The arguments opposed to the theory of Natural Selection, have been
discussed in my 'Origin of Species,' as far as the size of that work
permitted, under the following heads: the difficulty in understanding how
very simple organs have been converted by small and graduated steps into
highly perfect and complex organs; the marvellous facts of Instinct; the
whole question of Hybridity; and, lastly, the absence in our known
geological formations of innumerable links connecting all allied species.
Although some of these difficulties are of great weight, we shall see that
many of them are explicable on the theory of natural selection, and are
otherwise inexplicable.

In scientific investigations it is permitted to invent any hypothesis, and
if it explains various large and independent classes of facts it rises to
the rank of a well-grounded theory. The undulations of the ether and even
its existence are hypothetical, yet every one now admits the undulatory
theory of light. The principle of natural selection may be looked at as a
mere hypothesis, but rendered in some degree probable by what we positively
know of the variability of organic beings in a state of nature,--by what we
positively know of the struggle for existence, and the consequent almost
inevitable preservation of favourable variations,--and from the analogical
formation of domestic races. Now this hypothesis may be tested,--and this
seems to me the only fair and legitimate manner of considering the whole
question,--by trying whether it explains several large and independent
classes of facts; such as the geological succession of organic beings,
their distribution in past and present times, and their mutual affinities
and homologies. If the principle of natural selection does explain these
and other large bodies of facts, it ought to be received. On the ordinary
view of each species having been independently created, we gain no
scientific explanation of any one of these facts. We can only say that it
has so pleased the Creator to command that the past and present inhabitants