Allen.) 216 [June 19, 
more plainly colored than the average for the family. II. In aecord- 
ance with the increase in the size of the bill at the southward, all 
the species that have this member enormously developed are tropical 
or semi-tropical, not only such families as have the beak at its maxi- 
mum of development, as the toucans.and hornbills, but in all groups 
in which it is unusually large, the extreme development is reached in 
the intertropical regions. III: In respect to the tail, with very 
few exceptions, all long-tailed- forms reach their highest develop- 
ment within or near the equatorial regions. 
The facts indicated above in respect to the inosculation of forms 
formerly regarded as specifically differentiated, will evidently require 
modifications of the hitherto accepted nomenclature. Evidently 
many of these forms are sg strongly marked that they should be in 
some manner recognized in nomenclature, though admittedly of less 
than specific rank. Most naturalists now practically recognize as 
species such groups of individuals as are not known to graduate by 
nearly imperceptible stages into any other similar group; and as va- 
rieties, such groups of individuals as occur at certain localities, or 
over certain areas, which differ more or less from other groups inhab- 
iting other (generally contiguous) localities, with which there is evi- 
dence that they do, more or less fully, intergrade. Convenience 
seems to demand such a course, in order to enable the naturalist to 
specify what particular variety or race of a species inhabits a given 
section of country—a method, in fact, already more or less generally 
practiced. . 
Finally, what is the bearing of these facts of geosraphical varia- 
tion upon the question of origin of genera and species? Having 
approached the subject from a geographical standpoint, my own im- 
pression of the importance of the conditions of environment in modi- 
fying the characteristics of animals may have unduly impressed me; 
yet that they exercise a greater influence than is currently recognized 
I think must be admitted. How, for instance, can natural or sexual 
selection satisfactorily account for the occurence of pallid forms in 
arid, semi-desert regions, and of-brighter colored forms in contiguous 
humid districts, or the generally increased intensity of color south- 
ward, and its maximum development only toward and within the 
tropical regions? In many cases, it is true, the change in color may 
be protective, as it doubtless is in the assimilation of the pale tints 
of birds and other animals inhabiting arid plains to the generally 
gray color of the vegetation and the earth itself in such locali- 
