The second period, with which we have now to deal, lasts about a
century--that is to say, until the appearance of Darwin's Origin of
Species, which brought about a change in the very foundations of
biology, and, in particular, of embryology. The third period begins
with Darwin. When we say that the second period lasted a full century,
we must remember that Wolff's work had remained almost unnoticed
during half the time--namely, until the year 1812. During the whole of
these fifty-three years not a single book that appeared followed up
the path that Wolff had opened, or extended his theory of embryonic
development. We merely find his views--perfectly correct views, based
on extensive observations of fact--mentioned here and there as
erroneous; their opponents, who adhered to the dominant theory of
preformation, did not even deign to reply to them. This unjust
treatment was chiefly due to the extraordinary authority of Albrecht
von Haller; it is one of the most astonishing instances of a great
authority, as such, preventing for a long time the recognition of
established facts.

The general ignorance of Wolff's work was so great that at the
beginning of the nineteenth century two scientists of Jena, Oken
(1806) and Kieser (1810), began independent research into the
development of the alimentary canal of the chick, and hit upon the
right clue to the embryonic puzzle, without knowing a word about
Wolff's important treatise on the same subject. They were treading in
his very footsteps without suspecting it. This can be easily proved
from the fact that they did not travel as far as Wolff. It was not
until Meckel translated into German Wolff's book on the alimentary
system, and pointed out its great importance, that the eyes of
anatomists and physiologists were suddenly opened. At once a number of
biologists instituted fresh embryological inquiries, and began to