sted as part of an
organ, resembling a bird's head, or an irregular box or hood. It is
interesting to see two such widely different organs developed from a
common origin; and as the movable lip of the cell serves as a protection
to the zooid, there is no difficulty in believing that all the
gradations, by which the lip became converted first into the lower
mandible of an avicularium, and then into an elongated bristle,
likewise served as a protection in different ways and under different
circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely
the structure of the flowers of orchids, and the movements of climbing
plants. With respect to the former, he says: "The explanation of their
ORIGIN is deemed thoroughly unsatisfactory--utterly insufficient to
explain the incipient, infinitesimal beginnings of structures which are
of utility only when they are considerably developed." As I have fully
treated this subject in another work, I will here give only a few
details on one alone of the most striking peculiarities of the flowers
of orchids, namely, their pollinia. A pollinium, when highly developed,
consists of a mass of pollen-grains, affixed to an elastic foot-stalk
or caudicle, and this to a little mass of extremely viscid matter. The
pollinia are by this means transported by insects from one flower to
the stigma of another. In some orchids there is no caudicle to the
pollen-masses, and the grains are merely tied together by fine threads;
but as these are not confined to orchids, they need not here be
considered; yet I may mention that at the base of the orchidaceous
series, in Cypripedium, we can see how the threads were probably
first developed. In other orchids the threads cohere at one end of the
pollen-masses; and this forms the first or nascent trace of a caudicle.
That this is the origin of the caudicle, even when of considerable
length and highly developed, we have good evidence in the aborted
pollen-grains which can sometimes be detected embedded within the
central and solid parts.

With respect to the second chief peculiarity, namely, the little mass
of viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In most
flowers belonging to other orders the stigma secretes a little viscid
matter. Now, in certain orchids similar viscid matter is secreted, but
in much larger quantities by one alone of the three