308 BIOLOGY AND ITS MAKERS 
theories founded upon the idea of pangens were destined to 
be replaced by others founded on the conception of geminal 
continuity—the central idea in nineteenth-century biology. 
The four chief steps which have led to the advancement 
of the knowledge of heredity, as suggested by Thomson, are 
as follows: ‘ (a) The exposition of the doctrine of germinal 
continuity. (b) More precise investigation of the material 
basis of inheritance. (c) Suspicions regarding the inherit- 
ance of acquired characteristics. (d) Application of statis- 
tical methods which have led to the formulation of the law 
of ancestral heredity.” We shall take these up in order. 
Exposition of the Doctrine of Germinal Continuity.--— 
From parent to. offspring there passes some hereditary sub- 
stance; although small in amount, it is the only living thread 
that connects one generation with another. It thus appears 
that there enters into the building of the body of a new organ- 
ism some of the actual substance of both parents, and that 
this transmitted substance must be the bearer of hereditary 
qualities. Does it also contain some characteristics inherited 
from grandparents and previous generations? If so, how 
far back in the history of the race does unbroken continuity 
extend ? 
Briefly stated, genetic continuity means that the ovum 
and its fertilizing agent are derived by continuous cell-lineage 
from the fertilized ovum of previous generations, extending 
back to the beginning of life. The first clear exposition of 
this theory occurs in the classical work of Virchow on Cellular 
Pathology, published in 1858. Virchow (1821-1902), the 
distinguished professor of the University of Berlin, has al- 
ready been spoken of in connection with the development 
of histology. He took the step of overthrowing the theory 
of free cell-formation, and replacing it by the doctrine of 
cell-succession. According to the theory of Schleiden and 
Schwann, cells arose from a blastema by a condensation of 
