THE PLANT SPECIES 



IN THEORY AND PRACTICE 



VERNE GRANT: rancho santa ana botanic garden and 



CLAREMONT GRADUATE SCHOOL, CLAREMONT, CALIFORNIA 



Biological Units 



A philosophical tendency in modern biology, which has been 

 in process of formulation for a century or more, is the view that 

 the world of life is composed of a series of units organized into 

 successively higher levels of complexity. The elementary bio- 

 logical units, gene, chromosome, cell, individual, deme (or local 

 population), society, species, biotic community, represent differ- 

 ent levels of organization of biological materials. All these units, 

 in spite of numerous differences, share three common properties: 

 an internal integration, the power of reproduction, and the ability 

 to become modified in time. It would be correct to describe them 

 all as integrated wholes, as reproductive units, and as entities 

 capable of development. 



The process of intellectual criticism has sharpened the concept 

 of each one of the biological units. The progress of thinking fol- 

 lows a pattern. The unit in question is discovered and its auton- 

 omy and unity are frequently overemphasized by the earlier 

 investigators in the field. With the growth of knowledge a reac- 

 tion sets in. Some biologist in a later period is certain to maintain 

 that the entity has no well-marked boundaries and hence has no 

 definable unity, or even that it has no objective existence at all. 

 This heresy forces a reexamination of the concept hitherto held 

 and of the evidence upon which it was based. A controversy 

 over "the problem of the gene" or "the concept of the association" 

 ensues, which may eventually culminate in a deeper understand- 

 ing of the nature of the particular unit. 



One of the most common adjustments which has to be made 



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