shown that variations do not commonly supervene at a very early period of
embryonic growth, and on these two principles we can understand that most
wonderful fact in the whole circuit of natural history, namely, the close
similarity of the embryos within the same great class--for instance, those
of mammals, birds, reptiles, and fish.

It is the consideration and explanation of such facts as these which has
convinced me that the theory of descent with modification by means of
natural selection is in the main true. These facts have as yet received no
explanation on the theory of independent Creation; they cannot be grouped
together under one point of view, but each has to be considered as an
ultimate fact. As the first origin of life on this earth, as well as the
continued life of each individual, is at present quite beyond the scope of
science, I do not wish to lay much stress on the greater simplicity of the
view of a few forms or of only one form having been originally created,
instead of innumerable miraculous creations having been necessary at
innumerable periods; though this more simple view accords well with
Maupertuis's philosophical axiom of "least action."

In considering how far the theory of natural selection may be extended,
--that is, in determining from how many progenitors the inhabitants of the
world have descended,--we may conclude that at least all the members of the
same class have descended from a single ancestor. A number of organic
beings are included in the same class, because they present, independently
of their habits of life, the same fundamental type of structure, and
because they graduate into each other. Moreover, members of the same class
can in most cases be shown to be closely alike at an early embryonic age.
These facts can be explained on the belief of their descent from a common
form; therefore it may be safely admitted that all the members of the same
class are descended from one progenitor. But as the members of quite