TO J.D.HOOKER.

Down [1854].
With respect to "highness" and "lowness," my ideas are only eclectic and not very clear.

It appears to me that an unavoidable wish to compare all animals with men, as supreme, causes some confusion; and I think that nothing besides some such vague comparison is intended, or perhaps is even possible, when the question is whether two kingdoms such as the Articulata or Mollusca are the highest.

Within the same kingdom I am inclined to think that "highest" usually means that form which has undergone most "morphological differentiation" from the common embryo or archetype of the class; but then every now and then one is bothered (as Milne Edwards has remarked) by "retrograde development," i.e., the mature animal having fewer and less important organs than its own embryo.

The specialisation of parts to different functions, or "the division of physiological labour" (35/1.