o become highly organized? Members of a high group might even
become, and this apparently has occurred, fitted for simpler conditions of
life; and in this case natural selection would tend to simplify or degrade
the organization, for complicated mechanism for simple actions would be
useless or even disadvantageous.

In a second work, after treating of the Variation of organisms in a state
of nature, of the Struggle for Existence and the principle of Natural
Selection, I shall discuss the difficulties which are opposed to the
theory. These difficulties may be classed under the following heads:--the
apparent impossibility in some cases of a very simple organ graduating by
small steps into a highly perfect organ; the marvellous facts of Instinct;
the whole question of Hybridity; and, lastly, the absence, at the present
time and in our 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 {9} 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 pa