THE NATURALIST S OCCUPATION. 43 



itself have not yet been fully answered ; but the inves- 

 tigation of the last ten years has heaped up affirmative 

 evidences until the final answer has been in the main 

 anticipated. Although a Dutch embryologist, Van Wijhe, 

 has shown that Bell's law must be modified for the cra- 

 nial nerves, yet we know from the researches that started 

 with His, Balfour, and Marshall, that these nerves fol- 

 low the same general law of development as the spinal 

 nerves. We find posterior nerves with ganglia and ante- 

 rior nerves without ganglia ; and the latter are purely 

 motor as in the trunk, while the former are sensory. 

 Some of these posterior cranial nerves, however, are 

 mixed nerves ; that is, they have in addition to the regu- 

 lar sensory fibres motor fibres, and in this respect they 

 appear to depart from the spinal nerve type. But this 

 difficulty, which still remains to be cleared up, loses its 

 force as an objection, when placed beside an overwhelm- 

 ing amount of evidence in favor of the homology of the 

 two sets of nerves. The posterior nerves of the head 

 and trunk have the same origin ; and the early develop- 

 ment runs so exactly parallel in both cases, that their 

 fundamental equivalence can no longer be seriously ques- 

 tioned. The cranial ganglia, according to the researches 

 of Beard, receive, secondarily, some elements that are 

 not added to the spinal ganglia ; but homologies are 

 settled by original conditions, not by adventitious differ- 

 ences, and hence no objection can be raised on this 

 score to the identification of the nerves. That the 

 cranial nerves agree with the spinal in having a metam- 

 eric arrangement is made evident by their relations to 

 undoubted segmental structures of the head, such as 

 the gill-arches and the head-cavities. 



