Wilder.] 404 [December 20, 



we not likewise conclude that the visceral arches are membra joined 

 on the middle line; that the flukes of cetacea are membra; that 

 the lateral ridges at the root of the tail in some selachians are also mem- 

 bra; and finally that the carapax of tortoises, (Agassiz, 200, 2, 562,) 

 represents a continuous series of undistinguishable membra above the 

 ordinary pairs ; this is almost a reductio ad absurdum, and is to my 

 mind sufficient evidence that in this connection at least, relative posi- 

 tion is of greater importance than apparent identity of primitive 

 structure; and that we are entitled to recognize in the vertebrate only 

 two pairs of real membra. 



There is one other fact which serves to distinguish the membra from 

 the median fins; the latter always appear as a continuous dorsal and 

 ventral ridge, which may persist in some fishes and batrachians, but 

 which is generally absorbed at intervals so as to leave certain portions 

 to form the permanent fins ; now if the membra were wholly in the 

 same category with these median fins, why should they not be formed 

 in like manner? The fact is that they never are so formed, even in 

 the skate, where, as shown by Wyman (317, p. 35 and Fig. 4), the 

 pectoral and ventral fins commence as slight ridges in the same plane 

 and in close juxtaposition, yet not continuous with each other. 



Nevertheless the opinions of the above-mentioned authorities are 

 entitled to great respect, and it can hardly be assumed that the ques- 

 tion as to the number of membra is decided; indeed, perhaps a recog- 

 nition of three or more " potential pairs of limbs" is not necessarily 

 incompatible with the idea of a meketropic relation between the armus 

 and skelos, which all agree to be homologous in some way ; but it is 

 evident that such a conception as Owen's archetype skeleton, (63, 1, 

 30), in which the diverging appendages all point backward, could not 

 co-exist with a distinct idea of meketropy ; and neither he nor Cle- 

 land, nor Humphrey, nor Parker have ever admitted such a principle 

 of organization so far as the skeleton is concerned ; it is manifestly 

 more easy to regard the membra as themselves antitropically related 

 if we can show that there are but two pairs, the one belonging to the 

 cephalic the other to the caudal half of the trunk, as seen in Fig. 4. 



The early and enormous increase of the head in the higher verte- 

 brates leaves the armal buds at about the middle of the embryo; the 

 balance is only restored when a long tail is. formed at the other end; 

 in either case the armi would seem to be most intimately connected 

 with the cervical region, and the skelea with the lumbar; but here 



