DEVELOPMENT AND EMBRYOLOGY. 437 



honey ; so that they now more closely resemble the ordinary 

 larvae of insects ; ultimately they undergo a further transfor- 

 mation, 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 in- 

 sects, 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 naupliusform ; 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 con- 

 nected together by fine gradations, the best, and, if our col- 

 lections 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 under- 

 stand how it is that, in the eyes of most naturalists, th« 

 structure of the embryo is even more important for classifi- 

 cation 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 embryoaid 



