40 THE PLANT SPECIES 



in the development of a unit concept in biology is the recogni- 

 tion that the unit does not enjoy an independent and uncondi- 

 tioned existence but that it forms a constituent part of a higher 

 organization. Neither the extreme position that the unit is auton- 

 omous (and therefore objectively defined), nor the opposite 

 position that it is merely an integral part of a more complex unit 

 (which alone is objectively demarcated), is likely to prove cor- 

 rect in the final synthesis. The truth is more likely to lie in the 

 middle ground that the elementary unit possesses an objective 

 existence which, however, is subject to certain limitations due to 

 its integration into a higher system. 



Thus in nineteenth century biology the adherents of the cell 

 theory were forced to retreat from their original position under 

 the pressure of the knowledge that the organism as a whole con- 

 stitutes a physiological unit and not a mere aggregation of cells. 

 The compromise solution embodied in the now generally ac- 

 cepted organismal theory did not, however, negate, but merely 

 qualified, the unitary nature of the cell. The individual in turn 

 is now recognized as an elementary component of a more com- 

 plex unit, the breeding population, in cross-fertilizing organisms. 

 The facts of modern population genetics which make this con- 

 clusion inescapable can be accepted without denying the exist- 

 ence of the individual as a subordinate but real unit in its own 

 right. The acceptance of these facts, moreover, is fatal to the 

 opposite extreme view, still held in some quarters, that the in- 

 dividual is the ultimate biological unit. 



A second adjustment which is frequently necessary in the 

 formulation of a satisfactory concept of a biological unit is the 

 redrawing of the boundaries. A unit as originally defined may 

 require subdivision in the light of subsequent evidence. The 

 concept of the gene seems to be going through this process at 

 the present time. There is accumulating evidence that the 

 Mendelian gene, which is the unit of crossing over, is not equiva- 

 lent to the unit of biochemical activity in the chromosomes. The 

 theoretical difficulty of delimiting individuals in clonally repro- 

 ducing organisms has led to the recognition of two units in 

 place of one: the individual, as defined by physiological auton- 



