THE ORIGIN OF SPECIES 



similarly used for sweeping insects out of crevices in the bark of trees. But 

 the lower mandible was much shorter and thicker. This was used in wood- 

 pecker-fashion to pry or chisel bits of bark to expose its prey. This species 

 rarely visited flowers because its bill was not adapted to sucking the nec- 

 tar. H. wilsoni, which is still extant, differs from the others in that its 

 lower mandible is very heavy and chisel-like. It uses its lower mandible 

 to break away wood and bark, then sweeps its upper mandible through 

 the crevices so exposed. It never visits flowers. A closely related genus, 

 Pseudonestor, has a heavy bill which is adapted for crushing dead twigs 

 to expose beetle larvae. In a fourth genus, Psittarosfra, which appears to 

 be closely related to Loxops, the bills are finch-like, that is, they are short, 

 heavy structures for cracking hard seeds. Thus there is within this family 

 a very wide range of bill structure and feeding habits, and this has been 

 achieved within a short span of time. Differentiation of other structures 

 has been much slower. Competition for a limited food supply by closely 

 related birds has given a strong adaptive value to variations which tend 

 to open up new food sources to them. Thus there has resulted a rapid 

 adaptive radiation. This is an excellent example of character displacement 

 (see Chapter 18). 



ENDEMISM 



Closely related to the phenomena of island life is that of endemism. En- 

 demic is defined as restricted to or prevalent in a particular district. Thus 

 defined, all species are endemic, for all are confined to a definite area, 

 even though that area may be very great. For practical purposes, a species 

 is regarded as endemic if its distribution is very much more restricted 

 than that of typical species. Thus, the northern white pine, Piniis strobus, 

 is widely distributed over northeastern United States and Canada, and is 

 not regarded as endemic; but the redwood. Sequoia sempervirens, which 

 is confined to the coastal valleys of California, is an endemic. A still more 

 restricted endemic is the recently discovered living member of the genus 

 Metasequoia, most of the members of which are extinct. This species is 

 confined to a single valley in central China. Two different types of en- 

 demic species are recognized. A species may have a very restricted distri- 

 bution because it is a young species, and has not had time to expand its 

 range. Or a species may have a restricted range because it is the last 

 remnant of an old group nearing extinction. Some biologists reserve the 

 term endemics for the first type, while calling the second epibiotics. Yet 

 most species of restricted distribution fall into the latter class. They have 

 few living close relatives, and they are often well represented in paleon- 

 tologic series, as for example, Sequoia. 



Island life, particularly that of oceanic islands, is replete with endemics. 

 It has been estimated that more than 90 per cent of the flora of the Hawai- 

 ian Islands is endemic, and it is probable that the figure for animals would 

 be comparable if calculated. As already mentioned, the high proportion 

 of endemics in the Galapagos Islands was among the factors which di- 

 rected Darwin's thoughts toward the possibility of the transformation of 



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