If the views here advocated are correct, as to the various influences that have determined the specialities of every bird's nest, and the general colouration of female birds, with their action and reaction on each other, we can hardly expect to find evidence more complete than that here set forth. Nature is such a tangled web of complex relations, that a series of correspondences running through hundreds of species, genera, and families, in every part of the system, can hardly fail to indicate a true casual connexion; and when, of the two factors in the problem, one can be shown to be dependent on the most deeply seated and the most stable facts of structure and conditions of life, while the other is a character universally admitted to be superficial and easily modified, there can be little doubt as to which is cause and which effect. _Various modes of Protection of Animals._ But the explanation of the phenomenon here attempted does not rest alone on the facts I have been able now to adduce. In the essay on "Mimicry," it is shown how important a part the necessity for protection has played, in determining the external form and colouration, and sometimes even the internal structure of animals. As illustrating this latter point, I may refer to the remarkable hooked, branched, or star-like spiculae in many sponges, which are believed to have the function chiefly, of rendering them unpalatable to other creatures. The Holothuridae or sea-cucumbers possess a similar protection, many of them having anchor-shaped spicules embedded in their skin, as the Synapta; while others (Cuviera squamata) are covered with a hard calcareous pavement. Many of these are of a bright red or purple colour, and are very conspicuous, while the allied Trepang, or Beche-de-mer (Holothuria edulis), which is not armed with any such defensive weapons, is of a dull sand-or mud-colour, so as hardly to be distinguished from the sea bed on which it reposes. Many of the smaller marine animals are protected by their almost invisible transparency, while those that are most brightly coloured will be often found to have a special protection, either in stinging tentacles like Physalia, or in a hard calcareous crust, as in the star fishes. _Females of some Groups require and obtain more Protection than the Males._