242 COOK 



such as the Leguminosae or Composita?. There can be little 

 doubt in such cases that the evolution of the later phases of the 

 parasites have taken place in coincidence with the advancing 

 development of their host-plants to which they are so strictly 

 confined. 



GROWTH SPECIALIZATIONS ARISING FROM SOCIAL ORGANIZA- 

 TION (politism). 



Just as cells have become diverse by specialization in the build- 

 ing up of compound cellular structures, so individual organisms 

 of the same species may become diverse under conditions of 

 social organization, that is, when the individual organisms do 

 not live singly and independently, but in groups, colonies or 

 compound individuals. The bionomic unit of such species is 

 no longer the individual but the colony, since it is only in the 

 colony form that it meets its environmental problems or enters 

 into relations with other species. A good illustration of politism 

 is to be found among the compound types of higher plants, those 

 which take the form of shrubs or trees and consist of aggregates 

 of large numbers of the individual twigs or branches which cor- 

 respond to whole individuals of simpler types. 



The primitive herbaceous types of flowering plants have a 

 root and a stem, the latter with a series of leaves and a flower 

 at the top. If this be considered an individual, larger plants 

 with many stems or branches and many flowers are compound 

 individuals. Each branch or flowering twig of a tree may be 

 thought of as corresponding to the small individual herb. 

 Usually the branch-individuals are all of one kind, or at least 

 equivalent and able to replace each other, but in some species 

 such as cacoa, coffee, cotton and the Central American rubber 

 tree (Castillo.) the branches are strictly dimorphic, that is, of two 

 or more distinct kinds with different forms, structures and func- 

 tions, and also taking definite positional relations in the building 

 up of the compound individual plant or tree. 



It is among the animals, however, that specializations of poli- 

 tism exist in vast variety, and the diversity becomes obvious 

 and familiar. In many different groups there have grown up 

 social organizations, so that all stages may be found between the 



