am fully convinced that species are not
immutable; but that those belonging to what are called the same genera
are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are the
descendants of that species. Furthermore, I am convinced that Natural
Selection has been the most important, but not the exclusive, means of
modification."

It should be especially noted that all which is here claimed is now
almost universally admitted, while the criticisms of Darwin's works
refer almost exclusively to those numerous questions which, as he
himself says, "will long remain obscure."

_The Darwinian Theory_.

As it will be necessary, in the following chapters, to set forth a
considerable body of facts in almost every department of natural
history, in order to establish the fundamental propositions on which the
theory of natural selection rests, I propose to give a preliminary
statement of what the theory really is, in order that the reader may
better appreciate the necessity for discussing so many details, and may
thus feel a more enlightened interest in them. Many of the facts to be
adduced are so novel and so curious that they are sure to be appreciated
by every one who takes an interest in nature, but unless the need of
them is clearly seen it may be thought that time is being wasted on mere
curious details and strange facts which have little bearing on the
question at issue.

The theory of natural selection rests on two main classes of facts which
apply to all organised beings without exception, and which thus take
rank as fundamental principles or laws. The first is, the power of rapid
multiplication in a geometrical progression; the second, that the
offspring always vary slightly from the parents, though generally very
closely resembling them. From the first fact or law there follows,
necessarily, a constant struggle for existence; because, while the
offspring always exceed the parents in number, generally to an enormous
extent, yet the total number of living organisms in the world does not,
and cannot, increase year by year. Consequently every year, on the
average, as many die as are born, plants as well as animals; and the
majority die premature deaths. They kill each other in a thousand
different ways; they starve each other by some consuming the food that
others want; they are destroyed largely by the powers of nature--by cold
and heat, by