The web-wing of bats involves much more than the fore-arm. The double fold of skin begins on the side of the neck, passes along the front of the arm, skips the thumb, and is continued over the elongated palm-bones and fingers to the sides of the body again, and to the hind-legs, and to the tail if there is a tail. It is interesting to find that the bones of the bat's skeleton tend to be lightly built as in birds, that the breast-bone has likewise a keel for the better insertion of the pectoral muscles, and that there is a solidifying of the vertebrae of the back, affording as in birds a firm basis for the wing action. Such similar adaptations to similar needs, occurring in animals not nearly related to one another, are called "convergences," and form a very interesting study. In addition to adaptations which the bat shares with the flying bird, it has many of its own. There are so many nerve-endings on the wing, and often also on special skin-leaves about the ears and nose, that the bat flying in the dusk does not knock against branches or other obstacles. Some say that it is helped by the echoes of its high-pitched voice, but there is no doubt as to its exquisite tactility. That it usually produces only a single young one at a time is a clear adaptation to flight, and similarly the sharp, mountain-top-like cusps on the back teeth are adapted in insectivorous bats for crunching insects. Whether we think of the triumphant flight of birds, reaching a climax in migration, or of the marvel that a creature of the earth--as a mammal essentially is--should evolve such a mastery of the air as we see in bats, or even of the repeated but splendid failures which parachuting animals illustrate, we gain an impression of the insurgence of living creatures in their characteristic endeavour after fuller well-being. We have said enough to show how well adapted many animals are to meet the particular difficulties of the haunt which they tenant. But difficulties and limitations are ever arising afresh, and so one fitness follows on another. It is natural, therefore, to pass to the frequent occurrence of protective resemblance, camouflage, and mimicry--the subject of the next article. BIBLIOGRAPHY ELMHIRST, R., _Animals of the Shore_. FLATTELY AND WALTON, _The Biology of the Shore_ (1921). FURNEAUX, _Life of Ponds and Streams_. HICKSON, S. J., _Story of Life in the Seas_ and _Fauna of the Deep Sea_. JOHNSTONE, J., _Life in the Sea_ (Cambridge Manual of Science). MIALL, L. C., _Aquatic Insects_. MURRAY, SIR JOHN, _The Ocean_ (Home University Library). MURRAY, SIR JOHN AND HJORT, DR. J., _The Depths of the Ocean_. NEWBIGIN, M. I., _Life by the Sea Shore_. PYCRAFT, W. P., _History of Birds_. SCHARFF, R. F., _History of the European Fauna_ (Contemp. Sci. Series). THOMSON, J. ARTHUR, _The Wonder of Life_ (1914) and _The Haunts of Life_ (1921). IV THE STRUGGLE FOR EXISTENCE ANIMAL AND BIRD MIMICRY AND DISGUISE Sec. 1