220 fertilization and Hybridization 



Everywhere this law is confirmed, that the child in- 

 herits one part of its nature from the father, the other 

 from the mother. The child is, therefore, on the whole, 

 a double being, with twofold qualities, more or less dis- 

 tinctly separated, that may still be traced back to their ori- 

 gin. This principle of duality, as we might call it, domi- 

 nates the entire theory of heredity ; it forms the thread that 

 binds together apparently separated cases; it serves as a 

 guidance for the whole investigation. 



This investigation occupies two different fields. On 

 the one hand we have experimental research, on the other 

 hand microscopical. Physiology ascertains the relations 

 of the offspring to their parents ; it analyzes their charac- 

 teristics into their individual units, and tries to demon- 

 strate their origin. The history of development discloses 

 to us the corresponding microscopic processes; it looks 

 for the smallest visible bearers of heredity in the cell, and 

 investigates how they are maintained during life, and how, 

 during fertilization, they pass on from father and mother 

 to the offspring. 



Few investigators master both provinces ; their extent 

 is much too great for that. And especially has the study 

 of hybrids so greatly advanced in recent years, that even 

 here a division of labor will soon be necessary. Both lines 

 of work have therefore developed more or less indepen- 

 dently of each other. In both, the main features of the 

 problem begin gradually to arise out of the abundance of 

 individual phenomena. And thereby there is disclosed, 

 one might almost say, beyond all expectation, an agree- 

 ment in the results of both lines of investigation, which 

 is so great, that almost everywhere the physiological pro- 

 cesses are reflected in the microscopically visible changes. 



It is true that the final analysis lies yet beyond the 



