PDF - Wallace Online

136 NATURAL SELECTION ^viis such a tangled web of complex relations, that a series ofcorrespondences running through hundreds of species, genera,and families, in every part of the system, can hardly fail toindicate a true casual connection ;and when, of the twofactors in the problem, one can be shown to be dependent onthe most deeply seated and the most stable facts of structureand conditions of life, while the other is a character universallyadmitted to be superficial and easily modified, there canbe little doubt as to which is cause and which effect.Various modes of Protection of AnimalsBut the explanation of the phenomenon here attempteddoes not rest alone on the facts I have been able now toadduce. In the essay on "Mimicry"it is shown how importanta part the necessity for protection has played, indetermining the external form and coloration, and sometimeseven the internal structure of animals.As illustrating this latter point, I may refer to the remarkablehooked, branched, or star-like spiculae in many sponges,which are believed to have the function chiefly of renderingthem unpalatable to other creatures. The Holothuridae orsea -cucumbers possess a similar protection, many of themhaving anchor-shaped spicules embedded in their skin, as theSynapta ;while others (Cuviera squamata) are covered with ahard calcareous pavement. Many of these are of a bright redor purple colour, and are very conspicuous, while the alliedTrepang, or Beche-de-mer (Holothuria edulis), which is notarmed with any such defensive weapons, is of a dull sand ormud colour, so as hardly to be distinguished from the sea-bedon which it reposes. Many of the smaller marine animals areprotected by their almost invisible transparency, Avhile thosethat are most brightly coloured will be often found to have aspecial 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 Protectionthan the MalesIn the struggle for existence incessantly going on, protectionor concealment is one of the most general and mosteffectual means of maintaining life ;and it isby modifications