16 PROCEEDINGS OF THE NATIONAL MUSEUM VoL. 125 
probably in part filled by animals that are phylogenetically dis- 
tantly related to lizards, but ecologically the former should be con- 
sidered with the latter. The lizards do not make an ecological unit. 
(7) The low population densities within the forest are such that 
interspecific competition may be relatively unimportant and ecological 
specialization may be the result of intraspecific competition for maxi- 
mum efficiency of resource exploitation. Selection pressure for this 
must always exist, but the tropical rain forest, because of its great 
continuous extent and stability, probably generates unusually few 
counterselection pressures and, consequently, a high relative selection 
pressure toward specialization. 
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