CiiAr. XIH. AND EMBIIYOLOGY. -40:} 



Afterward these lan'flQ undergo a complete change ; their eyes 

 disapjiear ; tlicir legs and autenna3 become rudimentary-, and 

 tl ley feed on honey ; so that they now more closely reseniljlc 

 the ordinary larva? of insects ; ultimately they undergo a fur- 

 ther transformation, and hnally 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, their course of development would probably be 

 M-idely dillerent from what it now is ; and the hrst larval stage 

 certainly would not represent the former condition of any 

 adult and ancient insect. 



On the other hand, it is highly probable that with many 

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

 coni])letely, the state of the progenitor of the whole group in 

 its adult condition. In the great class of the Crustacea, forms 

 wondeifully distinct from each other, namely, suctorial para- 

 sites, cirripedes, entomostraca, and even the malacostraca, ap- 

 pear at iirst as larva; imder the nauplius-form ; and as these 

 larva; feed and live in the open sea, and are not adapted for 

 any peruliar habits of life, and from other reasons assigned by 

 Fritz Midler, it is probable that an independent adult animal, 

 i-estMubling the nauplius, existed at some very remote period, 

 and subsef(ncntly produced, along several diverg(;nt lines of 

 descent, the several above-named great Crustacean groups. 

 So again it is pr(»l:)able, fi-om what we know of the emljryos of 

 mannnals, birds, fishes, and reptiles, that these animals are the 

 niodilied descendants of some one ancient progenitor, which 

 Avas furnished in its adult state with branchia', a swim-1)ladder, 

 four simple 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, formerly 

 licen connected together by fine gradations, the best, and, if 

 our collections were nearly perfect, the only possible arrange- 

 ment, would be genealogical; descent being the hidden bond 

 (if connection which naturalists have been seeking vmder the 

 term of the Natural System. On tliis view we can under- 

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

 structure of the eml)ryo is even more important for classifica- 

 lion than that of the adult. In two or more groups of ani- 

 mals, however much they may dilTer from each other in struct- 

 ure and habits, if they pass tlirough closely similar embryonic 

 stagt'S, we may feel assured that they all arc descended from one 



