II.] INCIPIENT STRUCTURES. 51 



we see in sea-snakes, 21 which may be the rudiment of a tail 

 formed strictly to aid in swimming. Also that a mere rough- 

 ness of the skin might be useful to a swimming animal by 

 holding the water better, that thus minute processes might 

 be selected and preserved, and that, in the same way, these 

 might be gradually increased into limbs. But it is, to say 

 the least, very questionable whether a roughness of the 

 skin, or minute processes, would be useful to a swimming 

 animal ; the motion of which they would as much impede 

 as aid, unless they were at once capable of a suitable and 

 appropriate action, which is against the hypothesis. Again, 

 the change from mere indefinite and accidental processes to 

 two regular pairs of symmetrical limbs, as the result of 

 merely fortuitous, favoring variations, is a step the feasibil- 

 ity of which hardly commends itself to the reason, seeing 

 the very different positions assumed by the ventral fins in 

 different fishes. If the above suggestion made in opposi- 

 tion to the views here asserted be true, then the general 

 constancy of position of the limbs of vertebrata may be 

 considered as due to the position assumed by the primitive 

 rugosities from which those limbs were generated. Clearly 

 only two pairs of rugosities were so preserved and devel- 

 oped, and all limbs (on this view) are descendants of the 

 same two pairs, as all have so similar a fundamental struct- 

 ure. Yet we find in many fishes the pair of fins, which 

 correspond to the hinder limbs of other animals, placed so 

 far forward as to be either on the same level with, or actu- 

 ally in front of, the normally anterior pair of limbs ; and 

 such fishes are from this circumstance called " thoracic," or 

 " jugular " fishes respectively, as the weaver-fishes and the 

 cod. This is a wonderful contrast to the fixity of position 

 of vertebrate limbs generally. If, then, such a change can 



21 It is hardly necessary to observe that these " sea-snakes " have no 

 relation to the often-talked-of " sea-serpent." They are small, venomous 

 reptiles, which abound in the Indian seas. 



