could be given of characters derived from parts which must be considered of very trifling physiological importance, but which are universally admitted as highly serviceable in the definition of whole groups. For instance, whether or not there is an open passage from the nostrils to the mouth, the only character, according to Owen, which absolutely distinguishes fishes and reptiles — the inflection of the angle of the lower jaw in Marsupials — the manner in which the wings of insects are folded — mere colour in certain Algae — mere pubescence on parts of the flower in grasses — the nature of the dermal covering, as hair or feathers, in the Vertebrata.

If the Ornithorhynchus had been covered with feathers instead of hair, this external and trifling character would have been considered by