Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 253 



peculiar habits of life, and from other reasons assigned 

 by Fritz Miiller, it is probable that at some very remote 

 period an independent adult animal, resembling the 

 Nauplius, existed, and subsequently produced, along sev- 

 eral divergent lines of descent, the above-named great 

 Crustacean groups. So. again it is probable, from what 

 we know of the embryos of mammals, birds, fishes, and 

 reptiles, that these animals are the modified descen- 

 dants of some ancient progenitor, which was furnished 

 in its adult state with branchiae, a swim-bladder, four 

 fin-like limbs, and a long tail, all fitted for an aquatic 

 life. 



As all the organic beings, extinct and recent, which 

 have ever lived, can be arranged within a few great 

 classes; and as all within each class have, according to 

 our theory, been connected together by fine gradations, 

 the best, and, if our collections were nearly perfect, the 

 only possible arrangement, would be genealogical; de- 

 scent being the hidden bond of connexion which natural- 

 ists have been seeking under the term of the N"atural 

 System. On this view we can understand how it is that, 

 in the eyes of most naturalists, the structure of the em- 

 bryo is even more important for classification than that 

 of the adult. In two or more groups of animals, how- 

 ever 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 em- 

 bryonic 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 



