3o6 BIOLOGY AXD 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 cjuestion, 

 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 mediaeval 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- 

 oi)liers 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 



