almost
every naturalist admits the great principle of evolution. There are,
however, some who still think that species have suddenly given birth,
through quite unexplained means, to new and totally different forms.
But, as I have attempted to show, weighty evidence can be opposed to the
admission of great and abrupt modifications. Under a scientific point of
view, and as leading to further investigation, but little advantage is
gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the old
belief in the creation of species from the dust of the earth.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of
whole classes are connected together by a chain of affinities, and all
can be classed on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between
existing orders.

Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed condition, and this in some cases
implies an enormous amount of modification in the descendants.
Throughout whole classes various structures are formed on the same
pattern, and at a very early age the embryos closely resemble each
other. Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same great class or
kingdom. I believe that animals are descended from at most only four or
five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their cellular structure, their
laws of growth, and their liability to injurious influences. We see this
even in so trifling a fact as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak tree. With all
organic beings, excepting perhaps some of the very lowest, sexual
reproduction seems to