136 NATURAL SELECTION ^ vi 



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 connection ; 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 univer- 

 sally 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 im- 

 portant a part the necessity for protection has played, in 

 determining the external form and coloration, and sometimes 

 even the internal structure of animals. 



As illustrating this latter point, I may refer to the remark- 

 able 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, Avhile 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 



In the struggle for existence incessantly going on, pro- 

 tection or concealment is one of the most general and most 

 effectual means of maintaining life ; and it is by modifications 



