204 "Journal of Comparative Neurology and Psychology. 



conditions due to the action of similar environniiental agencies or 

 any other cause can be regarded as having weight in determining 

 homology. The question then narroAvs itself down to the problem 

 of the recognition and evaluation of evidences of genetic relation- 

 ship among organs. 



There is theoretically no limit to the diversity of the forms which 

 homologous parts may show in their transformation from type to 

 type in the course of phylogenetic history. So long as the sequence 

 can be traced in unbroken series with no admixture of foreign 

 elements into the organ complex the homology remains perfect. 

 Practically, however, such ideal relations seldom prevail, for most 

 organs are complexes of diverse tissile elements, some of which 

 may disappear in the course of a long phylogenetic history to be 

 replaced perhaps by others originally foreign to this organ. How 

 far this process of substitution may be carried and leave the indi- 

 viduality of the organ unimpaired is certainly a debatable point. 



A peripheral nerve, for instance, may show extreme variation 

 in its distribution without change in its functional composition and 

 present no difficulties to the morphologist so far as its homologies 

 are concerned. But a nciwe whose composition varies from species 

 to species may be incapable of any simple morphological treatment 

 and homology, even though its area of peripheral distribution is 

 practically constant through the whole animal series. This would 

 be the case if some functional systems represented in the nerve in 

 the higher members of the scries can be shown not to have been 

 developed out of those of the lower members, but to have entered 

 the nerve as alien structures. 



Thus, the ramus lateralis vagi is a nerve of simple composition 

 which is present throughout the Ichthyopsida, but with the widest 

 possible variation in the details of its distribution. Its homology 

 throughout the entire series is free from uncertainty in all but a 

 very few cases. When, however, we find that the ramus lateralis 

 accessorius of the facial nerve, which when present in teleosts 

 typically runs an entirely separate course into the body, in some 

 cyprinoid fishes is joined by an intra-cranial anastomosis to the 

 ramus lateral vagi and that the two nerves pass into the body fused 



