THE RISE OF EMBRYOLOGY 225 
substance between all living organisms, and that the egg and 
the sperm are endowed with an inherited organization of 
great complexity, has become the basis for all current theories 
of heredity and development. So much is involved in this 
conception that, in the present decade, it has been designated 
(Whitman) “the central fact of modern biology.” The first 
clear expression of it is found in Virchow’s Cellular Pa- 
thology, published in 1858. It was not, however, until the 
Fic. 68.—A. Kowa.Levsky, 1840-1901. 
period of Balfour, and through the work of Fol, Van Beneden 
(chromosomes, 1883), Boveri, Hertwig, and others, that the 
great importance of this conception began to be appreciated, 
and came to be woven into the fundamental ideas of de- 
velopment. 
Influence of the Doctrine of Organic Evolution.—This 
doctrine, although founded in its modern sense by Lamarck 
in the early part of the nineteenth century, lay dormant until 
Darwin, in 1859, brought a new feature into its discussion 
15 
