and the
attempts to explain how this happens; for we might safely have
transferred the proposition to the breeds or species naturally selected;
and the ultimate effect would necessarily have been that in a number of
races or species descended from a common stock and forming several
genera and families the embryos would have resembled each other more
closely than full-grown animals. Whatever may have been the form or
habits of the parent-stock of the Vertebrata, in whatever course the
arteries ran and branched, the selection of variations, supervening
after the first formation of the arteries in the embryo, would not tend
from variations supervening at corresponding periods to alter their
course at that period: hence, the similar course of the arteries in the
mammal, bird, reptile and fish, must be looked at as a most ancient
record of the embryonic structure of the common parent-stock of these
four great classes.

{477} _Origin_, Ed. i. p. 444, vi. p. 611.

A long course of selection might cause a form to become more simple, as
well as more complicated; thus the adaptation of a crustaceous{478}
animal to live attached during its whole life to the body of a fish,
might permit with advantage great simplification of structure, and on
this view the singular fact of an embryo being more complex than its
parent is at once explained.

{478} _Origin_, Ed. i. p. 441, vi. p. 607.

_On the graduated complexity in each great class._

I may take this opportunity of remarking that naturalists have observed
that in most of the great classes a series exists from very complicated
to very simple beings; thus in Fish, what a range there is between the
sand-eel and shark,--in the Articulata, between the common crab and the
Daphnia{479},--between the Aphis and butterfly, and between a mite and a
spider{480}. Now the observation just made, namely, that selection might
tend to simplify, as well as to complicate, explains this; for we can
see that during the endless geologico-geographical changes, and
consequent isolation of species, a station occupied in other districts
by less complicated animals might be left unfilled, and be occupied by a
degraded form of a higher or more complicated class; and it would by no
means follow that, when the two regions became united, the degraded
organism would give way to the aboriginally lower organism. According to
our theory, there is obviously no power tending constantly to exalt
specie