Embryonic Vestiges 183 
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; eg. 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 develop- 
ment. 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 external 
conditions. Of what use to the diamond is its high specific gravity 
and high refrangibility, and to gold of its yellow colour and great 
weight? These substances continue to exist in virtue of other 
properties than these. It is impossible to suppose that the properties 
of living matter at its first appearance were all useful to it, for even 
now after aeons of elimination we find that it possesses many useless 
organs and that many of its relations to the external world are 
capable of considerable improvement. 
