466 DEVELOPMENT AND EMBBTOLOar. 



malacostraca, appear at first as larvae under the nauplius- 

 form; and as these larvae live and feed in the open 

 sea, and are not adapted lor any peculiar habits of 

 life, and from other reasons assigned by Fritz Muller, it 

 is probable that at some very remote period an independ- 

 ent 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 descendants 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; descent being the 

 hidden bond of connection 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 embryonic 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 

 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, ai-e at once known by their larvae to belong to 

 the great class of crustaceans. As the embryo often shows 



