There is only one safe path which the naturalist may follow who would tell the story of the meaning and nature of an extinct type of animal life, and that is to compare it as fully as possible in its several bones, and as a whole, with other animals, especially with those which survive. It is easy to fix the place in nature of living animals and determine their mutual relations to each other, because all the organs--vital as well as locomotive--are available for comparison. On such evidence they are grouped together into the large divisions of Beasts, Birds, and Reptiles; as well as placed in smaller divisions termed Orders, which are based upon less important modifications of fundamental structures. All these characteristic organs have usually disappeared in the fossil. Hence a new method of study of the hard parts of the skeleton, which alone are preserved, is used in the endeavour to discover how the Flying Reptile or other extinct animal is to be classified, and how it acquired its characters or came into existence. VARIATIONS OF BONES AMONG MAMMALIA [Illustration: FIG. 14. THE FORE LIMB IN FOUR TYPES OF MAMMALS Comparison of the fore limb in mammals, showing variation of form of the bones with function] Resemblances and differences in the bones are easily over-estimated in importance as evidence of pedigree relationship. The Mammalia show, by means of such skeletons as are exhibited in any Natural History Museum, how small is the importance to be attached to even the existence of any group of bones in determining its grade of organisation. The whole Whale tribe suckle their young and conform to the distinctive characters in brain and lungs which mark them as being mammals. But if there is one part of the skeleton more than another which distinguishes the Mammalia, it is the girdle of bones at the hips which supports the hind limbs. It is characterised by the bone named the ilium being uniformly directed forward. Yet in the Whale tribe the hip-girdle and the hind limb which it usually supports are so faintly indicated as to be practically lost; while the fore limb becomes a paddle without distinction of digits, and is therefore devoid of hoofs or claws, which are usual terminations of the extremities in mammals. Yet this swimming paddle, with its ill-defined bones--sometimes astonishing in number, as well as in fewness of the finger bones--is represented by the burrowing fore limb of the Mole, which lives underground; by the elongated hoofed legs of the Giraffe, which lives on plains; and the extended arm and finger bones of the Bat, which are equally mammals with the Whale. From such comparison it is seen that no proportion, or form, or length, or use of the bones of the limbs, or even the presence of limbs, is necessarily characteristic of a mammal. No limitation can be placed upon the possible diversity of form or development of bones in unknown animals, when they are considered in the light of such experience of varied