Wilder.] 330 [June2L 



2. That plan of structure depends upon the relative normal posi- 

 tion of important organs ; while form depends upon the relative size of 

 these and other organs. If, then, it is true that the branch is determined 

 by the plan of structure, and the family by the form, it follows that rel- 

 ative size is of less morphical value than relative normal position ; it 

 seems probable, too, that the natural attitude of organs must be simi- 

 lar within the same family, since the membra of a family have not 

 only the same form, but the same mode of locomotion ; if this is true, 

 then this character also is of less morphical value than normal posi- 

 tion. 



3. I am also convinced that segmentation, or numerical composi- 

 tion, is of less morphical value than either of the characters above 

 named ; but this has been already considered. 



Tn support of the general conclusion which is expressed by the di- 

 agram (page 179), that internal characters are more valuable in the 

 determination of the more comprehensive groups, while external 

 characters are more valuable for the determination of lesser groups, 

 which would ascribe to the former more, and to the latter less, mor- 

 phical value, I can bring little direct evidence; but the following 

 passage from an eminent conchologist shows that the idea is not con- 

 fined to myself ; and I am inclined to believe that it must have been 

 in practice, at least, recognized to some extent by all who have 

 sought to reach a natural classification. 



" In all attempts to characterize the groups of animals, we find 

 that in advancing from the smaller to the larger combinations, many 

 of the most obvious external features become of less avail, and we 

 are compelled to seek for more constant and comprehensive signs in 

 the phases of embryonic development, and the condition of the cir- 

 culatory, respiratory, and nervous systems." 1 



The above is in part confirmed by Agassiz's view that the genera of 

 turtles are based upon the voluntary organs of nutrition, the jaws and 

 other muscles (200, 1, 422), and by Owen's view that the primary 

 subdivisions of the mammalia are characterized by the condition of 

 the brain (63, 2, 270 2 ), and further by the general acknowledgement 

 that osteological characters alone are often insufficient for the dis- 



1 Woodward; Recent and Fossil Shells., p. 56. 



2 Flower (Phil. Trans. 1865, p. 647), remarks of the brains of monotremes and 

 marsupials as contrasted with those of other Mammalia. "The appearance of 

 either a transverse or longitudinal section would leave no doubt as to which group 

 the brain belonged. 



