404 THE ORIGIN OF SPECIES 



sons assigned by Fritz Miiller, it is probable that at some very 

 remote period an independent adult animal, resembling the Nau- 

 plius, 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 mam- 

 mals, 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 nat- 

 uralists, 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 struc- 

 ture 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 re- 

 lated. Thus, community in embryonic structure reveals com- 

 munity 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, cr 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 larvse; we have seen, for 

 instance, that cirripedes, though externally so like shell-fish, are 

 at once known by their larvae to belong to the great class of crusta- 

 ceans. As the embryo often shows us more or less plainly the struc- 

 ture of the less modified and ancient progenitor of the group, we 

 can see why ancient and extinct forms so often resemble in their 

 adult state the embryos of existing species of the same class. 

 Agassiz believes this to be a universal law of nature; and we may 

 hope hereafter to see the law proved true. It can, however, be 

 proved true only in those cases in which the ancient state of the 

 progenitor of the group has not been wholly obliterated, either 

 by successive variations having supervened at a very early period 

 of growth, or by such variations having been inherited at an earlier 



