30 THE RIDDLE OF THE UNIVERSE. 



in every case : in the front limb the three groups of 

 bones of the fore-foot (or " hand ") correspond in all 

 classes of the tetrapoda : (1) the carpus, (2) the meta- 

 carpus, (3) the five fingers {digiti anteriores) ; in the rear 

 limb, similarly, we have always the same three osseous 

 groups of the hind-foot : (1) the tarsus, (2) the meta- 

 tarsus, and (3) the five toes (digiti posteriores) . It was 

 a very difficult task to reduce all these little bones to 

 one primitive type, and to establish the equivalence (or 

 homology) of the separate parts in all cases; they 

 present extreme variations of form and construction in 

 detail, sometimes being partly fused together and losing 

 their individuality. This great task was first success- 

 fully achieved by the most eminent comparative - 

 anatomist of our time, Carl Gegenbaur. He pointed 

 out, in his Researches into the Comparative Anatomy of 

 the Vertebrata (1864), how this characteristic " five-toed 

 leg " of the land-tetrapods originally (not before the 

 Carboniferous period) arose out of the radiating fin (the 

 breast-fin, or the belly-fin) of the ancient fishes. He 

 had also, in his famous Researches into the Skull of the 

 Vertebrata (1872), deduced the younger skull of the 

 tetrapods from the oldest cranial form among the 

 fishes, that of the shark. 



It is especially remarkable that the original number 

 of the toes (five) on each of the four feet, which first 

 appeared in the old amphibia of the Carboniferous 

 period, has, in virtue of a strict heredity, been pre- 

 served even to the present day in man. Also, naturally 

 and harmoniously, the typical construction of the 

 joints, ligaments, muscles, and nerves of the two pairs 

 of legs has, in the main, remained the same as in the 

 rest of the " four-footed." In all these important 

 relations man is a true tetrapod. 



