6 THE WILSON BU LLETIN— March, 1921 



eal nester ou the iiiainland, and the couditioiis of environ- 

 ment made further changes imijossible. 



The eminent anatomist Pvcroft in his paper entitled 

 " The !?5igniticauce of the Condition of the Young at Birth," 

 (Popular /Science Monthly, l.ii'i, 108 and further elaborated 

 in "The Infancy of Animals/') , however, considers that 

 birds were originally arboreal. He states that tlie struct- 

 ure of the feet of the Archwopteryx (the earliest bird known 

 to science) would prove it strictly arboreal and suggests 

 the conclusion that the reptilian stock from which the 

 Aves are descended was probably also arboreal, and con- 

 siders that we prol>ably have in tlie arboreal Soutli Amer- 

 ican Hoactzin ( the young of which scrambles about the 

 branches in a truly reptilian manner) a direct survival of 

 the proavian ty^je of nesting; -the uidifugous young dif- 

 fering from other nidifugous young in the prehensile char- 

 acter of its wings. He claims that the facts justify the 

 theory, ( 1 1 that birds were originally arboreal and their 

 young nidifugous; (2) that the nidicolous habits and! 

 helplessness of young birds are specialized adaptations to 

 an arboreal or gregarous mode of life, and ( o ) that the 

 young of Gallinaceous birds form a link in the chain of 

 evolution of nidifugous Imbits. The free finger tips and 

 arrested development of tlie outer quill-feathers point to a 

 prior arboreal habit, whilst the accelerated develoi)ment of 

 the inner quill-feathers indicates an a<laptation to enable 

 the young to escape from enemies surrounding a terrestrial 

 nursery. 



He believes that systematists attacli too much im- 

 portance to the diverse conditions presented by the young 

 of different groups of birds at birth and that the signifi- 

 cance of these conditions has been misunderstood. The 

 real explanation seems to turn to expediency, designied to 

 reduce infant mortality: (1) by depositing tlie egg upon 

 the ground, or (2) curtailing the activity of the young. 

 "One great disadvantage attendant ou precocious develop- 

 ment of the young whose nursery is in the tree tops is 

 obvious — the nestling would be constantly in danger of 

 falling to the uroniid. and a larue number would indeed 



