132 PsycJiolvgical Distinctions 



to my own satisfaction, that, as a general rule, it is in autumn 

 mainly influenced by decline of temperature, and in spring 

 by the breeding stimulus: the period of the incidence of 

 which latter (though, undoubtedly, somewhat affected by tem- 

 perature) is primarily dependent on specific peculiarity, and, 

 secondarily, on constitutional vigour.* It is not the erratic 

 impulse, however, so much as the guiding principle, that we 

 have here to do with; that wonderful, most inexplicable prin- 

 ciple, on which a diurnal migratory bird is not only, and by 



* The direct influence of decline of temperature in prompting the 

 equatorial movements of the feathered race, may be observed in the 

 fluctuations in intensity of the erratic impulse, throughout the greater 

 part of winter, exhibited by migrant birds in a state of confinement; such 

 variations being constantly found to accord with thermometrical changes. 

 It may be added, that the degree of temperature which incites them to 

 migrate varies considerably in different species ; and in some instances, 

 also, it must not be concealed, that the impulse to quit the breeding 

 station is entirely independent of decrease of temperature; as is exempli- 

 fied by the swift and adult cuckoo retiring southward at the hottest period 

 of the year: so powerful, too, is this impetus in the first-named species, 

 and others of the Ilirundinidae, that these have been many times known to 

 leave a brood of half-fledged nestlings to perish. As regards the polar 

 movement, the proximate cause will appear on consideration of the fol- 

 lowing facts: — It is known that, in the feathered race, the enlargement of 

 particular organs in spring superinduces, in most groups, some consider- 

 able change in the external aspect; frequently altering, for instance, the 

 colour of the bill, and occasioning (in single-moulting species) the rapid 

 disappearance of those deciduous edgings to the feathers, which oftentimes 

 conceal, for a while, the brighter tints of summer ; which latter, also, are, 

 in addition, commonly more or less heightened at this period. Now, all 

 these changes are observable in two nearly allied species, the chaffinch 

 and bramble finch, both of which pass the winter in the same localities ; 

 but it uniformly happens that the vernal change takes place in the former 

 species several weeks earlier than in the latter. In the beginning of 

 March, every chaffinch is found to exhibit its complete summer aspect ; 

 whereas, late in April, I have watched, with a glass, a flock of bramble 

 finches feeding on elm blossoms, in none of which had the bill acquired 

 its blue colour ; coincident with which change this species always leaves 

 the country. The fact is equally noticeable when they are kept in con- 

 finement. Fieldfares and redwings, also, linger in our fields till long after 

 their resident congeners have been engaged inbreeding; and it is found, 

 on dissecting these, at this period, that they are comparatively very back- 

 ward in their seasonal developements, the attainment of which immediately 

 prompts the migrative impulse. Of course, the breeding station is the 

 proper home of a species, and thereto do all its adaptations directly refer; 

 and thus we find that even the genial influence of a more equatorial abode 

 fails to excite the breeding energies of migrant birds, until such time as 

 their distant summer haunts become fitted for their reception. To con- 

 clude this subject, it may be added, that the migratory restlessness in 

 caged birds does not dissipate in spring, at the time of the reappearance 

 of their wild brethren, but is occasionally evinced throughout the summer, 

 till its cessation follows the decrease of those organs which had all along 

 stimulated its manifestation ; a constitutional change which likewise puts 

 a stop to song, and brings about the autumnal renovation of plumage. 



