GEOGRAPHICAL DISTRIBUTION. 31 



In regard to the enlargement of peripheral parts, to the southward, it 

 seefcis not unreasonable to suppose that the increase of temperature in stimu- 

 lating the circulation in these exposed members may have something to do 

 with it, especially in view of the evidence afforded by mammals, which, in 

 general, present climatic modifications parallel with those of birds. 



" Whatever may be the cause of the above modifications of structure 

 and color, at different localities, we certainly find the following coincidences : 



" 1st. In accordance with the increase in the intensity of color in individu- 

 als of the same species from the north southward, in the northern hemisphere, 

 the brighter colored species in general represented in both the temperate and 

 tropical regions occur, as a general rule, at the southward ; the same fact 

 holding good also for sub-families. In cosmopolitan genera, families, etc., 

 the tropical species are almost always brighter colored than the extra-tropical 

 ones. All the most gorgeously colored families of birds are either exclu- 

 sively tropical or semi-tropical, with generally the outlying species more 

 plainly colored than the average for the family. 



" 2d. In accordance with the increase in the size of the bill at the south- 

 ward, all the species that have this member enormously developed are tropical 

 or semi-tropical ; not only such families as have the beak at its maximum 

 of development, as the toucans and hornbills, but in all groups in which it 

 is unusually large, the extreme development is reached in the inter-tropical 

 regions. 



" 3d. In respect to the tail, with very few exceptions, all long-tailed 

 forms reach their highest development within or near the equatorial regions. 



" The facts indicated above in respect to the inosculation of forms form- 

 erly regarded as specifically differentiated will evidently require modifications 

 of the hitherto accepted nomenclature. Evidently, many of these forms 

 are so strongly marked that they should be, in some manner, recognized in 

 nomenclature, though admittedly of less than specific rank. Most natural- 

 ists now practically recognize as species such groups of individuals as are 

 not known to graduate by nearly imperceptible stages into any other simi- 

 lar group, and as varieties such groups of individuals as occur at certain 

 localities, or over certain areas, which differ more or less from other groups 

 inhabiting other (generally contiguous) localities, with which there is evi- 



