154 LECTURE VI. 



But note what a hindrance such a fettering of the movements of the 

 caudal vertebra3 would be to creatures which progress by alternate 

 vigorous inflections of a muscular tail. A sacrum is a consolidation 

 of a greater or less proportion of the vertebral axis of the body, 

 for the transference of more or less of the weight of the body 

 upon limbs organised for its support on dry land ; such a modifi- 

 cation would have been useless to the fish, and not only useless, 

 but a hindrance and a defect. 



The pectoral fins, those curtailed prototypes of the fore -limbs of 

 other Vertebrata, with the last segment, or hand, alone projecting 

 freely from the trunk, and swathed in a common undivided tegu- 

 mentary sheath, present a condition analogous to that of the embryo 

 buds of the homologous members in the higher Vertebrata. But 

 what would have been the effect if both arm and fore-arm had 

 also extended freely from the side of the fish, and dangled as 

 a long flexible many-jointed appendage in the water ? This higher 

 development, as it is termed, in relation to the prehensile limb of the 

 denizen of dry land, would have been an imperfection in the structure 

 of the creature which is to cleave the liquid element : in it, therefore, 

 the fore limb is reduced to the smallest proportions consistent with 

 its required functions : the brachial and antibrachial segments are 

 abrogated, or hidden in the trunk : the hand alone projects and can 

 be applied, when the fish darts forwards, prone and flat, by flexion of 

 the wrist, to the side of the trunk ; or it may be extended at right 

 angles, with its flat surfaces turned forwards and backwards, so as 

 to check and arrest more or less suddenly, according to its degree 

 of extension, the progress of the fish ; its breadth may also be dimi- 

 nished or increased by approximating or divaricating the rays. In 

 the act of flexion, the fin slightly rotates and gives an oblique stroke 

 to the water. For these functions, however, the hand requires as 

 much extra development in breadth, as reduction in length and 

 thickness; and mark how this is given to the so-called embryo or 

 rudimental fore-limb : it is gained by the addition of ten, twenty, or 

 it may be even a hundred digital rays, beyond the number to which 

 the fingers are restricted, in the hand of the higher classes of 

 Vertebrata. We find, moreover, as numerous and striking modi- 

 fications of the pectoral fins, in adjustment to the peculiar ha- 

 bits of the species in Fishes, as we do of the fore limbs in any 

 of the higher classes. Tliis fin may wield a formidable and 

 special weapon of ofience, as in many Siluroid fishes. But the modi- 

 fied hands have a more constant secondary ofiice, that of touch, and 

 are applied to ascertain the nature of surrounding objects, and par- 

 ticularly the character of the bottom of the water in which the fish 



