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 

 several 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 ; 

 desce nt being t lie_Jii dden bond of connexion which 

 naturalists have been seeking under the term of the 

 Natural System. On this view we can understand how 

 it is that, in the eyes of most naturalists, the structure 

 of the embryo is even more important for classification 

 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 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 



