1887.] NATURAL SCIENCES OF PHILADELPHIA. 355 



gion iu the bird, and Esox is no exception to this rule. In the Bird 

 (Chick and Sparrow), I find, just as was to have been expected, 

 that the pectoral limb-bud (wing) does not grow out from the sides 

 of the cervical or anterior region of the trunk as in Fishes, but some 

 distance behind it. In fact, a corresponding number of somites in- 

 tervene between the hindermost limit of the cranium and the ante- 

 rior margin of the fore-limb of the embryo bird. This gives to young 

 embryos of birds their peculiar long necked appearance, because 

 their necks are absolutely longer for morphological reasons than 

 those of other warm-blooded vertebrates. We find indeed, that the 

 fore limbs of birds are separated by a wider interval from the oral 

 opening or the base of the skull, than those of Mammals. This fact 

 will be palpably brought out, if two series of embryos of Birds and 

 Mammals of approximately the same relative stages are laid side by 

 side and compared. It will then be seen that tli^ first traces of limb- 

 buds in Birds arise farther back from the head than those of Mam- 

 mals, thus clearly showing that the fore limbs, in the two cases, do 

 not arise from serially equivalent somites. 



The fore-limb of the Bird, therefore, grows out and develops at a 

 point posterior to that at which the fore-limb of the Fish is devel- 

 oped ; the segmental elements w hich enter into the formation of the 

 fore-limb (pectoral) of the fish, are therefore not homonymous with 

 those which enter into the formation of the fore-limb (wing) of the 

 bird. Nor can less be said when we come to compare the early stages 

 of the Bird and Mammal. 



This failure of the limbs to arise from an exactly homonymous 

 series of successive segments in different groups, shows in the clearest 

 manner that such difference in origin unquestionably implies the ex- 

 istence of inexact homology, with all the morphological consequence 

 Avhich must follow. 



We have also seen that sudden or rapid forward translocation of 

 the hind limbs during their early stages, occurs only amongst Fhy- 

 soclists, where it has been repeatedly observed in different genera. 

 In Cetacea, I have elsewhere sought to explain the peculiar method 

 of their backward displacement in another manner, while the cases 

 in which it is more than probable that the fore-limbs undergo rear- 

 ward displacement include only one type of Fishes viz : the Lyomeri. 

 In no others except Pinnipeds does there appear to be the slightest 

 evidence of the occurrence of translocation. The defenders of the 

 Archipterygium hypothesis are, therefore, so far as they would avail 



