GREAT STEPS IN ORGANIC EVOLUTION 885 



the thumb and the index finger bear claws, which are nsed in a 

 quaintly reptilian fashion for crawling about on the branches. Now 

 Archa^opteryx had three well-developed clawed digits, the meta- 

 carpals are free from one another, and there seem to be four distinct 

 carpal bones. There is thus hardly room for doubting that Archse- 

 opteryx used its wings in clambering as well as in flight. Different 

 investigators of the two fossils have given different counts (8-12) 

 of the primary feathers, but, whatever the precise number, they 

 were certainly well developed. 



As to the hip-girdle, it is more reptilian than avian; thus the 

 number of vertebrae gripped by the ilium is only four, whereas the 

 "syn-sacrum" of modern birds has never fewer than eleven, and 

 sometimes as many as twenty-three. This remarkable increase in 

 numbers is an adaptation to perfected bipedal progression; for so 

 much of the bird's body is in front of a perpendicular dropped from 

 the hip-socket (acetabulum), that there is a great balancing advantage 

 in the long and strong grip that the hip-girdle has taken of the 

 backbone. The toppling-forward tendency, seen in an incipient biped 

 like the Australian Collared Lizard (Chlamydosaurus), and in some 

 very young birds, is corrected by the distribution of the strain over 

 a wide area of backbone. It may be noted that from the width of 

 the pelvis of Archseopteryx it has been possible to calculate the size 

 of the egg, the result being that while Archaeopteryx was about the 

 size of the common fowl, its unknown egg could be scarcely a 

 quarter of the size of a hen's. The whole hip-girdle of Archaeopteryx 

 is not even half that possessed by an average hen. 



As to the hind-leg, it is usually regarded as bird-like, except that 

 the three bones below the ankle-joint (the metatarsals), which are 

 always coalesced in modern birds, appear to have been separate, as 

 they are in reptiles; and Heilmann maintains that there were two 

 free tarsals, while no tarsals remain as such in modern birds, the 

 proximal ones fusing to the lower end of the tibia (forming the tibio- 

 tarsus), and the distal ones fusing to the upper end of the three 

 coalesced metatarsals (forming the tarso-metatarsus). 



Heilmann, whose survey of the facts is the latest, regards Archae- 

 opteryx as "a warm-blooded reptile disguised as a bird", and some 

 other zoologists also rule it out of the bird class. This seems to us 

 somewhat perverse, since there is no known hint of feathers in any 

 reptile, and the two words "feathered bipeds" serve as an exclusive 

 definition for the entire class of living birds. We therefore agree with 

 those who regard Archaeopteryx as a primitive bird with numerous 

 lingering reptilian features. 



Heilmann has redrawn all the bones of Archaeopteryx with so 

 much loving care that they have come to life; and his imaginative 

 reconstruction includes even the courtship! He has literally pictured 

 an island in the deep blue Jurassic sea, with giant sea-lizards basking 



