Emhryonic 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; e.g. the muscle-plate 

 coelom of Aves, the primitive streak and the ncurenteric 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 docs 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 foi-m 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 tlian 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 diliered 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 

 weiglit? These substances continue to exist in virtue of other 

 properties than these. It is impossible to supi)ose that the properties 

 of living matter at its fii-st appearance were all useful to it, for even 

 now after aeons of elimination we find that it i)osse8ses many useless 

 organs and that many of its relations to tlie external world arc 

 capable of considerable improvement. 



