136 NATURAL SELECTION vl 
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 spicule in many sponges, 
which are believed to have the function chiefly of rendering 
them unpalatable to other creatures. The Holothuride 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 
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 
