38 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 



