438 DEVELOPMENT AND EMBRYOLOGY] 



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 modi- 

 fied through adaptation to new habits of life as to be no 

 longer recognizable. 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 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 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 descend- 

 ants ; 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 

 the principle of variations in the many descendants from 

 some one ancient progenitor, having appeared at a not very 

 early period of life, and having been inherited at a corre- 

 sponding period. Embryology rises greatly in interest, 

 when we look at the embryo as a picture, more or less ob- 

 scured, of the progenitor, either in its adult or larval state, 

 of all the members of the same great class. 



