CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 371 



their eyes disappear ; their legs and antennae become rudimentary, 

 and they feed on honey ; so that they now more closely resemble 

 the ordinary larvoa of insects ; ultimately they undergo a further 

 transformation, and finally emerge as the perfect beetle. Now, 

 if an insect, undergoing transformations like those of the Sitaris, 

 were to become the progenitor of a whole new class of insects, 

 the course of development of the new class would be widely 

 different from that of our existing insects ; and the first larval 

 stage certainly would not represent the former condition of any 

 adult and ancient form. 



On the other hand it is highly probable that with many 

 animals the embryonic or larval stages show us, more or less 

 completely, the condition of the progenitor of the whole group 

 in its adult state. In the great class of the Crustacea, forms 

 wonderfully distinct from each other, namely, suctorial parasites, 

 cirripedes, entomostraca, and even the 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 for any 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 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 genea- 

 logical; descent being the hidden 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 embryonic structure 

 reveals community of descent; but dissimilarity in embryonic 

 development does not prove discommunity of descent, for in one 

 of two grouDs the developmental stages may have been suppressed 



13*" 



