39^ Development and Embryology. Chap. xiv. 



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 embryonic structure reveals community of descent; but dissi- 

 milarity 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 ex- 

 treme 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 larva? to 

 belong to the great class of crustaceans. As the embryo 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 believes 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 inhe- 

 rited at an earlier age than that at which they first appeared. 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 demon- 

 stration. The law will not strictly 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 such larvae 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 the principle of 

 variations in the many descendants from some one ancient proge- 

 nitor, having appeared at a not very early period of life, and having 

 been inherited at a corresponding period. Embryology rises greatly 

 in interest, when we look at the embryo 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. 



