274 A HISTORY OF BIRDS 



ones : and by this token we are often enabled to trace relation- 

 ships between forms which ma}^, in adult life, show no very 

 great similarity. 



No better instance of this can be found than that afforded 

 by the young of the Thrush and Flycatcher tribe where the 

 young are invariably spotted, while the adults are for the most 

 part whole coloured above, save among a comparatively few 

 species, <'.^., Thrushes of the Genus Geochichla, the adults of 

 which are also spotted. These last, then, we may assume, 

 represent the older type of coloration once common to all the 

 species of these groups but now reappearing in the more 

 specialised species only during the earlier stages of the life- 

 history. By way of corroboration we find that it is only among 

 the species which are spotted when adult, species which have 

 never lost the primitive type of coloration, that vestiges of 

 basipterygoid processes appear in the skull. These processes 

 are heritages from reptilian ancestors, and are largest in the 

 most primitive groups, e.g., Ostrich tribe. 



The Thrushes and Flycatchers are, however, not alone in 

 passing from an immature spotted to an adult whole-coloured 

 plumage. A striking example of this fact is furnished by the 

 Common Gannet, wherein the first teleoptyle plumage is of 

 a dark-brown colour heavily spotted with white, while the adult 

 dress is pure white tinged with buff on the head and neck. 

 Here, however, though the spotted plumage may, and probably 

 does, represent an earlier adult livery, there is no evidence at 

 present which would justify the assumption that this spotted 

 livery was once common to all the Gannets, inasmuch as all 

 the other species of this Family when immature are whole 

 coloured and dark. 



A striped plumage no less than a spotted, when it appears 

 in immature birds, must, it would seem, be regarded as a heritage 

 from originally striped ancestors, and because, in so many 

 cases, these stripes are later replaced by transverse bars. This 

 has long been recognised in the case of the diurnal birds of prey, 

 for example. Thus, in the Falcons young birds have the 

 under parts conspicuously striped with broad longitudinal lines, 

 while in the adult these give place to transverse barrings. 



The period of adolescence can be measured only, in the 

 case of birds, by the diff"erences in plumage between the 



