DEVELOPMENT AND EMBRYOLOGY 489 



structure of the embryo is even more important for classi- 

 fication than that of the adult. In two or more groups of 

 animals, however much they may differ from each other in 

 structure and habits in their adult condition, if they pass 

 through closely similar embryonic stages, we may feel assured 

 that they all are descended from one parent-form, and are 

 therefore closely related. Thus, community in embrj'onic 

 structure reveals community of descent ; but dissimilarity in 

 embryonic development does not prove discommunity of 

 descent, 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 be 

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

 have been modified to an extreme degree, community of 

 origin is often revealed by the structure of the larvae; we 

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

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

 the great class of crustaceans. As the embrj-o often shows 

 us more or less plainly the structure of the less modified 

 and ancient progenitor of the group, we can see why ancient 

 and extinct forms so often resemble in their adult state the 

 embryos of existing species of the same class. Agassiz be- 

 lieves this to be a universal law of nature ; and we may hope 

 hereafter to see the law proved true. It can, however, be 

 proved true only in those cases in which the ancient state of 

 the progenitor of the group has not been wholly obliterated, 

 either by successive variations having supervened at a very 

 early period of growth, or by such variations having been 

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

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

 be true, but yet, owing to the geological record not extending 

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

 for ever, incapable of demonstration. The law will not 

 strictly hold good in those cases in which an ancient form 

 became adapted in its larvae state to some special line of life, 

 and transmitted the same larval state to a whole group of 

 descendants ; for such larval will not resemble any still more 

 ancient form in its adult state. 



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

 which are second to none in importance, are explained on 



