nal development. When for any cause reduction
begins, it is affected at all stages of its growth, unless it has
functional importance in the larva, and in some cases its life is
shortened at one or both ends. In cases, as in that of the whale's
teeth, in which it entirely disappears in the adult, the latter part
of its life is cut off; in others, the beginning of its life may
be deferred. This happens, for instance, with the spiracle of many
Elasmobranchs, which makes its appearance after the hyobranchial cleft,
not before it as it should do, being anterior to it in position, and
as it does in the Amniota in which it shows no reduction in size as
compared with the other pharyngeal clefts. In those Elasmobranchs
in which it is absent in the adult but present in the embryo (e.g.
Carcharias) its life is shortened at both ends. Many more instances
of organs, of which the beginning and end have been cut off, might be
mentioned; e.g. the muscle-plate coelom of Aves, the primitive streak
and the neurenteric canal of amniote blastoderms. In yet other cases in
which the reduced organ is almost on the verge of disappearance, it
may appear for a moment and disappear more than once in the course of
development. As an instance of this striking phenomenon I may mention
the neurenteric canal of avine embryos, and the anterior neuropore of
Ascidians. Lastly the reduced organ may disappear in the developing
stages before it does so in the adult. As an instance of this may be
mentioned the mandibular palp of those Crustacea with zoaea larvae. This
structure disappears in the larva only to reappear in a reduced form in
later stages. In all these cases we are dealing with an organ which, we
imagine, attained a fuller functional development at some previous stage
in race-history, but in most of them we have no proof that it did so. It
may be, and the possibility must not be lost sight of, that these organs
never were anything else than functionless and that though they have
been got rid of in the adult by elimination in the course of time, they
have been able to persist in embryonic stages which are protected from
the full action of natural selection. There is no reason to suppose that
living matter at its first appearance differed from non-living matter
in possessing only properties conducive to its well-being and prolonged
existence. No one thinks that the properties of the various forms of
inorganic matter are all strictly related to exter