the ovum. Forty years ago no one dared attack the question of the
CAUSES of these phenomena. For fully a century, from the year 1759,
when Wolff's solid Theoria generationis appeared, until 1859, when
Darwin published his famous Origin of Species, the real causes of the
embryonic processes were quite unknown. No one thought of seeking the
agencies that effected this marvellous succession of structures. The
task was thought to be so difficult as almost to pass beyond the
limits of human thought. It was reserved for Charles Darwin to
initiate us into the knowledge of these causes. This compels us to
recognise in this great genius, who wrought a complete revolution in
the whole field of biology, a founder at the same time of a new period
in embryology. It is true that Darwin occupied himself very little
with direct embryological research, and even in his chief work he only
touches incidentally on the embryonic phenomena; but by his reform of
the theory of descent and the founding of the theory of selection he
has given us the means of attaining to a real knowledge of the causes
of embryonic formation. That is, in my opinion, the chief feature in
Darwin's incalculable influence on the whole science of evolution.

When we turn our attention to this latest period of embryological
research, we pass into the second division of organic
evolution--stem-evolution, or phylogeny. I have already indicated in
Chapter 1.1 the important and intimate causal connection between these
two sections of the science of evolution--between the evolution of the
individual and that of his ancestors. We have formulated this
connection in the biogenetic law; the shorter evolution, that of the
individual, or ontogenesis, is a rapid and summary repetition, a
condensed recapitulation, of the larger evolution, or that of the
species. In this principle we express all the essential points
relating to the causes of evolution; and we shall seek throughout this