On the Genesis of Species | Page 5

will in
the long run be preserved, and will transmit its favourable peculiarity to
some of its offspring, which peculiarity will thus become intensified {6}
till it reaches the maximum degree of utility. On the other hand,
individuals presenting unfavourable peculiarities will be ruthlessly
destroyed. The action of this law of Natural Selection may thus be well
represented by the convenient expression "survival of the fittest."[3]

Now this conception of Mr. Darwin's is perhaps the most interesting
theory, in relation to natural science, which has been promulgated
during the present century. Remarkable, indeed, is the way in which it
groups together such a vast and varied series of biological[4] facts, and
even paradoxes, which it appears more or less clearly to explain, as the
following instances will show. By this theory of "Natural Selection,"
light is thrown on the more singular facts relating to the geographical
distribution of animals and plants; for example, on the resemblance
between the past and present inhabitants of different parts of the earth's
surface. Thus in Australia remains have been found of creatures closely
allied to kangaroos and other kinds of pouched beasts, which in the
present day exist nowhere but in the Australian region. Similarly in
South America, and nowhere else, are found sloths and armadillos, and
in that same part of the world have been discovered bones 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, though the number and proportion
of parts may{8} more or less differ. Again, the butterfly and the shrimp,
different as they are in appearance and mode of life, are yet constructed
on the same common plan, of which they constitute diverging
manifestations. No a priori reason is conceivable why such similarities
should be necessary, but they are readily explicable on the assumption
of a genetic relationship and affinity between the animals in question,
assuming, that is, that they are the modified descendants of some
ancient form--their common ancestor.
That remarkable series of changes which animals undergo before they
attain their adult condition, which is called their process of
development, and during which they more or less closely resemble
other animals during the early stages of the same process, has also great
light thrown on it from the same source. The question as to the
singularly complex resemblances borne by every adult animal and plant
to a certain number of other animals and plants--resemblances by
means of which the adopted zoological and botanical systems of
classification have been possible--finds its solution in a similar manner,
classification becoming the expression of a genealogical relationship.