. The structure and life of birds . wonderful dimensions, yet No. Iis more than half its length. And each of these twocarries at its end a very big claw. Contrast with themthe digits of a Swifts wing ; there we find the firsthardly more than a quarter of the length of the second,and there is no sign of a claw on In the 1 Shufeldt, Ibis, vol. ii., 1890. 2 The hand of the embryo Hoatzin is here compared withthat of an adult Swift. The hand of an embryo Swift wouldprobably present a very great, though not so great, a contrast. IX YOUTH, MATURITY, AND ACE 28c nestling Hoatzin the hand is
. The structure and life of birds . wonderful dimensions, yet No. Iis more than half its length. And each of these twocarries at its end a very big claw. Contrast with themthe digits of a Swifts wing ; there we find the firsthardly more than a quarter of the length of the second,and there is no sign of a claw on In the 1 Shufeldt, Ibis, vol. ii., 1890. 2 The hand of the embryo Hoatzin is here compared withthat of an adult Swift. The hand of an embryo Swift wouldprobably present a very great, though not so great, a contrast. IX YOUTH, MATURITY, AND ACE 28c nestling Hoatzin the hand is longer than the forearm ;gradually it grows shorter, while the other parts of thewing lengthen, till, in the fledged bird, the forearmsurpasses it (Fig. 72 a, b). The feathers, too, adaptthemselves to changing circumstances ; in the nest-ling the growth of the two outermost primaries iscompletely arrested, so that the use of the clawsmay not be impeded ; when it is fledged and canfly, they begin again to grow and attain their full. ^Ar^l Fig. 72.—Wing of Hoatzin—(a) young, (/>) mature—after Pycraft. length. With maturity, too, the claw on digit No. Igrows small, while that on No. 2 is lost pause made by the growing quills was firstnoticed by Mr. Pycraft, and he has further pointedout that in the common chick we have traces of asimilar development. There too we find the hand ofthe young bird longer, though only by a little, thanthe forearm ; the second digit has a claw only in theembryo stage ; the first is much reduced, but retains U 290 THE STRUCTURE AND LIFE OF BIRDS chap. its claw. But the most wonderful point of all is thatthe three outermost primaries have their growth arrestedwhile the others advance in length ; later on theygrow to their full size. This can be well seen in theseries of young chickens in the hall at the SouthKensington Museum. And so the chicken, a thor-oughly modernized relative of the archaic Hoatzin,still retains this queer trac
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