3:>0 Annals of the Carnegie Museum. 



are always abundant in the localities suitable to their habits? Or why 

 should certain cotingas as Pachyrhamphus versicolor costaricensis, 

 Platypsaris aglaice, and Laniocera rufescens be almost impossible to 

 collect, while Lathria unirufa clara, Procnias tricarumulata, Quei'iila 

 cruenta, and others are met with in large numbers throughout their 

 range? Is it because Costa Rica lies on the edge of their range, or 

 are they nowhere to be found in abundance ? 



The first hypothesis doubtless explains the rarity of a few species in 

 Costa Rica, but there are others which are not only rare in Costa 

 Rica, but are few in numbers or entirely wanting in all other places, 

 together with their near relatives. I believe there are three explana- 

 tions of this scarcity, each applying to different species : 



First, there are some species which to all appearances are rare, but 

 in reality are not so, their habitat being in places inaccessible or easily 

 overlooked on account of their retiring habits : For example, I think 

 it very probable that some of the cotingas, being almost entirely fruit- 

 eaters, rarely descend to a point below the tops of the tall forest-trees, 

 and in consequence are impossible to be seen from the ground beneath. 

 Others inhabit the almost impenetrable jungle of the Caribbean low- 

 lands, such as Gymnocichla cheiroleuca and PJicenostictus macleannani 

 saturatus, which are rarely seen outside of the dense growths of wild 

 plantains and cane found so abundantly in those regions. Second, 

 some species have a most extraordinarily local range, and may be 

 fairly abundant in one small locality, but the collector may never hap- 

 pen to locate this spot, or may only approach the edge of it, where he 

 will pick up a few stragglers. Such birds as Agyrtria boucardi, Tangara 

 florida, T. icterocephala, Buthraupis coeruleigularis, Myrmelastes Icevio- 

 stictus, CatJiarus mexicaiius, Thryorchilus ridgwayi, and Eutoxeres aquila 

 heterura belong to this group. Third, there are some species the appar- 

 ent rarity of which can be explained from none of the above causes and 

 it seems to me that there remains but one possible explanation, that 

 is, that they belong to a vanishing fauna, and for reasons of high 

 specialization, interbreeding, or inability to hold their own in the ever 

 present struggle for the survival of the fittest, are slowly disappearing. 

 Formicarius rufipectus and its allies, found in Panama, Ecuador, and 

 Colombia are splendid illustrations of this group, as are also Leucopter- 

 nis princeps, Formicarius nigricapillus, Pittasoma michleri, Grallari- 

 cula jlavirostris, Xiphocolaptes emigrans, Pseudocolaptes lawrencii, 

 Troglodytes oc/iraceus, and others. Types of this group are met with 



