iO\ RUDIMENTARY, ATEOPIIIED, Chap. XHI. 



parent-form, and are therefore closely related. Thus, coumumi- 

 iy in emlHyonic structure reveals community of descent ; but 

 dissimilarity in embryonic development does not prove discom- 

 munity of ilescent, for in one of two groups the developmental 

 stages may have been suppressed, or may have been so greatly 

 modified through adaptation to new habits of life, as to bo 

 no longer recognizaljle. Even in groups, in which the adults 

 have been moditied to an extreme degree, community of 

 origin is often revealed by the structure of the larva? ; we have 

 seen, for instance, that cirripedes, though externally so like 

 shell-fish, are at once known by their larvae to belong to the 

 gTeat class of crustaceans. As the structure of the cmbrj-o 

 generally shows us more or less plainly the structure of its less 

 modified and ancient progenitor, Ave can see why ancient and 

 extinct forms so often resemble the embryos of existing spe- 

 cies in the same class. Agassiz believes this to be a univer- 

 sal law of Nature ; and I hope to see it hereafter shown in 

 most cases true. It can, however, be proved true only in 

 those cases in which the ancient state has not been wholly ob- 

 literated either by successive variations being supervened at a 

 very early period of growth, or by such variations having been 

 inherited at an earlier period than that at which they first ap- 

 peared. It should also be borne in mind, that the law may be 

 true, but yet, o\ving to the geological record not extending far 

 enough back in time, may remain for a long period, or for ever, 

 incapable of demonstration. The laAV will not hold good in those 

 cases in which an ancient form became adapted in its larval 

 state to some special line of life, and transmitted the same larval 

 state to a whole group of descendants ; for these in their lar^'al 

 condition will not resemble any ancient form in its adult state. 

 Thus, as it seems to me, the leading facts in embryology, 

 M'hich are second to none in importance, are explained on the 

 principle of modifications in the many descendants from some 

 one ancient progenitor, not having appeared at an early period 

 of life, and having been inherited at a corresponding period. 

 Embryology rises greatly in interest, when we look at the em- 

 bryo as a picture, more or less obscured, of the progenitor, either 

 in its adult or larval state, of all the members of the same 

 great class. 



liudimentart/, Atrophied, and Aborted Organs. 



Organs or jiarts in this strange condition, bearing the plain 

 stamp of inutihtv, arc extremely common, or even general 



