1871.] 333 [Wilder. 



it is not merely an exception to the generalizations applicable to all 

 other fishes, but which seem to constitute it a distinct class, coequal 

 with the Myzonts, Selachians, Ganoids, and Teleosts; still there 

 can be no question that this simply organized vertebrate, pre-> 

 senting the fewest organs, and the simplest functions, really does 

 come the nearest to being the realization of the ideal plan of struc- 

 ture of the branch. Now the Amphioxus may be said to be the zoo- 

 logical counterpart of the embryonic state of the higher vertebrates, 

 and to hold within the branch a central position, surrounded by the 

 more specialized organisms, as the central and constant organs of a 

 single individual are encompassed by peripheral and variable ones. 



A still better illustration will be furnished by the very question 

 now under consideration, in case it is decided in favor of antitropy ; 

 a glance at the manus and pes of most animals indicates a general 

 correspondence between them ; but they alone would furnish no sure 

 guide to the principle upon which they are to be compared in detail; 

 at any rate, even if we are not right now, the total disagreement for 

 a century is sufficient evidence of our proposition, and of the need of 

 appealing to more central and reliable parts of the membra, and even 

 to the trunk itself. 



From the foregoing considerations, there arises the suggestion that 

 the morphical value of a part of an animal, is in an inverse ratio to 

 its telical importance ; that reliability is inversely to variability ; and, 

 that hence, in determining morphical relations, we should regard pri- 

 marily, those parts which are constant in position and function, and 

 secondarily, those which are variable and inconstant, whether zoo- 

 logically, physiologically or teratically. 



The variability of the two extremes of the vertebral column is re- 

 marked by Owen, (63, 1, 94,) and Bell 1 connects peripheral variabil- 

 ity with diversity of function in language the more suggestive, as 

 coming from so " untranscendental " an anatomist. 



It is generally admitted that multiple organs, whether animal or 

 vegetal, are liable to variation, and many authors have remarked the 

 variability of the membra ; Owen refers to it in many places 2 ; T. 

 Rymer Jones 3 suggestively associates peripheral position with varia- 

 bility in number and appearance ; and Pouchet 4 goes so far as to 



1 On the Hand ; close of chap. 2. 



2 Trans. Zool. Soc, 1835, p. 353; 20, 333; 20, 269; 63, 2, 254. 

 s Cycl. of Anat. and Phys., 3, 841 and 843. 



* Plurality of races, p. 47. 



