In a sense, every phenomenon is unique. No two objects can occupy 

 the same space and time. Sets of energy relations, if recurring with 

 exact precision, at least differ in time. However, the perceptual uni- 

 verse is one of ordered uniqueness. The human mind is an apparatus 

 that functions by imposing relationships upon unique events. A col- 

 lection of objects having characteristics in common are grouped into 

 a class (e.g., table, race, butterfly), and this group concept is use- 

 ful for communication. Indeed, the existence of virtually all organ- 

 isms depends upon their ability to generalize in some sense from 

 collections of unique events. A completely unique event, one for 

 which there could be perceived no relationship with any other event, 

 would be totally without tneaning. 



All human understanding is based upon populations of things 

 and events and the patterns of interrelationship thought to exist 

 among them. In order to understand the workings of cells, a bio- 

 chemist studies the populations of chemical constituents and proc- 

 esses within the cell. For insight into the organization of organ- 

 isms, physiologists and embryologists study populations of cells and 

 tissues and the interactions among them. At the highest level of 

 biological organization, the population biologist investigates popu- 

 lations of organisms and the relationships within and among them. 

 In this book the term population will be restricted to aggregations 

 of individual organisms, the sense in which it customarily is used by 

 evolutionary biologists. Population biology deals, then, with the 

 properties of aggregations of organisms, particularly those emergent 

 properties not possessed by the individual constituents of the popu- 

 lations. Populations rarely can be studied in their entirety but must 

 be sampled at one or more points in time. Unfortunately, it is not 

 possible to sample the same population twice. 



INDIVIDUALS AND COLONIES 



The first problem arises with the definition of an individual organ- 

 ism. At first sight this appears to be easy, since familiar plants and 

 animals exist as discrete units. However, the situation is compli- 

 cated by the existence of forms such as lichens. These plants consist 

 of a fungus now known to be parasitic upon algal cells included in 

 its thallus. Different lichens have different morphological and bio- 

 chemical characteristics, but these fail to appear unless the correct 

 combination of alga and fungus occurs. The alga and fungus repro- 

 duce separately, but the lichen reproduces as well, with propagules | 73 



