50 THE STORY OF REPTILE LIFE. 



Perhaps the most important of these facts is that 

 which concerns the relative lengths of the fore 

 and hind limbs. In the fossil forms in question 

 the hind-limbs are conspicuously the longest ; 

 and this continues to hold good until Tertiary 

 times, since when the differences have greatly 

 diminished. From this we may gather that 

 among the earlier members of the family the 

 hind legs played a more important part in 

 swimming than now. Gradually, however, this 

 work became thrown upon the tail, and in pro- 

 portion as it took up the work of the legs the 

 latter diminished, according to the law of the 

 substitution of organs. That is to say, the tail 

 developed at the expense of the legs. 



The fore-limb reveals a still more startling 

 piece of evidence concerning the past history of 

 the group. This time we glean our information 

 not from the remains of bygone days, but from the 

 developing embryo of living Crocodiles. Herein 

 we find that the small bones or phalanges of the 

 fourth or fifth fingers are more numerous than 

 in the adult. Thus, in the fourth finger we find 

 seven, and in the fifth finger six phalanges in place 

 of four. Before development is complete these 

 additional phalanges have disappeared. To the 

 initiated the discovery of these little temporary 

 bones threw a ray of light upon the gloom which 

 enshrouded the early origin of the Crocodiles, 

 using the word in its widest sense, and not merely 

 to include the former, which had lived from the 

 Jurassic onwards. They showed that the stock 

 from which this illustrious house derives its origin 

 was probably of more strictly aquatic habits than 



