e led to its escape from sharp-sighted enemies. In certain
species of whales there is a tendency to the formation of irregular
little points of horn on the palate; and it seems to be quite within the
scope of natural selection to preserve all favourable variations, until
the points were converted, first into lamellated knobs or teeth, like
those on the beak of a goose--then into short lamellae, like those of
the domestic ducks--and then into lamellae, as perfect as those of the
shoveller-duck--and finally into the gigantic plates of baleen, as
in the mouth of the Greenland whale. In the family of the ducks, the
lamellae are first used as teeth, then partly as teeth and partly as
a sifting apparatus, and at last almost exclusively for this latter
purpose.

With such structures as the above lamellae of horn or whalebone, habit
or use can have done little or nothing, as far as we can judge, towards
their development. On the other hand, the transportal of the lower eye
of a flat-fish to the upper side of the head, and the formation of
a prehensile tail, may be attributed almost wholly to continued use,
together with inheritance. With respect to the mammae of the higher
animals, the most probable conjecture is that primordially the cutaneous
glands over the whole surface of a marsupial sack secreted a nutritious
fluid; and that these glands were improved in function through natural
selection, and concentrated into a confined area, in which case they
would have formed a mamma. There is no more difficulty in understanding
how the branched spines of some ancient Echinoderm, which served as
a defence, became developed through natural selection into tridactyle
pedicellariae, than in understanding the development of the pincers of
crustaceans, through slight, serviceable modifications in the ultimate
and penultimate segments of a limb, which was at first used solely
for locomotion. In the avicularia and vibracula of the Polyzoa we have
organs widely different in appearance developed from the same source;
and with the vibracula we can understand how the successive gradations
might have been of service. With the pollinia of orchids, the threads
which originally served to tie together the pollen-grains, can be traced
cohering into caudicles; and the steps can likewise be followed by which
viscid matter, such as that secreted by the stigmas of ordinary flowers,
and still subserving nearly but not quite the same purpose, became
attache