(20) 



two series, one consisting of the northern forms 0. a. alpestris, hoijti and arcticola, 

 of which the breeding areas are north of parallel 47°N.; and the other of the central 

 forms (). a. strigata, merrilli, leucolne.ma, enthi/mia and praticola, almost confined to 

 the belt between 37 and 54 N. The three other migrator)- forms — namely, 0. a. 

 aclustu, ammopliila and occidfiifulis — occnjiy restricted breeding areas in the south- 

 west of the United States. Of the ten apjiarently resident forms, 0. a. ijiraudi, 

 peregrina, chrysolaema, oaxacae, aphrasta and diaphora, have ranges to the sonth of 

 any of the migratory forms, extending from about 32' N. to close to the Equator. 

 The four remaining forms, 0. a. insularig, actia, rubta and leuca)tsi])tila, occur in 

 the west and south-west of the United States. (Cf. Map, PI. XLVIl.) 

 AVe arrive, therefore, at the following results : — 



1. All the Horned Larks which breed north of 41 N. hit. are migratory. 



2. All the forms south of 30 N. are resident. 



3. Between these parallels are fonnd forms apparently strictly resident, such as 

 0. alpestris rubea, which is confined to the Sacramento Valley, California ; and forms 

 distinctly migratory, such as 0. alpestris arliista. 



Here we have an assemblage of closely allied forms, some of which are 

 eminently migratory, while others are sedentary, and between the extremes are 

 other forms which exhibit no "regular nor well-defined movement," although 

 " there exists a greater or less individual inclination to wander during the winter " 

 (p. S02). There is some likelihood that the northern forms developed the impulse 

 of migration in consequence of a gradual extension of range. 



Most instructive instances of the development of migratory habits in con- 

 sequence of extension of range are fonnd in the almost exclusively tropical family 

 Trockilidue, one species, Selatop/iorus rufus, extending north-west in summer to 

 Cr N., while on the other hand Eusteplianus yalerifus "visits the inhospitable 

 shores of Tierra-del-Fuego, where it has been seen visiting the flowers of fuchsias in 

 a snowstorm, while it spends the winter in the warmer parts of ("hili and Bolivia" 

 fA. R. Wallace, Troincal Sature, ed. 1891, p. 323). 



