354 PROCEEDINGS OF THE ACADEMY OF [1887. 



lumbo-sacral nerve is found, as would be expected, in Phoccena, where 

 it aggregates 16 pairs, according to the same table. We are accord- 

 ingly forced to admit that the nerve supply going to a given limb 

 is correlated with the position along the axis, at which it first grew 

 out in the embryo. 



If it is still insisted that these comparisons are unfair, I shall now 

 propose some fresh difficulties to be disposed of by objectors to my 

 thesis, that, pairs of limbs which are apparently exactly homologous 

 upon superficial inspection are not necessarily the exact homologues 

 of each other. 



Take the cases of Coitus and Esox, if you please. We find here 

 that in the first there is a continuous series of not more than seven 

 pairs of nerves, reckoning from the occiput, which supply both the 

 pectoral and pelvic fins. Turning now to Esox, we find five pairs 

 of post-occipital nerves, which send a nerve supply to the pectoral, 

 then follows an interval of twelve paii's of ''intercostals," and it is 

 only when we reach the 18th jDOst-occipital pair, that we first find 

 nerves which pass to the pelvic limb ; eight pairs in all sending 

 branches to that limb, so that, according to the old view, we have 

 the preposterous conclusion that, the 25th pair of spinal nerves in 

 Esox are the homologues of the 7th pair in Coitus! 



We find in these two cases, moreover, that the rudiments of the 

 pelvic limbs do not grow out at the same point, in respect to the 

 median axis, but in the embryos of Coitus far in advance of the point 

 of origin of the same fin in the embryo of Esox. And in proof of 

 the fact that the pelvic fin of Coitus has not been derived by its mi- 

 gration forwards in the embryo, from a more posterior position sim- 

 ilar to that in the embryo of Esox, we have the fact that we have no 

 embryological evidence whatever, to show that such a tianslocation 

 occurs. In fact, the rudiments of the pelvic fins grow out from the 

 sides of the embryo in both genera in exactly the j^osition required 

 by the position of the nerve supj^ly in the adults. 



Further, is it to be supposed that in a Bird, where there are about 

 ten post-occipital pairs of nerves which have nothing to do with in- 

 nervating the wings, it would be fair to compare the first five of 

 these which have no direct relation to the fore-limb, with the first 

 five post-occipital pairs in Esoxf 



Here again, the embryological evidence is conclusive, since in 

 Fishes generally, the pectoral fin-fold grows out immediately behind 

 the last branchial arches and from what would be the cervical re- 



