of animals different indeed from existing sloths and
armadillos, but yet much more nearly related to them than to any other
kinds whatever. Such coincidences between the existing and antecedent
geographical distribution of forms are numerous. Again, "Natural Selection"
serves to explain the circumstance that often in adjacent islands we find
animals closely resembling, and appearing to represent, each other; while
if certain of these islands show signs (by depth of surrounding sea or what
not) of more ancient separation, the animals inhabiting them exhibit a {7}
corresponding divergence.[5] The explanation consists in representing the
forms inhabiting the islands as being the modified descendants of a common
stock, the modification being greatest where the separation has been the
most prolonged.

"Rudimentary structures" also receive an explanation by means of this
theory. These structures are parts which are apparently functionless and
useless where they occur, but which represent similar parts of large size
and functional importance in other animals. Examples of such "rudimentary
structures" are the foetal teeth of whales, and of the front part of the
jaw of ruminating quadrupeds. These foetal structures are minute in size,
and never cut the gum, but are reabsorbed without ever coming into use,
while no other teeth succeed them or represent them in the adult condition
of those animals. The mammary glands of all male beasts constitute another
example, as also does the wing of the apteryx--a New Zealand bird utterly
incapable of flight, and with the wing in a quite rudimentary condition
(whence the name of the animal). Yet this rudimentary wing contains bones
which are miniature representatives of the ordinary wing-bones of birds of
flight. Now, the presence of these useless bones and teeth is explained if
they may be considered as actually being the inherited diminished
representatives of parts of large size and functional importance in the
remote ancestors of these various animals.

Again, the singular facts of "homology" are capable of a similar
explanation. "Homology" is the name applied to the investigation of those
profound resemblances which have so often been found to underlie
superficial differences between animals of very different form and habit.
Thus man, the horse, the whale, and the bat, all have the pectoral limb,
whether it be the arm, or fore-leg, or paddle, or wing, formed on
essentially the same type