Morphology. 65 
of a mollusk. In other words, its peculiar structure 
is not specially in keeping with its present habits, 
although so curiously allied to the similar structure 
of certain other crabs of totally different habits, in 
relation to which the peculiarities are of plain and 
obvious significance. 
I will devote the remainder of this chapter to 
considering another branch of the argument from 
morphology, to which the case of Birgus serves as 
a suitable introduction: I mean the argument from 
rudimentary structures. 
Throughout both the animal and vegetable king- 
doms we constantly meet with dwarfed and useless 
representatives of organs, which in other and allied 
kinds of animals and plants are of large size and 
functional utility. Thus, for instance, the unborn 
whale has rudimentary teeth, which are never des- 
tined to cut the gums; and throughout its life this 
animal retains, in a similarly rudimentary condition, 
a number of organs which never could have been of use 
to any kind of creature save a terrestrial quadruped. 
The whole anatomy of its internal ear, for example, 
has reference to hearing in air—or, as Hunter long ago 
remarked, “is constructed upon the same principle as 
in the quadruped”’; yet, as Owen says, “the outer 
opening and passage leading therefrom to the tym- 
panum can rarely be affected by sonorous vibrations 
of the atmosphere, and indeed they are reduced, or 
have degenerated, to a degree which makes it difficult 
to conceive how such vibrations can be propagated to 
the ear-drum during the brief moments in which the 
opening may be raised above the water.” 
ba F 
