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 state; and this in some instances necessarily implies
an enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at an
embryonic age the species closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification embraces all
the members of the same class. I believe that animals have 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 have 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 germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance 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.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some
one primordial form, into which life was first breathed. When the views
entertained in this volume on the origin of species, or when analogous
views are generally admitted, we can dimly foresee that there will be a
considerable revolution in natural history. Systematists will be able
to pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in essence
a species. This I feel sure, and I speak after experience, will be no
slight relief. The endless disputes whether or not some fifty species
of British brambles are true species will cease. Systematists will
have only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a
far more essential consideration than it is at present; for differences,
however slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as suf