation of the cavity containing the lungs, and the investment of the partition so formed with muscular fibres, renders the action of the lungs more effective; and there are other instances among the Mammalia of greater complexity of the various parts and organs of the body when compared with lower forms, which help to justify the term "highest" generally applied to these creatures.
Complexity and finish of structure are often accompanied by large size; and the Mammalia are, on the whole, larger than any other Vertebrates, and also contain the most colossal species. The huge Dinosaurs of the Mesozoic epoch, though among the largest of animals, are exceeded by the Whales; and the latter group includes the mightiest creature that exists or has ever existed, the eighty-five-feet-long Sibbald's Rorqual. Confining ourselves rigidly to facts, and avoiding all theorising on the possible relation between complexity and nicety of build and the capacity for increase in bulk, it is plain from the history of more than one group of mammals that increase in bulk accompanies specialisation of structure. The huge Dinocerata when compared with the ancestral Pantolambda teach us this, as do many similar examples. Within the mammalian group, as in the case of other Vertebrates, difference of size has a certain rough correspondence with difference of habitat. The Whales not only contain the largest of animals, but their average size is great; so too with the equally aquatic Sirenia and very aquatic Pinnipedia. Here the support offered by the water and the consequent decreased need for muscular power to neutralise the effects of gravity permit of an increase in bulk. Purely terrestrial animals come next; and finally arboreal, and, still more, "flying" mammals are of small size, since the maintenance of the position when moving and feeding needs enormous muscular effort.
The Mammals are more easily to be separated from the Vertebrates lying lower in the series than any of the latter are from each other in ascending order. A large number of characters might be used in addition to those which will be made use of in the following brief catalogue of essential mammalian features, were it not for the low-placed Monotremata on the one hand and the highly specialised Whales on the other. Including those forms, the Mammalia are to be distinguished from all other Vertebrates by the following series of structural features, which will be expanded later into a short disquisition upon the general structure of the Mammalia. The class Mammalia may, in fact, be thus defined:—
Hair-clad Vertebrates, with cutaneous glands in the female, secreting milk for the nourishment of the young. Skull without prefrontal, postfrontal, quadrato-jugal, and some other bones, and with two occipital condyles formed entirely by the exoccipitals. Lower jaw composed of dentary bone only, articulating only with the squamosal. Ear bones a chain of three or four separate bonelets. Cervical vertebrae sharply distinguished from the dorsals, and if with free ribs, showing no transition between these and the thoracic ribs. Brain with four optic lobes. Lungs and heart separated from abdominal cavity by a muscular diaphragm. Heart with a single left aortic arch. Red blood-corpuscles non-nucleate.
The following characters are also very nearly universal, and in any case absolutely distinctive:—Cervical vertebrae, seven; vertebrae with epiphyses. Ankle-joint "cruro-tarsal," i.e. between the leg and the ankle, and not in the middle of the ankle. Attachment of the pelvis to the vertebral column pre-acetabular in position.
The Mammalia since they are hot-blooded creatures are more independent of temperature than reptiles; they are thus found spread over a wider area of the earth's surface. As however, though hot-blooded, they have not the powers of locomotion possessed by birds, they are not quite so widely distributed as are those animals. The Mammalia range up into the extreme north, but, excepting only forms mainly aquatic, such as the Sea Lions, are not known to occur on the Antarctic continent. With the exception of the flying Bats, indigenous mammals are totally absent from New Zealand; and it seems to be doubtful whether those supposed oceanic islands which have a mammalian fauna are really oceanic in origin. The continents and oceans are peopled by rather over three thousand species of Mammalia, a number which is considerably less than that of either birds or reptiles. It seems clear that, so far at any rate as concerns the numbers of families and genera, the mammalian fauna of to-day is less varied than it was during the Mid-tertiary period, the heyday of mammalian life. It is rather remarkable to contrast in this way the mammals and the birds. The two classes of the animal kingdom seem to have come into being at about the same period; but the birds either have reached their culminating point to-day, or have not yet reached it. The Mammalia, on the other hand, multiplied to an extraordinary extent during the Eocene and the Miocene periods, and have since dwindled. The break is most marked at the close of the Pleistocene, and may be in part due to the direct influence of man. At present man exercises so enormous an effect, both directly and indirectly, that the future history of the Mammalia is probably foreshadowed by the instances of the White Rhinoceros and the Quagga. On the other hand, the economic usefulness of the Mammalia is greater than that of any other animals; and the next most important era in their history will be probably that of domesticity and "preservation."
- The degeneration of the hind-limb in Whales and Sirenia forbids the use of this character as a distinctive one on the principles advocated by the selection of the above list. But it would be absurd to leave out hair.