CHAPTER XV 



BIOLOGICAL CONCEPTIONS IN THE LIGHT OF 

 MENDELIAN DISCOVERIES. 



Nature of Units — Natttre of Segregation — Moment of 

 Segregation — Differentiation of Parts compared with 

 Segregation — Reversion and Va^'iation. '* Bush " and 

 *' Ctcpid'' Sweet Peas — Mendelian Segregation and 

 Species — Discontinuity in Variation — Mendelism and 

 Natttral Selection, 



The purpose of the preceding chapters has been to 

 show the method of Mendehan analysis in apphcation to a 

 variety of problems, and to describe the concrete discoveries 

 to which that method has already led. To discuss the 

 bearino^ of the new facts on biolos^ical science is now a 

 considerable undertaking and all that will be attempted 

 within the limits of this volume is a slight sketch of the 

 possibilities which these facts suggest. 



Natttre of the Units. 



With the recognition of unit-characters our general 

 conceptions of the structure and properties of living things 

 inevitably undergo a change. We begin to perceive outlines 

 where previously all was vague, nor can we doubt that 

 those outlines will very soon become clearer. What the 

 physical nature of the units may be we cannot yet tell, but 

 the consequences of their presence is in so many instances 

 comparable with the effects produced by ferments, that with 

 some confidence we suspect that the operations of some 

 units are in an essential way carried out by the formation 

 of definite substances acting as ferments. 



