NORTH AMERICAN PORIFER^. PART II. 501 



ticular geographical regions. Thus there is found to be a gi^adual loss of color among indi- 

 viduals of the same species, in passing from the heavily wooded region of the eastern 

 half of the United States to the treeless plains, and the still more arid regions beyond 

 the Rocky Mountains, particularly in portions of the Great Basin and the desert tracts of 

 the Gila and Great Colorado Rivers ; the most pallid tints occurring where the vegetation 

 is most scanty, and the climate most arid, — where, in short, the protective conditions as 

 respects color are at a minimum. On the other hand, a remarkable accession of color 

 is seen over the heavily wooded, rainy region of the northwest coast. There is again a 

 marked accentuation of color to the southward, accompanying increased climatic humidity 

 and a higher temperature, tliese conditions evidently favoring in themselves an increase in 

 intensity, while they give rise to protective conditions, such as luxuriant vegetation and 

 a partially overcast sky, which favor the retention of color. It is certainly found that in 

 individuals of the same species the various shades of red, yellow, green, blue and black, 

 become stronger or more intensified in passing from the higher latitudes towards the trop- 

 ics ; that dusky and black spots and bars, or other blackish areas, increase in extent, while 

 light areas, particularly white spots and bars, become correspondingly reduced in area ; and 

 that, at the same time, there is an increase in lustre and iridescence, in species tending 

 to such phases of coloration. There is thus a general increase in the brightness and extent 

 of the stronger tints of coloration in the direction of the intertro^aical regions, where all 

 forms of animal life, as a rule, become most brilliantly colored.^ 



" In respect to the increased size of particular organs, it is found that, especially among 

 birds, there is a strong tendency to the disproportionate enlargement of peripheral parts in 

 passing from the colder parts of the North America continent toward the tropics. This is 

 particularly noticeable in resjject to the bill and tail, both of which often increase in size 

 in passing southward, either with relation to the general size of the individual, or absolutely. 

 In birds having a short thick bill, as the finches, crows, etc., the bill becomes generally 

 larger, while in slendei'-billed forms, as in some of the warblers, thrushes, wrens, the 

 grackles, etc., the bill becomes longer, slenderer, and often more decurved. In mammals 

 the pelage becomes shorter and harsher, with less underfur ; the soles of the feet often, in 

 great part, loose their furry covering, while in numerous well-marked instances the ears 

 and the feet become noticably increased in size. This may be presuniably considered as 

 due to the higher temperature of the lower latitudes favoring an increased circulation in 

 the peripheral parts of the body, and their consequent greater nourishment. Fui'thermore, 

 the increase in the size of the bill and length of the tail is in the direction of the region 

 where these parts of the ornithic organism almost universally reach their greatest devel- 

 opment. 



" In respect to the variation in size, there is generally, in both birds and mammals, a 

 marked increase toward the northern and more elevated parts of the continent, with a few 

 exceptions, however, of an equally marked increase in size toward the tropics. In general 

 terms, the law of increase in size, as modified by recent researches, may be stated as an 



'This l.aw also applies to sponges; the brighter colored greens, and yellows, with intense shades of puqilo and pur- 

 forms being generally littoral where the temperature of the plish brown or black. See, also, for increase of peripheral 

 ■water in any given locality is highest : and in the tropics parts and coloration of Insects, "Revision of the Qidipo- 

 there are comparatively few of the dull colored species so dida," by S. H. Scudder, Proc. Bost. Soc. Nat. Hist, Vol 

 abundant at the north, but a prevalence of brilliant reds, xvii, p. 482-83. 



MEMOIBS BOST. SOC. NAT. HIST. VOL. II. 126 



