388 PHYSICAL CONDITIONS IN GENESIS OF SPECIES* 



northern South America, runs into races in wliich the abdomen is 

 bright yellow. At the same time the black markings about the head 

 increase in extent and purity and the general size becomes larger, 

 the group liaving its metropolis in the tropical regions. In conse- 

 quence of these variations in color and size this species at the sontli- 

 ward becomes differentiated into several more or less well-marked 

 subspecies (formally accorded full specific rank), which are con- 

 nected Iw an unbroken series of intergradations. 



In the great-crested flycatcher (J/yuirchus crinitus) of the United 

 States the yellow of the abdominal region is much the stronger in 

 the southern birds, while the same is true of several of the western 

 species of the same genus, which at the sonthwarcl also pass into sev- 

 eral recognizable subspecies. 



The western goldfinch (Chrysomtris psaltria) affords a well-known 

 instance of increase of black. This species is found in the western 

 half of North America from about the parallel of 40° southward to 

 Ecuador. The northern form has the black of the upper parts 

 mainly restricted to the head, wing, and tail, the rest of the dorsal 

 surface being olive green. In northern Mexico the back begins to 

 be more or less clouded with black, which tint increases in extent in 

 Central America till it wholly replaces the olive green, while in 

 northern South America it becomes more intense and lustrous. In 

 northern specimens the tail is marked with white spots, which either 

 decrease greatly in size or become wholly obsolete in the southern 

 races. The extremes, as may well be imagined, are widely diverse in 

 their coloration and, though formally regarded as entirely distinct 

 species, have been found so thoroughly to intergrade that it is im- 

 possible to draw any lines of separation between the several races. 

 Lawrences's flycatcher {Myiarchvs lawrencei) affords also a striking 

 example of southward increase in the area and intensity of black. 

 At the north Avard this species has a grayish-black crown, which grad- 

 ually passes southward into a form Avith the crown wholly deep 

 black. With the increase southward of the area and intensity of 

 black markings there is also in this, as in other species, a general 

 increase in the intensity or depth of the other accompanying tints. 



The red-bellied, or Carolina, woodpecker (Centurus caroUnvs), a 

 common bird of the United States, shows a strong increase of red on 

 the head and lower surface of the body at the southward, in wdiich 

 this tint is not only much brighter, but also much more extended in 

 the south Florida birds than in those from the Northern States. At 

 the same time it presents, in common with other species of the same 

 family, a marked southward decrease in the size of the white trans- 

 verse bars and spots of the dorsal plumage. 



In the southern portion of the Mississippi Valley the variation is 



