A Theory of Birds' Nests 87 



cumstances in the habits or structure of the species that suffici- 

 ently explain them. If the views here advocated are correct 

 as to the various influences that have determined the special- 

 ities 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 indi- 

 cate a true causal connexion ; and when, of the two factors in 

 the problem, one can be shewn to be dependent on the most 

 deeply seated and the most stable facts of structure and con- 

 ditions 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. 



But the explanation of the phenomenon here attempted does 

 not rest alone on the facts I have been able now to adduce. In 

 the article on " Mimicry " already referred to, it is shewn how im- 

 portant a part the necessity for protection has played in determin- 

 ing the external form and colouration, and sometimes even the in- 

 ternal structure of animals. 



As illustrating this latter point, I may refer to the remarkable 

 hooked, branched, or star-like spiculse in many sponges, which are 

 believed to have the function chiefly of rendering them unpalatable 

 to other creatures. The Holothuridse or sea-cucumbers possess a si- 

 milar protection, many of them having anchor-shaped spicules em- 

 bedded in their skin, as the Synapta ; while others (Cuviera squam- 

 ata) 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 some pro- 

 tection either in stinging tentacles like Physalia, or in a hard cal- 

 careous crust, as in the star fishes. 



In the struggle for existence incessantly going on, protection or 

 concealment is one of the most general and most effectual means 

 of maintaining life, and it is by modifications of colour that this 

 protection can be most readily obtained, since no other character 



