Moby-Dick Questions Ch. 32: What is the purpose of the cetology chapter

What is the purpose of the cetology chapter?  Why does Melville say it is so difficult to classify whales?  By what criteria does Ishmael classify different whales?  What qualities does he ascribe to the sperm whale?  What color are “almost all whales”? (153 [121])  The final sentence of the next-to-last paragraph (“but signifying nothing”) is an allusion to Macbeth’s speech after he discovers that Lady Macbeth has killed herself; Shakespeare lovers, speculate on what meaning is implied here.  Why does Ishmael organize the chapter in the manner he does, and why does he leave his “cetological System standing thus unfinished”?  (157 [125])  In what ways does this last paragraph relate to the spirit of the broader narrative?