306 BIOLOGY AND ITS MAKERS 
although microscopic in size, have a very complex organiza- 
tion. The discovery of this organization must depend upon 
microscopic examination. Knowledge regarding the physical 
basis of heredity has been greatly advanced by critical studies 
of cells under the microscope and by the application of ex- 
perimental methods, while other phases of the problems of 
inheritance have been elucidated by the analysis of statistics 
regarding hereditary transmissions. The whole question, 
however, is so recent that a clear formulation of the direction 
of the main currents of progress will be more helpful than 
any attempt to estimate critically the underlying principles. 
Early Theories.—There were speculations regarding the 
nature of inheritance in ancient and medieval times. To 
mention any of them prior to the eighteenth century would 
serve no useful purpose, since they were vague and did not 
form the foundation upon which the modern theories were 
built. The controversies over pre-formation and epigenesis 
(see Chapter X) of the eighteenth century embodied some 
ideas that have been revived. The recent conclusion that 
there is in the germinal elements an inherited organization 
of great complexity which conditions inheritance seems, at 
first, to be a return to the doctrine of pre-formation, but closer 
examination shows that there is merely a general resemblance 
between the ideas expressed by Haller, Bonnet, and philos- 
ophers of their time and those current at the present time. 
Inherited organization, as now understood, is founded on 
the idea of germinal continuity and is vastly different from 
the old theory of pre-formation. ‘The meaning of epigenesis, 
as expressed by Wolff, has also been modified to include the 
conception of pre-localization of hereditary qualities within 
particular parts of the egg. It has come now to mean that 
development is a process of differentiation of certain qualities 
already laid down in the germinal elements. 
Darwin’s Theory of Pangenesis.--In attempting to 
