Oak, varieties of, [50].
Onites apelles, [135].
Orchis, pollen of, [193].
Organs of extreme perfection, [186].
——, electric, of fishes, [192].
—— of little importance, [194].
——, homologous, [434].
——, rudiments of, and nascent, [450].
Ornithorhynchus, [107], [416].
Ostrich not capable of flight, [134].