THE HUMAN FACTOR. 301 



progress are distinctly traceable in the history of Vertebrates. 

 The first vertebrate was a prophecy of man. In the skeletal 

 structure of the humble Devonian fish, man existed in poten- 

 tiality. The whole general plan was destined to endure 

 through the history of life, and unfold in man. The Amphib- 

 ian, the Reptile, the Bird, and the Quadruped are only suc- 

 cessive modifications of the vertebrate conception embodied in 

 On'chus and Onych'odus. Still more striking are the successive 

 modifications of vertebrate limbs. In the pectoral fin of the 

 fish, we have a number of bony, articulated rays, which an- 

 swer to digits; and above, are carpal and metacarpal bones, 

 and finally, radius and ulna, humerus and scapula, exactly as 

 in all other vertebrates. When, with the commencement of 

 the purification of the air, the situation was suited to low air- 

 breathers, the pectoral fin was modified into a five-toed fore- 

 foot. When, later, the situation demanded a more perfect 

 air-breather, the reptilian limb appeared. If the reptile was 

 appointed to swim in the sea, its hand was shaped into a pad- 

 dle, sometimes with six digits and many phalanges, as in the 

 fish. If it was destined to fly, a finger was enormously elon- 

 gated, and a skinny membrane was stretched from finger to 

 hind-limb and tail. When the time came for the fitting of a 

 vertebrate to make its home in the air, the bones were made 

 hollow, to combine lightness and strength. The hand and 

 fingers were abbreviated and consolidated ; cartilage was at- 

 tached along side, and in this were inserted the broad light 

 quills which form the expansion of the wing. But here we 

 find all the structures contained in the reptilian foot, except 

 so far as changed function demanded modification. When 

 locomotive appendages for an animal of mammalian rank were 

 needed, the same set of bones was further modified to meet 

 the exigencies of different situations, instincts, and habits. 

 The paddle of the whale, the shovel of the mole, the clawed 

 foot of the cat, the cloven hoof of the ox, the solid hoof of 

 the horse, the wing-frame of the bat all are but modifications 

 of the same bony elements, adjusted in the same way, but 

 modified in relative development. All this was prophetic of 



