supposed criterion.

If, weary of the endless difficulties involved in the determination of
species, the investigator, contenting himself with the rough practical
distinction of separable kinds, endeavours to study them as they occur
in nature--to ascertain their relations to the conditions which
surround them, their mutual harmonies and discordances of structure,
the bond of union of their parts and their past history, he finds
himself, according to the received notions, in a mighty maze, and with,
at most, the dimmest adumbration of a plan. If he starts with any one
clear conviction, it is that every part of a living creature is
cunningly adapted to some special use in its life. Has not his Paley
told him that that seemingly useless organ, the spleen, is beautifully
adjusted as so much packing between the other organs? And yet, at the
outset of his studies, he finds that no adaptive reason whatsoever can
be given for one-half of the peculiarities of vegetable structure; he
also discovers rudimentary teeth, which are never used, in the gums of
the young calf and in those of the foetal whale; insects which never
bite have rudimental jaws, and others which never fly have rudimental
wings; naturally blind creatures have rudimental eyes; and the halt
have rudimentary limbs. So, again, no animal or plant puts on its
perfect form at once, but all have to start from the same point,
however various the course which each has to pursue. Not only men and
horses, and cats and dogs, lobsters and beetles, periwinkles and
mussels, but even the very sponges and animalcules commence their
existence under forms which are essentially undistinguishable; and this
is true of all the infinite variety of plants. Nay, more, all living
beings march side by side along the high road of development, and
separate the later the more like they are; like people leaving church,
who all go down the aisle, but having reached the door some turn into