CREATION BY EVOLUTION
extended the distance covered in these leaps by stretching out their freely movable fore legs so that they acted like the wings of an aeroplane and came to support more and more of the weight of the body. Their value as wings may have been greatly increased by the fact that they were already covered with feathers, which formed a fringe along the hinder edge of the arm, exactly as the scales do to the hind leg of a crocodile. The use of the fore legs as wings in this way, however, merely supports the front end of the body and head and leaves the tail trailing behind. But just as scales cover the whole of a lizard so feathers covered the whole of the bird ancestor, including its tail, and if these feathers retained the arrangement which they had as scales and followed the crocodile pattern they formed lateral keels along that organ. These keels will also act as wings, and if they become large enough are quite capable of supporting the weight of the hinder half of the animal.
Even after the bird ancestor had progressed to the point where it could rapidly move by a series of leaps, whose length was increased by gliding, while it gained speed by an increase in the power of the hind legs, the transition to true flight was a great step, which depended on the adoption of a flapping action of the wings. Such flapping, to be effective, must be regulated in a definite way, the course taken by the wing in its down stroke being different from that which it follows as it is brought up again to begin a new stroke. This perfect regulation of flight can be most easily assured by so shaping all the faces by which the bones move on one another and so arranging the ligaments by which they are tied together that no other movements are possible. In the modern birds this end is accomplished with great perfection by an arrangement that results in the fusion of all the originally separate bones of the palm of the hand and
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